WEBVTT 00:00:00.000 --> 00:00:02.640 align:middle line:84% The following content is provided under a Creative 00:00:02.640 --> 00:00:04.000 align:middle line:90% Commons license. 00:00:04.000 --> 00:00:06.400 align:middle line:84% Your support will help MIT OpenCourseWare 00:00:06.400 --> 00:00:10.080 align:middle line:84% continue to offer high-quality educational resources for free. 00:00:10.080 --> 00:00:13.320 align:middle line:84% To make a donation or view additional materials 00:00:13.320 --> 00:00:17.080 align:middle line:84% from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:17.080 --> 00:00:20.093 align:middle line:90% at ocw.mit.edu. 00:00:20.093 --> 00:00:21.760 align:middle line:84% SCOTT DONEY: We're going to transition-- 00:00:21.760 --> 00:00:23.560 align:middle line:84% we're going to have, now, a set of six 00:00:23.560 --> 00:00:26.120 align:middle line:90% lectures on the water column. 00:00:26.120 --> 00:00:32.119 align:middle line:84% So we've been-- if you like, we've 00:00:32.119 --> 00:00:34.640 align:middle line:84% been looking at all of the ways that material gets 00:00:34.640 --> 00:00:40.740 align:middle line:84% into and out of the ocean, so hydrothermal vents, rivers, 00:00:40.740 --> 00:00:46.693 align:middle line:90% air-sea-gas exchange, particles. 00:00:46.693 --> 00:00:48.360 align:middle line:84% Is there a thermostat in here we could-- 00:00:48.360 --> 00:00:50.100 align:middle line:84% AUDIENCE: He turned the heat off when he got here. 00:00:50.100 --> 00:00:50.480 align:middle line:90% SCOTT DONEY: Oh, he did? 00:00:50.480 --> 00:00:51.060 align:middle line:90% OK. 00:00:51.060 --> 00:00:54.720 align:middle line:90% 00:00:54.720 --> 00:00:57.280 align:middle line:84% What we're going to be focusing on next 00:00:57.280 --> 00:01:03.300 align:middle line:84% is actually the dynamics in the water column itself. 00:01:03.300 --> 00:01:06.980 align:middle line:84% And so we're going to start by having a set of lectures 00:01:06.980 --> 00:01:08.480 align:middle line:90% on photosynthesis. 00:01:08.480 --> 00:01:13.340 align:middle line:90% 00:01:13.340 --> 00:01:15.940 align:middle line:84% That will be the main focus for today, and a little bit 00:01:15.940 --> 00:01:18.020 align:middle line:90% into next Tuesday. 00:01:18.020 --> 00:01:21.340 align:middle line:90% 00:01:21.340 --> 00:01:24.660 align:middle line:84% And so the photosynthesis is in the upper, illuminated part 00:01:24.660 --> 00:01:26.460 align:middle line:90% of the water column. 00:01:26.460 --> 00:01:28.140 align:middle line:84% We'll then have some lectures where 00:01:28.140 --> 00:01:30.380 align:middle line:84% we look at what happens to that material 00:01:30.380 --> 00:01:33.180 align:middle line:84% once it's formed from photosynthesis. 00:01:33.180 --> 00:01:38.820 align:middle line:84% So we'll look at respiration terms, and then what's 00:01:38.820 --> 00:01:43.060 align:middle line:90% left over, which is the export. 00:01:43.060 --> 00:01:46.540 align:middle line:84% So organic matter is formed by photosynthesis. 00:01:46.540 --> 00:01:52.100 align:middle line:84% The bulk of it is actually respired in the surface layer. 00:01:52.100 --> 00:01:55.540 align:middle line:84% The part that isn't respired is either advected away 00:01:55.540 --> 00:01:57.450 align:middle line:90% or sinks as particles. 00:01:57.450 --> 00:02:00.650 align:middle line:84% We usually call the stuff left over as export because you 00:02:00.650 --> 00:02:02.770 align:middle line:84% can think of it as local production, 00:02:02.770 --> 00:02:06.090 align:middle line:84% and the material that is in excess 00:02:06.090 --> 00:02:09.889 align:middle line:84% gets exported out of those local conditions. 00:02:09.889 --> 00:02:17.770 align:middle line:84% That exported material is then going to respire at depth, 00:02:17.770 --> 00:02:22.650 align:middle line:84% and then some small part will end up in the sediments. 00:02:22.650 --> 00:02:25.410 align:middle line:84% We'll be starting out looking at-- 00:02:25.410 --> 00:02:29.290 align:middle line:90% the focus will be on carbon. 00:02:29.290 --> 00:02:35.990 align:middle line:84% But we will also fold in nitrogen, phosphorus, oxygen, 00:02:35.990 --> 00:02:38.690 align:middle line:84% some of the key limiting nutrients, and also oxygen 00:02:38.690 --> 00:02:42.370 align:middle line:84% since it's often used to understand photosynthesis 00:02:42.370 --> 00:02:44.570 align:middle line:90% and respiration. 00:02:44.570 --> 00:02:47.050 align:middle line:84% We'll have some talks that we'll look 00:02:47.050 --> 00:02:50.130 align:middle line:84% at the internal dynamics of calcium carbonate 00:02:50.130 --> 00:02:57.350 align:middle line:84% in the water column, and then also the silicate cycle, 00:02:57.350 --> 00:03:00.590 align:middle line:84% which is associated with biological formation 00:03:00.590 --> 00:03:03.430 align:middle line:90% of particles and sinking. 00:03:03.430 --> 00:03:07.190 align:middle line:84% So over the next-- over the next six lectures, 00:03:07.190 --> 00:03:10.230 align:middle line:84% we will cover all of these topics. 00:03:10.230 --> 00:03:14.710 align:middle line:90% The class notes are-- 00:03:14.710 --> 00:03:18.710 align:middle line:84% you'll get both PDFs of my lecture notes 00:03:18.710 --> 00:03:21.190 align:middle line:84% that I use to give the lecture, but there are also 00:03:21.190 --> 00:03:25.670 align:middle line:84% some older, typed lecture notes that 00:03:25.670 --> 00:03:29.030 align:middle line:90% have been posted on the web. 00:03:29.030 --> 00:03:31.750 align:middle line:84% We don't have the electronic figures 00:03:31.750 --> 00:03:35.190 align:middle line:84% that go with those because those are older notes 00:03:35.190 --> 00:03:37.550 align:middle line:84% from back in the prehistoric days 00:03:37.550 --> 00:03:42.570 align:middle line:84% when this course was taught on overheads and lots of xeroxes. 00:03:42.570 --> 00:03:46.030 align:middle line:90% 00:03:46.030 --> 00:03:50.030 align:middle line:84% The notes are really useful, but just be 00:03:50.030 --> 00:03:52.183 align:middle line:84% aware that they don't have figures. 00:03:52.183 --> 00:03:53.850 align:middle line:84% I'm trying to dig up and see if maybe we 00:03:53.850 --> 00:03:57.150 align:middle line:84% can get those figures scanned in to go with those lecture notes, 00:03:57.150 --> 00:04:00.730 align:middle line:90% but bear with us on that. 00:04:00.730 --> 00:04:03.890 align:middle line:84% So in terms of things for the exam and stuff, all 00:04:03.890 --> 00:04:07.150 align:middle line:84% you will need to know is what's being taught in the course, 00:04:07.150 --> 00:04:10.170 align:middle line:84% but you can use these older notes as background-- 00:04:10.170 --> 00:04:13.890 align:middle line:90% as background material. 00:04:13.890 --> 00:04:16.810 align:middle line:90% So today, we're going to cover-- 00:04:16.810 --> 00:04:21.029 align:middle line:84% start in on primary production, or photosynthesis. 00:04:21.029 --> 00:04:23.650 align:middle line:90% 00:04:23.650 --> 00:04:25.970 align:middle line:84% And if you're a biologist, bear with me 00:04:25.970 --> 00:04:28.930 align:middle line:84% because we're going to skim through a lot of biology 00:04:28.930 --> 00:04:31.930 align:middle line:84% in a fairly short period of time. 00:04:31.930 --> 00:04:34.630 align:middle line:84% Everything I say is hopefully correct, 00:04:34.630 --> 00:04:36.330 align:middle line:84% but it's not as detailed as you might 00:04:36.330 --> 00:04:41.530 align:middle line:84% learn in a full biological full course on marine ecology 00:04:41.530 --> 00:04:43.350 align:middle line:90% or biological oceanography. 00:04:43.350 --> 00:04:46.450 align:middle line:90% 00:04:46.450 --> 00:04:54.800 align:middle line:84% The basic reaction we're looking at here is the conversion of CO2 00:04:54.800 --> 00:05:00.340 align:middle line:84% plus water in the presence of light. 00:05:00.340 --> 00:05:03.200 align:middle line:90% 00:05:03.200 --> 00:05:17.280 align:middle line:84% So H nu, to some form of organic matter plus oxygen. 00:05:17.280 --> 00:05:21.520 align:middle line:90% This is typically called-- 00:05:21.520 --> 00:05:29.400 align:middle line:84% or can be called photolithotrophy or 00:05:29.400 --> 00:05:31.040 align:middle line:90% photolithoautotrophy. 00:05:31.040 --> 00:05:34.680 align:middle line:90% 00:05:34.680 --> 00:05:39.100 align:middle line:84% It's, photo, because the main energy is coming from light, 00:05:39.100 --> 00:05:42.360 align:middle line:90% so this is the energy source. 00:05:42.360 --> 00:05:45.540 align:middle line:84% It's, litho, because you're using-- 00:05:45.540 --> 00:05:48.320 align:middle line:90% 00:05:48.320 --> 00:05:53.200 align:middle line:84% the primary carbon source is inorganic. 00:05:53.200 --> 00:06:00.180 align:middle line:90% 00:06:00.180 --> 00:06:03.220 align:middle line:84% And it's autotrophic because you're 00:06:03.220 --> 00:06:05.740 align:middle line:90% creating that organic matter. 00:06:05.740 --> 00:06:11.100 align:middle line:84% If, instead, we had used, for example, chemotrophy-- 00:06:11.100 --> 00:06:18.340 align:middle line:90% 00:06:18.340 --> 00:06:22.220 align:middle line:84% would be a case where you were using-- 00:06:22.220 --> 00:06:24.600 align:middle line:84% instead of using light for the main energy source, 00:06:24.600 --> 00:06:27.400 align:middle line:84% you might be using some form of organic matter. 00:06:27.400 --> 00:06:30.380 align:middle line:84% So you could have chemolithotrophy, 00:06:30.380 --> 00:06:33.360 align:middle line:84% where your carbon was coming from inorganic forms, 00:06:33.360 --> 00:06:36.180 align:middle line:84% or you could have chemoorganotrophy, 00:06:36.180 --> 00:06:38.060 align:middle line:84% where you were actually taking your carbon 00:06:38.060 --> 00:06:43.860 align:middle line:84% source from organic matter as well as your energy source. 00:06:43.860 --> 00:06:46.180 align:middle line:84% So there's a whole series of these. 00:06:46.180 --> 00:06:48.360 align:middle line:84% In most of the upper ocean, we're 00:06:48.360 --> 00:06:51.600 align:middle line:84% going to be looking at photolithoautotrophy, 00:06:51.600 --> 00:06:54.360 align:middle line:84% often just called photoautotrophy, 00:06:54.360 --> 00:06:57.540 align:middle line:84% or even abbreviated as autotrophy. 00:06:57.540 --> 00:07:00.080 align:middle line:90% 00:07:00.080 --> 00:07:02.640 align:middle line:84% The reverse of this reaction, if we 00:07:02.640 --> 00:07:06.160 align:middle line:84% were to write that reaction backwards, 00:07:06.160 --> 00:07:15.460 align:middle line:84% the C organic plus O2 into CO2 plus water, is respiration. 00:07:15.460 --> 00:07:54.430 align:middle line:90% 00:07:54.430 --> 00:07:59.910 align:middle line:84% The main way that photoautotrophy is carried out 00:07:59.910 --> 00:08:01.250 align:middle line:90% in the surface ocean-- 00:08:01.250 --> 00:08:03.830 align:middle line:90% 00:08:03.830 --> 00:08:10.670 align:middle line:84% almost all of the organisms use chlorophyll 00:08:10.670 --> 00:08:14.750 align:middle line:84% as their main pigment for absorbing the light. 00:08:14.750 --> 00:08:20.310 align:middle line:84% So remember, the equation is you're taking light energy, 00:08:20.310 --> 00:08:23.070 align:middle line:84% and you're using that light energy to form organic matter 00:08:23.070 --> 00:08:25.830 align:middle line:90% out of inorganic carbon. 00:08:25.830 --> 00:08:31.390 align:middle line:84% The chlorophyll molecule-- put this over here so I can see it-- 00:08:31.390 --> 00:08:33.890 align:middle line:90% is a tetrapyrrole. 00:08:33.890 --> 00:08:42.630 align:middle line:90% 00:08:42.630 --> 00:08:44.910 align:middle line:84% Let's see if I can do this without-- 00:08:44.910 --> 00:08:54.410 align:middle line:90% 00:08:54.410 --> 00:09:03.050 align:middle line:84% so it's a magnesium ion surrounded by nitrogens, 00:09:03.050 --> 00:09:09.790 align:middle line:84% and then those nitrogens are in a-- 00:09:09.790 --> 00:09:28.770 align:middle line:90% 00:09:28.770 --> 00:09:31.050 align:middle line:90% in pyrroles. 00:09:31.050 --> 00:09:37.050 align:middle line:84% I guess the main thing you need to know about this 00:09:37.050 --> 00:09:39.730 align:middle line:90% is that the reason why-- 00:09:39.730 --> 00:09:45.230 align:middle line:84% and this is a long-- this is a long carbon chain down here. 00:09:45.230 --> 00:09:51.310 align:middle line:84% What makes this useful as an antenna for absorbing light 00:09:51.310 --> 00:09:58.910 align:middle line:84% is that the electrons in this organic framework surrounding 00:09:58.910 --> 00:10:03.550 align:middle line:90% the magnesium are distributed. 00:10:03.550 --> 00:10:04.930 align:middle line:90% These double bonds-- 00:10:04.930 --> 00:10:08.150 align:middle line:90% 00:10:08.150 --> 00:10:12.190 align:middle line:84% I could write this with a slightly different configuration 00:10:12.190 --> 00:10:17.230 align:middle line:84% where the double bonds were switched, moved over one. 00:10:17.230 --> 00:10:23.750 align:middle line:84% And it's actually a series of pi orbitals, 00:10:23.750 --> 00:10:31.150 align:middle line:84% where the electrons are actually distributed 00:10:31.150 --> 00:10:34.590 align:middle line:90% over the entire molecule. 00:10:34.590 --> 00:10:39.390 align:middle line:84% So the electrons are not isolated to a single bond. 00:10:39.390 --> 00:10:43.260 align:middle line:84% They're actually shared across the entire pyrrole. 00:10:43.260 --> 00:10:47.400 align:middle line:90% Because of that, it allows for-- 00:10:47.400 --> 00:10:51.120 align:middle line:84% it allows for easy electron transitions 00:10:51.120 --> 00:10:53.300 align:middle line:84% so you can get a photon that comes in 00:10:53.300 --> 00:10:56.660 align:middle line:84% and then bumps one of these electrons in the pi 00:10:56.660 --> 00:10:59.860 align:middle line:84% orbitals up to a higher electronic state. 00:10:59.860 --> 00:11:02.180 align:middle line:84% And so that's how you're absorbing-- 00:11:02.180 --> 00:11:09.980 align:middle line:84% you're absorbing a photon in the visible into the chlorophyll 00:11:09.980 --> 00:11:11.500 align:middle line:90% pigment. 00:11:11.500 --> 00:11:15.300 align:middle line:90% So light comes in. 00:11:15.300 --> 00:11:24.100 align:middle line:84% And you're converting light energy into an excited electron, 00:11:24.100 --> 00:11:28.220 align:middle line:84% and then that excited electron is transferred out 00:11:28.220 --> 00:11:32.320 align:middle line:84% of the chlorophyll through a series of enzymatic reactions, 00:11:32.320 --> 00:11:35.060 align:middle line:84% where it can then be-- that energy of the excited electron 00:11:35.060 --> 00:11:38.600 align:middle line:84% can be used to break and form chemical bonds. 00:11:38.600 --> 00:11:41.120 align:middle line:90% 00:11:41.120 --> 00:11:44.000 align:middle line:90% In addition to chlorophyll-- 00:11:44.000 --> 00:11:47.160 align:middle line:84% this is actually chlorophyll A. There's a whole series of what 00:11:47.160 --> 00:11:48.980 align:middle line:90% are called accessory pigments. 00:11:48.980 --> 00:11:58.440 align:middle line:90% 00:11:58.440 --> 00:12:00.400 align:middle line:84% Oh, I should just say you'll also 00:12:00.400 --> 00:12:06.420 align:middle line:84% sometimes hear this as, conjugated double bonds, 00:12:06.420 --> 00:12:07.420 align:middle line:90% double bonds. 00:12:07.420 --> 00:12:18.520 align:middle line:90% 00:12:18.520 --> 00:12:21.760 align:middle line:84% There's a whole series of accessory pigments. 00:12:21.760 --> 00:12:40.880 align:middle line:84% So for example, chlorophyll B, chlorophyll C, carotenoids, 00:12:40.880 --> 00:12:43.780 align:middle line:90% biliproteins. 00:12:43.780 --> 00:12:46.060 align:middle line:84% These accessory pigments do a variety 00:12:46.060 --> 00:12:47.900 align:middle line:84% of different things in the cells. 00:12:47.900 --> 00:12:51.020 align:middle line:84% The main antenna is typically chlorophyll, 00:12:51.020 --> 00:12:53.900 align:middle line:84% but sometimes there are accessory pigments that 00:12:53.900 --> 00:12:56.780 align:middle line:84% also absorb light at different wavelengths, 00:12:56.780 --> 00:12:59.660 align:middle line:84% and that light can then channel down and be 00:12:59.660 --> 00:13:03.780 align:middle line:84% used by the chlorophyll process because it's basically 00:13:03.780 --> 00:13:08.340 align:middle line:84% transferring the excited electron down into the reaction 00:13:08.340 --> 00:13:10.660 align:middle line:90% center of the antenna. 00:13:10.660 --> 00:13:14.940 align:middle line:84% They also have use for protection, for photoprotection. 00:13:14.940 --> 00:13:19.860 align:middle line:84% A lot of times, these pigments are there in the cell 00:13:19.860 --> 00:13:23.620 align:middle line:84% to protect the cell from either UV radiation 00:13:23.620 --> 00:13:26.300 align:middle line:84% or from other radiation from damaging 00:13:26.300 --> 00:13:29.200 align:middle line:90% the chlorophyll reaction center. 00:13:29.200 --> 00:13:31.820 align:middle line:90% 00:13:31.820 --> 00:13:34.500 align:middle line:84% So the main thing about accessory pigments 00:13:34.500 --> 00:13:45.370 align:middle line:84% is they absorb light at different wavelengths 00:13:45.370 --> 00:13:46.310 align:middle line:90% than chlorophyll. 00:13:46.310 --> 00:13:55.930 align:middle line:90% 00:13:55.930 --> 00:14:09.410 align:middle line:84% So if I were to draw a absorption 00:14:09.410 --> 00:14:18.130 align:middle line:84% spectrum for chlorophyll A, as a function of nanometers-- 00:14:18.130 --> 00:14:20.490 align:middle line:84% so this would run-- that's in nanometers. 00:14:20.490 --> 00:14:26.730 align:middle line:84% This would run, in the visible, from 400 to 700. 00:14:26.730 --> 00:14:31.650 align:middle line:84% You're going to see a peak in the blue 00:14:31.650 --> 00:14:34.650 align:middle line:84% and then another peak out in the red. 00:14:34.650 --> 00:14:38.190 align:middle line:90% 00:14:38.190 --> 00:14:41.430 align:middle line:84% So what's going to happen if you have a pigment that's 00:14:41.430 --> 00:14:43.590 align:middle line:90% absorbing in blue and red? 00:14:43.590 --> 00:14:45.210 align:middle line:90% What light is left over? 00:14:45.210 --> 00:14:47.390 align:middle line:84% What light are you actually going to see? 00:14:47.390 --> 00:14:48.690 align:middle line:90% Green, right? 00:14:48.690 --> 00:14:50.870 align:middle line:90% So this is blue. 00:14:50.870 --> 00:14:52.470 align:middle line:90% This is red. 00:14:52.470 --> 00:14:55.270 align:middle line:90% And then you have-- 00:14:55.270 --> 00:14:58.750 align:middle line:90% then you have green. 00:14:58.750 --> 00:15:03.630 align:middle line:84% And the accessory pigments can fill in in this middle region. 00:15:03.630 --> 00:15:06.190 align:middle line:84% And so they can actually channel photons 00:15:06.190 --> 00:15:08.390 align:middle line:84% that aren't absorbed by chlorophyll 00:15:08.390 --> 00:15:13.150 align:middle line:84% and provide energy into the cell in some cases. 00:15:13.150 --> 00:15:16.870 align:middle line:90% This band of, say, 400 to 700-- 00:15:16.870 --> 00:15:20.710 align:middle line:84% sometimes you'll see it written as 350 to 700-- 00:15:20.710 --> 00:15:27.830 align:middle line:90% is called PAR, which is the-- 00:15:27.830 --> 00:15:42.880 align:middle line:84% PAR is the photosynthetic available radiation. 00:15:42.880 --> 00:15:46.460 align:middle line:90% 00:15:46.460 --> 00:15:58.140 align:middle line:84% And it's typically 45% of total solar, 00:15:58.140 --> 00:16:01.220 align:middle line:90% somewhere around in that range. 00:16:01.220 --> 00:16:05.540 align:middle line:84% It does depend upon clouds and time of day and solar zenith 00:16:05.540 --> 00:16:09.120 align:middle line:84% angle, but that's a pretty decent approximation. 00:16:09.120 --> 00:16:12.460 align:middle line:84% So about half the solar light that's coming in 00:16:12.460 --> 00:16:15.540 align:middle line:84% can be used by the photoautotrophs. 00:16:15.540 --> 00:16:18.460 align:middle line:84% The other half basically goes to warming the-- 00:16:18.460 --> 00:16:21.764 align:middle line:90% warming the water. 00:16:21.764 --> 00:16:22.620 align:middle line:90% AUDIENCE: Wait. 00:16:22.620 --> 00:16:26.020 align:middle line:84% So about half can be used, or about half does get used? 00:16:26.020 --> 00:16:29.920 align:middle line:90% SCOTT DONEY: A half can be used. 00:16:29.920 --> 00:16:31.920 align:middle line:90% Yeah. 00:16:31.920 --> 00:16:35.060 align:middle line:84% Only a small fraction actually gets used in photosynthesis, 00:16:35.060 --> 00:16:37.380 align:middle line:84% but only about half is available. 00:16:37.380 --> 00:16:40.080 align:middle line:84% The other half is just-- it's gone 00:16:40.080 --> 00:16:46.040 align:middle line:84% because the photons don't have enough energy 00:16:46.040 --> 00:16:47.700 align:middle line:90% to be used in photosynthesis. 00:16:47.700 --> 00:16:57.560 align:middle line:90% 00:16:57.560 --> 00:17:04.720 align:middle line:84% You don't think of the cell as having dissolved chlorophyll. 00:17:04.720 --> 00:17:07.680 align:middle line:84% The chlorophyll is actually highly organized 00:17:07.680 --> 00:17:14.400 align:middle line:84% within the cells, within membrane structures. 00:17:14.400 --> 00:17:18.098 align:middle line:84% And in fact, the chlorophylls are an antenna, 00:17:18.098 --> 00:17:19.640 align:middle line:84% where you have something on the order 00:17:19.640 --> 00:17:28.040 align:middle line:84% of about 500 chlorophyll molecules for a single antenna. 00:17:28.040 --> 00:17:32.300 align:middle line:84% And within that antenna, most of the chlorophyll 00:17:32.300 --> 00:17:34.820 align:middle line:84% are just there to absorb light, and then 00:17:34.820 --> 00:17:37.340 align:middle line:84% they transfer these excited electrons 00:17:37.340 --> 00:17:40.140 align:middle line:84% down to what's called a reaction center. 00:17:40.140 --> 00:17:46.040 align:middle line:84% And it's at the reaction center that that electronically-- 00:17:46.040 --> 00:17:50.540 align:middle line:84% that excited electron is then transferred into enzyme systems 00:17:50.540 --> 00:17:53.780 align:middle line:84% that can do actual work for the-- 00:17:53.780 --> 00:17:55.820 align:middle line:90% work for the organism. 00:17:55.820 --> 00:18:05.060 align:middle line:84% There's two parts to the photosystem called Photosystem 00:18:05.060 --> 00:18:06.720 align:middle line:90% I and Photosystem II. 00:18:06.720 --> 00:18:14.580 align:middle line:90% 00:18:14.580 --> 00:18:17.460 align:middle line:84% And I don't expect you to know the full details of these 00:18:17.460 --> 00:18:23.180 align:middle line:84% because these get rather complicated. 00:18:23.180 --> 00:18:26.770 align:middle line:84% But you should know that there are these two 00:18:26.770 --> 00:18:33.250 align:middle line:84% different photosystems, and they have different functions. 00:18:33.250 --> 00:18:40.718 align:middle line:84% Photosystem I is involved in the reduction 00:18:40.718 --> 00:18:45.530 align:middle line:90% of a molecule called NADP. 00:18:45.530 --> 00:19:14.130 align:middle line:84% And NADP stands for nicotinamide adenine dinucleotide phosphate, 00:19:14.130 --> 00:19:18.810 align:middle line:84% which is a bit of a mouthful, which is why we typically call 00:19:18.810 --> 00:19:21.810 align:middle line:90% it-- typically call it NADP. 00:19:21.810 --> 00:19:29.630 align:middle line:84% So when you reduce or NADP, it goes to NADPH, 00:19:29.630 --> 00:19:32.130 align:middle line:84% where you've actually added a proton to it. 00:19:32.130 --> 00:19:34.790 align:middle line:90% 00:19:34.790 --> 00:19:44.350 align:middle line:84% Photosystem II is involved in the liberation of oxygen 00:19:44.350 --> 00:19:45.210 align:middle line:90% from water. 00:19:45.210 --> 00:19:49.190 align:middle line:90% 00:19:49.190 --> 00:19:51.830 align:middle line:84% And the two are actually coupled so 00:19:51.830 --> 00:19:55.270 align:middle line:84% that you can think of this as a chain, where 00:19:55.270 --> 00:19:58.070 align:middle line:84% you need both Photosystem I and Photosystem 00:19:58.070 --> 00:20:01.210 align:middle line:90% II to be functioning. 00:20:01.210 --> 00:20:10.230 align:middle line:90% 00:20:10.230 --> 00:20:18.710 align:middle line:84% If you combine them and look at the net effect of both of them, 00:20:18.710 --> 00:20:21.190 align:middle line:84% the reaction looks something like this 00:20:21.190 --> 00:20:22.730 align:middle line:90% under light conditions. 00:20:22.730 --> 00:20:27.130 align:middle line:90% 00:20:27.130 --> 00:20:42.610 align:middle line:84% You're taking two waters plus four NADP-plus with 00:20:42.610 --> 00:20:54.830 align:middle line:84% approximately eight photons, going to oxygen plus 2-- 00:20:54.830 --> 00:20:59.450 align:middle line:90% 00:20:59.450 --> 00:21:00.870 align:middle line:84% oh, why did I write 4 over there? 00:21:00.870 --> 00:21:01.390 align:middle line:90% Sorry. 00:21:01.390 --> 00:21:02.390 align:middle line:90% I'm having a brain fart. 00:21:02.390 --> 00:21:04.690 align:middle line:90% This is a 2. 00:21:04.690 --> 00:21:07.010 align:middle line:90% Can't even read my own notes. 00:21:07.010 --> 00:21:11.070 align:middle line:90% NADPH plus two protons. 00:21:11.070 --> 00:21:16.450 align:middle line:90% 00:21:16.450 --> 00:21:21.800 align:middle line:84% In addition to reducing two NADPs, 00:21:21.800 --> 00:21:29.000 align:middle line:90% you also generate four ATPs. 00:21:29.000 --> 00:21:34.440 align:middle line:84% So you take four ATPs plus four inorganic phosphate, 00:21:34.440 --> 00:21:40.520 align:middle line:90% and you form four ATPs. 00:21:40.520 --> 00:21:45.900 align:middle line:84% Now, ATP is the basic energy currency within the cell. 00:21:45.900 --> 00:21:49.600 align:middle line:90% 00:21:49.600 --> 00:21:56.580 align:middle line:90% ATP is adenosine triphosphate. 00:21:56.580 --> 00:22:05.240 align:middle line:90% 00:22:05.240 --> 00:22:08.200 align:middle line:84% ADP has two phosphate units on it. 00:22:08.200 --> 00:22:10.200 align:middle line:90% ATP has three. 00:22:10.200 --> 00:22:12.920 align:middle line:84% You're storing energy in that bond 00:22:12.920 --> 00:22:15.120 align:middle line:84% between that third phosphate unit, 00:22:15.120 --> 00:22:17.800 align:middle line:84% between the second and the third phosphate unit. 00:22:17.800 --> 00:22:20.780 align:middle line:84% And so in the process of forming ATP, 00:22:20.780 --> 00:22:23.120 align:middle line:84% you're storing energy in that ATP. 00:22:23.120 --> 00:22:26.060 align:middle line:84% That ATP can be used later by enzymes 00:22:26.060 --> 00:22:29.440 align:middle line:84% to drive reactions that require energy. 00:22:29.440 --> 00:22:34.820 align:middle line:90% 00:22:34.820 --> 00:22:37.680 align:middle line:84% So this is how the cell generates energy. 00:22:37.680 --> 00:22:40.820 align:middle line:84% So far, we've liberated oxygen, and we've 00:22:40.820 --> 00:22:46.540 align:middle line:84% generated reducing power and stored chemical energy. 00:22:46.540 --> 00:22:48.880 align:middle line:90% So this is your reducing power. 00:22:48.880 --> 00:22:51.820 align:middle line:90% 00:22:51.820 --> 00:22:57.780 align:middle line:90% This is your energy. 00:22:57.780 --> 00:23:02.580 align:middle line:84% But we haven't made any organic matter yet. 00:23:02.580 --> 00:23:05.180 align:middle line:84% To make organic matter, there's a series 00:23:05.180 --> 00:23:08.180 align:middle line:84% of what are called the dark reactions, 00:23:08.180 --> 00:23:09.920 align:middle line:90% or the Calvin-Benson cycle. 00:23:09.920 --> 00:23:17.320 align:middle line:90% 00:23:17.320 --> 00:23:21.480 align:middle line:84% And these are involved in the formation, the actual formation 00:23:21.480 --> 00:23:23.840 align:middle line:90% of organic matter. 00:23:23.840 --> 00:23:40.560 align:middle line:84% So the reaction is two protons plus CO2 plus two NADPH, 00:23:40.560 --> 00:23:44.900 align:middle line:84% goes to some generic carbohydrate. 00:23:44.900 --> 00:23:48.320 align:middle line:90% We'll get to that in a moment. 00:23:48.320 --> 00:23:53.440 align:middle line:84% So this is a generic carbohydrate plus water 00:23:53.440 --> 00:23:59.280 align:middle line:90% plus two NADP-plus. 00:23:59.280 --> 00:24:03.600 align:middle line:84% So essentially, you use your reducing power 00:24:03.600 --> 00:24:06.920 align:middle line:84% to reduce the oxidized form of carbon 00:24:06.920 --> 00:24:11.160 align:middle line:84% to a more reduced form in the form of the organic matter. 00:24:11.160 --> 00:24:14.340 align:middle line:90% And this uses up three ATPs-- 00:24:14.340 --> 00:24:20.230 align:middle line:90% 00:24:20.230 --> 00:24:29.290 align:middle line:84% ADP plus 3 P-I. Notice that there is a difference. 00:24:29.290 --> 00:24:30.690 align:middle line:90% This generates four ATP. 00:24:30.690 --> 00:24:34.230 align:middle line:84% This generates three ATP, or uses up three ATP. 00:24:34.230 --> 00:24:39.550 align:middle line:84% There is an extra ATP that can be used within the cell. 00:24:39.550 --> 00:24:41.470 align:middle line:84% And it depends on what you're synthesizing 00:24:41.470 --> 00:24:45.110 align:middle line:84% and all sorts of things like that. 00:24:45.110 --> 00:24:50.110 align:middle line:84% The net reaction, if you combine the light reactions 00:24:50.110 --> 00:25:01.530 align:middle line:84% and dark reactions, are CO2 plus two H2O goes to-- 00:25:01.530 --> 00:25:03.930 align:middle line:84% with approximately eight photons. 00:25:03.930 --> 00:25:13.190 align:middle line:90% 00:25:13.190 --> 00:25:16.170 align:middle line:84% Now, I say approximately eight photons 00:25:16.170 --> 00:25:20.410 align:middle line:84% because not all the photons absorbed end up 00:25:20.410 --> 00:25:22.710 align:middle line:90% being used biochemically. 00:25:22.710 --> 00:25:25.970 align:middle line:90% 00:25:25.970 --> 00:25:30.010 align:middle line:84% For example, you can lose photons by fluorescence. 00:25:30.010 --> 00:25:31.690 align:middle line:90% Chlorophyll fluoresces. 00:25:31.690 --> 00:25:33.690 align:middle line:84% In fact, that's one of the ways you 00:25:33.690 --> 00:25:36.130 align:middle line:84% make measurements of chlorophyll, 00:25:36.130 --> 00:25:42.090 align:middle line:84% is you shine a beam of light into a seawater sample. 00:25:42.090 --> 00:25:45.110 align:middle line:84% You shine a beam of light at one wavelength, 00:25:45.110 --> 00:25:47.610 align:middle line:90% at a high energy of photon. 00:25:47.610 --> 00:25:51.150 align:middle line:84% Some of those photons are absorbed by the chlorophyll, 00:25:51.150 --> 00:25:52.390 align:middle line:90% and then they fluoresce. 00:25:52.390 --> 00:25:59.250 align:middle line:84% They re-emit a photon at a different, lower wavelength. 00:25:59.250 --> 00:26:03.210 align:middle line:84% And that can be a very accurate or a very precise 00:26:03.210 --> 00:26:06.308 align:middle line:84% measurement because you're measuring photons 00:26:06.308 --> 00:26:07.850 align:middle line:84% at a different wavelength than you're 00:26:07.850 --> 00:26:09.770 align:middle line:90% putting them into the system. 00:26:09.770 --> 00:26:12.110 align:middle line:84% So it's not- you're not measuring the absorbance, where 00:26:12.110 --> 00:26:14.910 align:middle line:84% you have to worry about scattering of photons 00:26:14.910 --> 00:26:16.570 align:middle line:84% and all sorts of things like that. 00:26:16.570 --> 00:26:19.710 align:middle line:84% You're actually looking at a distinct signature that's 00:26:19.710 --> 00:26:22.750 align:middle line:84% specific to chlorophyll, which is that re-fluorescence 00:26:22.750 --> 00:26:24.910 align:middle line:90% at a different wavelength. 00:26:24.910 --> 00:26:28.510 align:middle line:84% But some of the photons absorbed by the chlorophyll, 00:26:28.510 --> 00:26:33.310 align:middle line:84% depending upon the conditions, might just get re-fluoresced, 00:26:33.310 --> 00:26:37.710 align:middle line:90% or they might be lost into heat. 00:26:37.710 --> 00:26:40.430 align:middle line:84% So not all the photons that get absorbed 00:26:40.430 --> 00:26:42.910 align:middle line:84% by the chlorophyll antenna end up 00:26:42.910 --> 00:26:45.010 align:middle line:90% driving this reaction forward. 00:26:45.010 --> 00:26:50.910 align:middle line:90% 00:26:50.910 --> 00:26:56.230 align:middle line:84% Another thing is I've written this generic carbohydrate, 00:26:56.230 --> 00:27:02.430 align:middle line:84% but the cell needs a whole bunch of things that are carbon-based. 00:27:02.430 --> 00:27:06.690 align:middle line:84% It needs to make sugars, which go into carbohydrates. 00:27:06.690 --> 00:27:11.660 align:middle line:90% 00:27:11.660 --> 00:27:19.080 align:middle line:84% It needs to make amino acids, which go into proteins. 00:27:19.080 --> 00:27:23.060 align:middle line:90% 00:27:23.060 --> 00:27:34.860 align:middle line:84% It needs to make fatty acids, that go into lipids. 00:27:34.860 --> 00:27:43.520 align:middle line:84% And it needs to make nucleotides, that go into DNA. 00:27:43.520 --> 00:27:46.960 align:middle line:90% 00:27:46.960 --> 00:27:49.900 align:middle line:84% The amino acids and the nucleotides 00:27:49.900 --> 00:27:52.660 align:middle line:90% have a lot of nitrogen in them. 00:27:52.660 --> 00:27:54.220 align:middle line:84% So in addition to carbon, you also 00:27:54.220 --> 00:27:59.220 align:middle line:84% need nitrogen to form amino acids and nucleotides. 00:27:59.220 --> 00:28:04.233 align:middle line:84% And if you actually look at the cell's biochemistry, 00:28:04.233 --> 00:28:05.900 align:middle line:84% they're going to have-- this equation is 00:28:05.900 --> 00:28:10.280 align:middle line:84% going to be slightly different, depending upon what it's making. 00:28:10.280 --> 00:28:22.760 align:middle line:84% So for example, if I were making a fatty acid-- 00:28:22.760 --> 00:28:24.360 align:middle line:84% oh, well, let's go back for a second. 00:28:24.360 --> 00:28:28.880 align:middle line:84% If I look at this reaction, I can define something called 00:28:28.880 --> 00:28:31.460 align:middle line:90% the photosynthetic quotient. 00:28:31.460 --> 00:28:42.000 align:middle line:90% 00:28:42.000 --> 00:28:49.400 align:middle line:84% And photosynthetic quotient is just the ratio of O2 to CO2. 00:28:49.400 --> 00:28:54.920 align:middle line:84% So it's the ratio of the O2 that you produce per CO2 molecule 00:28:54.920 --> 00:28:58.320 align:middle line:90% that you consume. 00:28:58.320 --> 00:29:07.820 align:middle line:84% And so for this reaction here, the PQ would be 1. 00:29:07.820 --> 00:29:16.180 align:middle line:84% But in fact, the PQ for most algae and photoautotrophs 00:29:16.180 --> 00:29:18.820 align:middle line:84% is greater than 1, and it's because you're 00:29:18.820 --> 00:29:23.620 align:middle line:84% forming, for example, fatty acids or other compounds. 00:29:23.620 --> 00:29:28.160 align:middle line:84% So a fatty acid might look like-- 00:29:28.160 --> 00:29:39.220 align:middle line:90% 00:29:39.220 --> 00:29:42.020 align:middle line:84% and the photosynthetic quotient for that 00:29:42.020 --> 00:29:58.860 align:middle line:84% would actually be more like around 1.42, rather than 1. 00:29:58.860 --> 00:30:01.920 align:middle line:84% And so there's been a lot of work 00:30:01.920 --> 00:30:05.010 align:middle line:84% within the community to try to come up 00:30:05.010 --> 00:30:13.770 align:middle line:84% with an estimate of the photosynthetic quotient 00:30:13.770 --> 00:30:15.030 align:middle line:90% for phytoplankton. 00:30:15.030 --> 00:30:16.950 align:middle line:90% And there's two ways to do that. 00:30:16.950 --> 00:30:24.010 align:middle line:90% 00:30:24.010 --> 00:30:31.170 align:middle line:84% One is just to take your algae and essentially burn them, 00:30:31.170 --> 00:30:33.770 align:middle line:84% add oxygen until you've taken all the organic matter 00:30:33.770 --> 00:30:35.790 align:middle line:90% and converted it into CO2. 00:30:35.790 --> 00:30:45.610 align:middle line:90% 00:30:45.610 --> 00:30:48.370 align:middle line:84% The other way is to look at the water column 00:30:48.370 --> 00:30:53.530 align:middle line:84% and to see what-- when you change inorganic carbon, what's 00:30:53.530 --> 00:30:56.330 align:middle line:84% the relative change in oxygen. And we'll talk about that 00:30:56.330 --> 00:30:58.050 align:middle line:84% more when we get to remineralization, 00:30:58.050 --> 00:31:01.570 align:middle line:84% about different techniques that have been used for that. 00:31:01.570 --> 00:31:11.630 align:middle line:84% But the canonical numbers are that the oxygen-to-CO2 ratio 00:31:11.630 --> 00:31:17.910 align:middle line:84% for algae is something in the range of. 00:31:17.910 --> 00:31:23.470 align:middle line:84% 150 to 170 over 106, where you have 00:31:23.470 --> 00:31:26.910 align:middle line:84% a ratio that looks something like this, where you have 00:31:26.910 --> 00:31:38.990 align:middle line:84% C 106, some number of hydrogens, oxygens, nitrogens and one 00:31:38.990 --> 00:31:40.010 align:middle line:90% phosphate. 00:31:40.010 --> 00:31:48.427 align:middle line:84% So this would be a canonical algae organic molecule. 00:31:48.427 --> 00:31:51.010 align:middle line:84% There is, of course, no organic molecule that looks like that, 00:31:51.010 --> 00:31:56.270 align:middle line:84% but that's what the composite might look like. 00:31:56.270 --> 00:32:02.650 align:middle line:84% A lot of this work dates back to Alfred Redfield, 00:32:02.650 --> 00:32:10.210 align:middle line:84% who was a professor of physiology at Harvard 00:32:10.210 --> 00:32:14.072 align:middle line:84% and was also one of the first marine chemists here at WHOI. 00:32:14.072 --> 00:32:15.530 align:middle line:84% And, in fact, the Redfield building 00:32:15.530 --> 00:32:18.690 align:middle line:90% is named after Alfred Redfield. 00:32:18.690 --> 00:32:21.270 align:middle line:84% These are sometimes called Redfield ratios. 00:32:21.270 --> 00:32:25.290 align:middle line:90% 00:32:25.290 --> 00:32:26.990 align:middle line:84% When we do nitrogen and phosphorus, 00:32:26.990 --> 00:32:29.250 align:middle line:84% we'll come back to this in more detail. 00:32:29.250 --> 00:32:32.530 align:middle line:84% Most of the focus has been on the nitrogen 00:32:32.530 --> 00:32:36.130 align:middle line:84% to carbon, the phosphorus to carbon, but the oxygen to carbon 00:32:36.130 --> 00:32:38.030 align:middle line:90% is equally important. 00:32:38.030 --> 00:32:41.350 align:middle line:84% And it's still not actually fully resolved. 00:32:41.350 --> 00:32:43.310 align:middle line:84% It probably varies in time and space, 00:32:43.310 --> 00:32:45.290 align:middle line:84% depending upon what the community-- 00:32:45.290 --> 00:32:51.010 align:middle line:84% who's there and what kind of environmental 00:32:51.010 --> 00:32:52.310 align:middle line:90% stresses they're under. 00:32:52.310 --> 00:32:55.650 align:middle line:90% 00:32:55.650 --> 00:32:59.360 align:middle line:90% So that's photosynthesis. 00:32:59.360 --> 00:33:09.480 align:middle line:84% The reaction, CO2 plus water, under light conditions, 00:33:09.480 --> 00:33:15.020 align:middle line:90% to organic matter. 00:33:15.020 --> 00:33:17.560 align:middle line:84% The forward version of this reaction 00:33:17.560 --> 00:33:24.340 align:middle line:84% is often called gross primary production. 00:33:24.340 --> 00:33:28.680 align:middle line:90% 00:33:28.680 --> 00:33:32.440 align:middle line:84% You'll sometimes hear it called GPP. 00:33:32.440 --> 00:33:34.340 align:middle line:90% And it's just jargon. 00:33:34.340 --> 00:33:37.520 align:middle line:90% 00:33:37.520 --> 00:33:40.280 align:middle line:84% The problem is typically, we don't 00:33:40.280 --> 00:33:47.920 align:middle line:84% measure gross primary production because the cells also 00:33:47.920 --> 00:33:55.720 align:middle line:84% need the autotrophs respire some of the carbon 00:33:55.720 --> 00:34:01.420 align:middle line:90% that they make for energy. 00:34:01.420 --> 00:34:03.500 align:middle line:84% So for example, at night-- during the day, 00:34:03.500 --> 00:34:05.000 align:middle line:90% they photosynthesize. 00:34:05.000 --> 00:34:07.980 align:middle line:84% At night there's a basal metabolism 00:34:07.980 --> 00:34:10.380 align:middle line:90% or a basal respiration. 00:34:10.380 --> 00:34:12.199 align:middle line:84% So if you were to watch a cell at night, 00:34:12.199 --> 00:34:15.020 align:middle line:84% the cell would actually be consuming oxygen 00:34:15.020 --> 00:34:16.980 align:middle line:90% and using organic matter. 00:34:16.980 --> 00:34:18.639 align:middle line:84% That's also going on during the day. 00:34:18.639 --> 00:34:22.340 align:middle line:84% It's just very hard to see with normal techniques. 00:34:22.340 --> 00:34:26.540 align:middle line:84% And we will talk, I think on Tuesday, 00:34:26.540 --> 00:34:32.659 align:middle line:84% more about how these measurements are actually made. 00:34:32.659 --> 00:34:36.139 align:middle line:84% So what we typically measure is something more 00:34:36.139 --> 00:34:45.179 align:middle line:84% like what's called net primary production, which 00:34:45.179 --> 00:34:54.860 align:middle line:84% is GPP minus the autotroph respiration. 00:34:54.860 --> 00:35:02.680 align:middle line:90% 00:35:02.680 --> 00:35:06.680 align:middle line:84% For most systems, the autotroph respiration is pretty small. 00:35:06.680 --> 00:35:11.240 align:middle line:84% They're fairly efficient, so it's often less than 00:35:11.240 --> 00:35:13.280 align:middle line:90% or equal to 0.1. 00:35:13.280 --> 00:35:22.240 align:middle line:84% The respiration is equal to less than or equal to 0.1 GPP. 00:35:22.240 --> 00:35:28.340 align:middle line:84% But for example, the are systems, for example, 00:35:28.340 --> 00:35:30.840 align:middle line:84% in the tropics, in oligotrophic regions, 00:35:30.840 --> 00:35:35.560 align:middle line:90% where it can get up to 0.4 GPP. 00:35:35.560 --> 00:35:38.960 align:middle line:84% And some of this is what's called photorespiration, 00:35:38.960 --> 00:35:42.880 align:middle line:90% which is simply the cells-- 00:35:42.880 --> 00:35:46.160 align:middle line:84% they have the antenna there for absorbing light. 00:35:46.160 --> 00:35:49.480 align:middle line:84% They're driving the reaction this way, 00:35:49.480 --> 00:35:51.880 align:middle line:84% but they don't have enough nutrients 00:35:51.880 --> 00:35:55.850 align:middle line:84% to form all the other parts of their cells that they need. 00:35:55.850 --> 00:35:58.110 align:middle line:84% They don't have enough nitrogen and phosphorus 00:35:58.110 --> 00:36:01.390 align:middle line:90% to build amino acids and DNA. 00:36:01.390 --> 00:36:04.170 align:middle line:84% So they can't replicate themselves. 00:36:04.170 --> 00:36:05.870 align:middle line:90% They can't grow new cells. 00:36:05.870 --> 00:36:08.270 align:middle line:84% And so they just churn out-- they just 00:36:08.270 --> 00:36:11.950 align:middle line:90% recycle that organic matter. 00:36:11.950 --> 00:36:13.710 align:middle line:84% So sometimes, in some environments, 00:36:13.710 --> 00:36:16.450 align:middle line:84% respiration can get rather large. 00:36:16.450 --> 00:36:23.150 align:middle line:90% 00:36:23.150 --> 00:36:27.190 align:middle line:90% Any questions so far? 00:36:27.190 --> 00:36:28.390 align:middle line:90% Nope. 00:36:28.390 --> 00:36:29.390 align:middle line:90% OK. 00:36:29.390 --> 00:36:32.050 align:middle line:90% So who are these autotrophs? 00:36:32.050 --> 00:36:38.470 align:middle line:90% 00:36:38.470 --> 00:36:50.110 align:middle line:84% There's something like about 20,000 phytoplankton species. 00:36:50.110 --> 00:36:56.610 align:middle line:84% And I'm going to put "species" in quotes because unlike mammals 00:36:56.610 --> 00:37:01.530 align:middle line:84% or reptiles, it's often hard to define what a species is. 00:37:01.530 --> 00:37:09.530 align:middle line:84% And in fact, one of the rather well-known and well-studied 00:37:09.530 --> 00:37:14.947 align:middle line:84% phytoplankton species, prochlorococcus-- 00:37:14.947 --> 00:37:16.530 align:middle line:84% when they actually went in and started 00:37:16.530 --> 00:37:19.250 align:middle line:84% to look at the genome of prochlorococcus, 00:37:19.250 --> 00:37:21.970 align:middle line:84% they found a variety of different strains, 00:37:21.970 --> 00:37:24.610 align:middle line:84% and the genomic differences between them 00:37:24.610 --> 00:37:26.570 align:middle line:90% were rather stunning. 00:37:26.570 --> 00:37:31.530 align:middle line:84% Huge amounts of differences in the DNA code. 00:37:31.530 --> 00:37:34.230 align:middle line:84% And then that raises the question of, really, 00:37:34.230 --> 00:37:36.090 align:middle line:90% are these actually-- 00:37:36.090 --> 00:37:41.210 align:middle line:84% are these truly ecotypes or variants of the same species, 00:37:41.210 --> 00:37:43.930 align:middle line:84% or are they very different species 00:37:43.930 --> 00:37:47.730 align:middle line:84% that just happen to look approximately the same to us 00:37:47.730 --> 00:37:51.790 align:middle line:84% because we're looking at very small cells, 00:37:51.790 --> 00:37:56.030 align:middle line:84% and the techniques we're using are rather crude to characterize 00:37:56.030 --> 00:37:57.510 align:middle line:90% what is a species? 00:37:57.510 --> 00:38:02.830 align:middle line:84% So the paradigm number is 20,000, 00:38:02.830 --> 00:38:06.690 align:middle line:84% but this is a really loose definition of what a species is, 00:38:06.690 --> 00:38:12.030 align:middle line:84% particularly because a lot of the bacterial species, 00:38:12.030 --> 00:38:15.910 align:middle line:84% in particular, are able to exchange genomic information 00:38:15.910 --> 00:38:19.430 align:middle line:84% across species boundaries quite readily. 00:38:19.430 --> 00:38:22.550 align:middle line:84% And so you'll see lateral gene transfer, 00:38:22.550 --> 00:38:24.910 align:middle line:84% which begs the question of, well, what's a species, 00:38:24.910 --> 00:38:31.310 align:middle line:84% then, if they're able to exchange genetic information? 00:38:31.310 --> 00:38:36.110 align:middle line:84% There are, however, some significant differences 00:38:36.110 --> 00:38:39.590 align:middle line:84% that allow you to at least group these species 00:38:39.590 --> 00:38:41.210 align:middle line:90% into some larger groups. 00:38:41.210 --> 00:38:43.990 align:middle line:90% 00:38:43.990 --> 00:38:47.710 align:middle line:84% One of the big ones is prokaryotes, 00:38:47.710 --> 00:38:51.680 align:middle line:84% or prokaryotic cells, versus eukaryotic cells. 00:38:51.680 --> 00:38:56.620 align:middle line:90% 00:38:56.620 --> 00:38:59.740 align:middle line:84% The prokaryotes don't have a defined nucleus 00:38:59.740 --> 00:39:00.840 align:middle line:90% within their cell. 00:39:00.840 --> 00:39:03.420 align:middle line:90% 00:39:03.420 --> 00:39:06.580 align:middle line:84% Eukaryotes do have a defined nucleus. 00:39:06.580 --> 00:39:09.260 align:middle line:84% And in fact, the best understanding 00:39:09.260 --> 00:39:12.020 align:middle line:84% for most of the phytoplankton is the way 00:39:12.020 --> 00:39:14.500 align:middle line:84% cells became eukaryotic was there 00:39:14.500 --> 00:39:18.740 align:middle line:84% was some form of endosymbiosis, where 00:39:18.740 --> 00:39:22.420 align:middle line:84% one prokaryotic cell basically swallowed 00:39:22.420 --> 00:39:25.900 align:middle line:84% or engulfed a second prokaryotic cell. 00:39:25.900 --> 00:39:31.820 align:middle line:84% And there's been a series of these endosymbiotic events 00:39:31.820 --> 00:39:34.140 align:middle line:90% through evolutionary time. 00:39:34.140 --> 00:39:38.360 align:middle line:84% And that's how not only did the eukaryotic cells get nucleuses, 00:39:38.360 --> 00:39:40.540 align:middle line:84% but they also got things like mitochondria 00:39:40.540 --> 00:39:45.920 align:middle line:84% and other internal organelles, chloroplasts, mitochondria. 00:39:45.920 --> 00:39:48.960 align:middle line:90% 00:39:48.960 --> 00:39:52.180 align:middle line:84% The big difference is, in the eukaryotic cells, 00:39:52.180 --> 00:39:55.940 align:middle line:84% the nucleus is where most of the genetic material is. 00:39:55.940 --> 00:40:00.840 align:middle line:84% The mitochondria actually can have their own genetic material. 00:40:00.840 --> 00:40:03.920 align:middle line:84% But you need some biochemical mechanism 00:40:03.920 --> 00:40:08.260 align:middle line:84% for transferring the proteins that are made in the nucleus, 00:40:08.260 --> 00:40:10.280 align:middle line:84% or, excuse me, the genetic information 00:40:10.280 --> 00:40:13.200 align:middle line:84% that's made into the nucleus back out into the cell. 00:40:13.200 --> 00:40:15.440 align:middle line:84% And it's a little bit more complicated 00:40:15.440 --> 00:40:18.480 align:middle line:84% in terms of the internal biogeochemistry. 00:40:18.480 --> 00:40:22.400 align:middle line:84% Prokaryotes tend to be smaller than eukaryotes, 00:40:22.400 --> 00:40:26.546 align:middle line:90% but that's not always true. 00:40:26.546 --> 00:40:29.080 align:middle line:84% There's one really bizarre prokaryote, 00:40:29.080 --> 00:40:31.920 align:middle line:84% and I can't remember its name, but it was found off 00:40:31.920 --> 00:40:34.880 align:middle line:84% of the sands of Namibia, and it's actually 00:40:34.880 --> 00:40:37.120 align:middle line:90% visible with the naked eye. 00:40:37.120 --> 00:40:41.180 align:middle line:84% It's like one of those weird things out of science fiction. 00:40:41.180 --> 00:40:48.270 align:middle line:90% 00:40:48.270 --> 00:40:54.350 align:middle line:84% With what you might then call the eukaryotes, 00:40:54.350 --> 00:41:00.070 align:middle line:84% there are a whole series of different groups 00:41:00.070 --> 00:41:04.590 align:middle line:84% that all are considered phytoplankton. 00:41:04.590 --> 00:41:09.390 align:middle line:84% They're not very closely related if you look at them genomically. 00:41:09.390 --> 00:41:10.170 align:middle line:90% Oh. 00:41:10.170 --> 00:41:13.750 align:middle line:84% Within the prokaryotic, let me just say, one of the big ones 00:41:13.750 --> 00:41:16.150 align:middle line:90% are cyanobacteria. 00:41:16.150 --> 00:41:18.170 align:middle line:84% You'll hear about cyanobacteria a lot. 00:41:18.170 --> 00:41:21.970 align:middle line:84% So for example, prochlorococcus is a cyanobacteria. 00:41:21.970 --> 00:41:24.950 align:middle line:84% Trichodesmium is a cyanobacteria. 00:41:24.950 --> 00:41:28.630 align:middle line:84% They're quite prevalent as photoautotrophs, 00:41:28.630 --> 00:41:34.030 align:middle line:84% particularly in oligotrophic regimes. 00:41:34.030 --> 00:41:38.870 align:middle line:84% In eukaryotes, you have both green and red. 00:41:38.870 --> 00:41:39.650 align:middle line:90% Green. 00:41:39.650 --> 00:41:40.710 align:middle line:90% That was green. 00:41:40.710 --> 00:41:49.770 align:middle line:84% Green and red algae are the diatoms. 00:41:49.770 --> 00:41:56.910 align:middle line:84% And dinoflagellates are part of the red algae. 00:41:56.910 --> 00:42:00.010 align:middle line:90% 00:42:00.010 --> 00:42:08.290 align:middle line:84% You also have haptophytes, which are where 00:42:08.290 --> 00:42:15.850 align:middle line:90% the coccolithophores fall. 00:42:15.850 --> 00:42:18.170 align:middle line:84% Coccolithophores are the ones that 00:42:18.170 --> 00:42:21.290 align:middle line:90% form little calcareous shells. 00:42:21.290 --> 00:42:22.470 align:middle line:90% And I have some-- 00:42:22.470 --> 00:42:26.170 align:middle line:90% 00:42:26.170 --> 00:42:30.930 align:middle line:84% I have some pictures of some of these things. 00:42:30.930 --> 00:42:37.410 align:middle line:84% So this top image is a synechococcus cell, which 00:42:37.410 --> 00:42:40.530 align:middle line:90% is a type of cyanobacteria. 00:42:40.530 --> 00:42:41.790 align:middle line:90% It's quite small. 00:42:41.790 --> 00:42:45.070 align:middle line:90% This is a half a micron. 00:42:45.070 --> 00:42:47.030 align:middle line:84% At the other end of the spectrum is 00:42:47.030 --> 00:42:52.230 align:middle line:84% trichodesmium, which is a colonial nitrogen-fixing 00:42:52.230 --> 00:42:54.190 align:middle line:90% cyanobacteria. 00:42:54.190 --> 00:42:57.230 align:middle line:84% These are actually visible by the naked eye, not 00:42:57.230 --> 00:42:59.250 align:middle line:84% individual cells but the whole colony, 00:42:59.250 --> 00:43:02.050 align:middle line:84% and you can actually pick them out by hand. 00:43:02.050 --> 00:43:07.750 align:middle line:90% 00:43:07.750 --> 00:43:11.070 align:middle line:84% This is the silica shell from a diatom. 00:43:11.070 --> 00:43:18.370 align:middle line:84% Diatoms form little, almost like pill boxes or hat boxes. 00:43:18.370 --> 00:43:22.570 align:middle line:84% There's two shells, silica shells, that nest in each other. 00:43:22.570 --> 00:43:24.210 align:middle line:90% This has a thickness. 00:43:24.210 --> 00:43:28.050 align:middle line:84% If you turn it on its side, it would look like a wafer. 00:43:28.050 --> 00:43:33.350 align:middle line:90% 00:43:33.350 --> 00:43:36.170 align:middle line:84% And then I should say diatoms come in all sorts of shapes. 00:43:36.170 --> 00:43:38.470 align:middle line:84% Some of them look like these little pill boxes. 00:43:38.470 --> 00:43:42.860 align:middle line:84% Other ones look more like elongated capsules. 00:43:42.860 --> 00:43:46.660 align:middle line:84% And they can be either-- live alone, or they can-- 00:43:46.660 --> 00:43:50.500 align:middle line:84% or they can live in colonies, in colonial form. 00:43:50.500 --> 00:43:54.780 align:middle line:90% They form long chains, often. 00:43:54.780 --> 00:44:00.460 align:middle line:84% This is an electron micrograph of a coccolithophore. 00:44:00.460 --> 00:44:02.500 align:middle line:90% So you see these little ovals. 00:44:02.500 --> 00:44:05.520 align:middle line:84% Those are actually little-- they're called coccoliths. 00:44:05.520 --> 00:44:11.300 align:middle line:84% They're little calcium carbonate plates that surround the shell, 00:44:11.300 --> 00:44:14.260 align:middle line:90% or surround the cell. 00:44:14.260 --> 00:44:17.420 align:middle line:84% And in fact, when you have a bloom 00:44:17.420 --> 00:44:20.700 align:middle line:84% of these, when the bloom starts to die off, 00:44:20.700 --> 00:44:23.220 align:middle line:84% often, they will lose these lifts. 00:44:23.220 --> 00:44:27.020 align:middle line:84% And it's like-- the water looks like-- 00:44:27.020 --> 00:44:32.480 align:middle line:84% water a off of a glacier that has really fine glacial flour. 00:44:32.480 --> 00:44:36.060 align:middle line:84% It gets this really pretty looking. 00:44:36.060 --> 00:44:38.960 align:middle line:90% greenish blue, milky color. 00:44:38.960 --> 00:44:41.200 align:middle line:84% And that's because these are scattering-- these are 00:44:41.200 --> 00:44:42.660 align:middle line:90% very good at scattering light. 00:44:42.660 --> 00:44:48.720 align:middle line:84% They're small, very fine bits of chalk ground up. 00:44:48.720 --> 00:44:51.400 align:middle line:84% Think of it as ground-up chalk in the water. 00:44:51.400 --> 00:44:53.920 align:middle line:84% And you can actually see these from space 00:44:53.920 --> 00:44:56.040 align:middle line:84% because of the amount of backscatter 00:44:56.040 --> 00:44:57.460 align:middle line:90% you get in the visible. 00:44:57.460 --> 00:45:04.240 align:middle line:90% 00:45:04.240 --> 00:45:09.560 align:middle line:84% And then there's a whole series of other smaller groups 00:45:09.560 --> 00:45:11.200 align:middle line:90% that have all-- 00:45:11.200 --> 00:45:15.280 align:middle line:84% they're all phytoplankton, but they haven't 00:45:15.280 --> 00:45:16.780 align:middle line:90% evolved from a single group. 00:45:16.780 --> 00:45:18.800 align:middle line:84% They've evolved multiple times from 00:45:18.800 --> 00:45:22.320 align:middle line:84% multiple different directions within the eukaryotes. 00:45:22.320 --> 00:45:33.960 align:middle line:84% So it's a very phylogenetically rich group of species. 00:45:33.960 --> 00:45:38.260 align:middle line:84% And so you need to be very careful about talking 00:45:38.260 --> 00:45:41.020 align:middle line:90% about the typical phytoplankton. 00:45:41.020 --> 00:45:43.040 align:middle line:84% And I will, for the rest of the course, 00:45:43.040 --> 00:45:46.820 align:middle line:84% talk about phytoplankton as if it were a single group 00:45:46.820 --> 00:45:49.980 align:middle line:90% of organisms, but it's not. 00:45:49.980 --> 00:45:51.320 align:middle line:90% It's quite diverse. 00:45:51.320 --> 00:45:55.340 align:middle line:90% 00:45:55.340 --> 00:45:59.240 align:middle line:84% Another way that you can look at phytoplankton is size. 00:45:59.240 --> 00:46:02.020 align:middle line:90% 00:46:02.020 --> 00:46:06.120 align:middle line:84% And size is quite important for a variety of reasons. 00:46:06.120 --> 00:46:09.020 align:middle line:90% One is grazing. 00:46:09.020 --> 00:46:12.540 align:middle line:84% Typically, but not always, grazers 00:46:12.540 --> 00:46:14.920 align:middle line:84% eat things that are smaller than them. 00:46:14.920 --> 00:46:18.180 align:middle line:90% 00:46:18.180 --> 00:46:20.420 align:middle line:84% Not always true, but for the most part. 00:46:20.420 --> 00:46:23.260 align:middle line:84% And so if you're big, one way to avoid grazing 00:46:23.260 --> 00:46:27.220 align:middle line:84% is to be bigger than the zooplankton or the heterotrophs 00:46:27.220 --> 00:46:29.020 align:middle line:90% that are there. 00:46:29.020 --> 00:46:33.140 align:middle line:90% A second is export. 00:46:33.140 --> 00:46:37.890 align:middle line:84% If you're really, really small and you die, 00:46:37.890 --> 00:46:40.290 align:middle line:90% you won't sink very fast. 00:46:40.290 --> 00:46:44.250 align:middle line:84% And so you don't contribute that much to export flux. 00:46:44.250 --> 00:46:48.010 align:middle line:84% Or even if you're gray, somebody eats you and turns you 00:46:48.010 --> 00:46:51.610 align:middle line:84% into a fecal pellet-- well, if you're small, 00:46:51.610 --> 00:46:53.730 align:middle line:84% your grazer is going to be small, 00:46:53.730 --> 00:46:56.470 align:middle line:84% and your grazer isn't going to produce a very big fecal pellet. 00:46:56.470 --> 00:46:59.730 align:middle line:84% Well, small fecal pellets aren't going to sink very fast either. 00:46:59.730 --> 00:47:03.450 align:middle line:84% And so size has a strong correlation 00:47:03.450 --> 00:47:04.950 align:middle line:90% with export production. 00:47:04.950 --> 00:47:08.370 align:middle line:90% 00:47:08.370 --> 00:47:16.170 align:middle line:84% There's a size classification that's based on diameter. 00:47:16.170 --> 00:47:21.170 align:middle line:84% And they go up by about a factor of 10 in diameter, 00:47:21.170 --> 00:47:23.010 align:middle line:84% but a factor of 10 in diameter is 00:47:23.010 --> 00:47:27.630 align:middle line:84% going to be a factor of what in volume? 00:47:27.630 --> 00:47:29.450 align:middle line:90% AUDIENCE: Roughly 100. 00:47:29.450 --> 00:47:30.890 align:middle line:90% SCOTT DONEY: Roughly 100. 00:47:30.890 --> 00:47:32.310 align:middle line:90% How about 1,000? 00:47:32.310 --> 00:47:34.550 align:middle line:90% Because it's going to be-- 00:47:34.550 --> 00:47:37.150 align:middle line:84% the simplest assumption would be it's a sphere. 00:47:37.150 --> 00:47:40.590 align:middle line:84% Of course, they're not always spherical. 00:47:40.590 --> 00:47:44.070 align:middle line:84% So even within these ranges, there's 00:47:44.070 --> 00:47:45.970 align:middle line:90% actually a huge range in cells. 00:47:45.970 --> 00:47:49.230 align:middle line:90% But picoplankton are about-- 00:47:49.230 --> 00:47:50.450 align:middle line:90% maybe 0.2. 00:47:50.450 --> 00:47:52.110 align:middle line:90% That's pretty small. 00:47:52.110 --> 00:47:57.870 align:middle line:84% Most bacteria-- you only get below a micron 00:47:57.870 --> 00:48:00.950 align:middle line:84% when you're a heterotrophic bacteria or a prochlorococcus 00:48:00.950 --> 00:48:05.910 align:middle line:84% or maybe a synechococcus, pretty small things, up to about 20 00:48:05.910 --> 00:48:07.550 align:middle line:90% microns. 00:48:07.550 --> 00:48:14.990 align:middle line:90% Nano is 2 to 20 microns. 00:48:14.990 --> 00:48:20.830 align:middle line:90% Micro is 20 to 200. 00:48:20.830 --> 00:48:24.550 align:middle line:84% And then you have meso and macro, et cetera. 00:48:24.550 --> 00:48:26.990 align:middle line:90% These are pretty loose. 00:48:26.990 --> 00:48:30.210 align:middle line:84% People will talk about the piconanoplankton, 00:48:30.210 --> 00:48:33.490 align:middle line:84% and they might not have gone out and actually measured 00:48:33.490 --> 00:48:37.970 align:middle line:84% the exact size, but it gives you a rough-- 00:48:37.970 --> 00:48:41.770 align:middle line:90% a rough ballpark figure. 00:48:41.770 --> 00:48:45.290 align:middle line:84% The last way we distinguish-- that was size. 00:48:45.290 --> 00:48:52.610 align:middle line:84% The last might be by what you might call guild or-- 00:48:52.610 --> 00:48:56.250 align:middle line:84% and this is-- some of this is ecological, but some of this 00:48:56.250 --> 00:48:59.530 align:middle line:90% is geochemical. 00:48:59.530 --> 00:49:02.330 align:middle line:84% For a geochemist, I might want to know everybody 00:49:02.330 --> 00:49:06.530 align:middle line:90% who can fix nitrogen-- 00:49:06.530 --> 00:49:09.430 align:middle line:90% can fix nitrogen, right? 00:49:09.430 --> 00:49:14.050 align:middle line:84% Because that's a very important geochemical process. 00:49:14.050 --> 00:49:18.670 align:middle line:84% Nitrogen fixation is converting into gas, 00:49:18.670 --> 00:49:21.950 align:middle line:84% which is unavailable to most organisms, 00:49:21.950 --> 00:49:25.570 align:middle line:84% into a form that can be used by most organisms, nitrate, 00:49:25.570 --> 00:49:28.200 align:middle line:90% ammonia. 00:49:28.200 --> 00:49:31.760 align:middle line:84% I might want to know about calcifiers. 00:49:31.760 --> 00:49:35.820 align:middle line:90% Who forms a calcareous shell? 00:49:35.820 --> 00:49:39.340 align:middle line:84% Totally different groups might be calcifiers, 00:49:39.340 --> 00:49:41.760 align:middle line:84% but I'm just interested in whether they're forming 00:49:41.760 --> 00:49:43.840 align:middle line:90% calcium, calcium carbonate. 00:49:43.840 --> 00:49:50.240 align:middle line:84% Similar, siliceous, shell users, et cetera. 00:49:50.240 --> 00:49:54.880 align:middle line:84% So you can also group things by these geochemical functional 00:49:54.880 --> 00:49:55.660 align:middle line:90% groups. 00:49:55.660 --> 00:50:05.240 align:middle line:90% 00:50:05.240 --> 00:50:08.600 align:middle line:90% So that's who's there. 00:50:08.600 --> 00:50:15.360 align:middle line:84% The next question is, what controls photosynthesis? 00:50:15.360 --> 00:50:21.660 align:middle line:84% Well, the first thing is just phytoplankton biomass. 00:50:21.660 --> 00:50:26.760 align:middle line:90% 00:50:26.760 --> 00:50:29.220 align:middle line:84% If there are more of these autotrophs there-- 00:50:29.220 --> 00:50:30.300 align:middle line:90% oh, and I should note-- 00:50:30.300 --> 00:50:34.540 align:middle line:84% I'm trying to be very careful and not call these, plants, 00:50:34.540 --> 00:50:38.220 align:middle line:84% because remember, we were talking about how many of 00:50:38.220 --> 00:50:39.820 align:middle line:90% them are prokaryotes? 00:50:39.820 --> 00:50:42.840 align:middle line:90% Well, the plants are only-- 00:50:42.840 --> 00:50:44.980 align:middle line:90% you can have eukaryotic plants. 00:50:44.980 --> 00:50:48.060 align:middle line:90% The prokaryotes are not plants. 00:50:48.060 --> 00:50:49.980 align:middle line:84% People get a little sloppy, and they call all 00:50:49.980 --> 00:50:52.620 align:middle line:90% of them, microscopic plants. 00:50:52.620 --> 00:50:56.380 align:middle line:84% Well in much of the ocean, plants are actually 00:50:56.380 --> 00:50:57.860 align:middle line:90% a minor contributor. 00:50:57.860 --> 00:50:59.100 align:middle line:90% I don't know. 00:50:59.100 --> 00:51:02.440 align:middle line:84% It's one of those little biological quibbles. 00:51:02.440 --> 00:51:07.940 align:middle line:84% But I think it actually is important to distinguish 00:51:07.940 --> 00:51:11.460 align:middle line:84% that most of the photosynthesis, for example, 00:51:11.460 --> 00:51:14.660 align:middle line:84% in the oligotrophic gyres is being done by prokaryotes, not 00:51:14.660 --> 00:51:16.440 align:middle line:90% by eukaryotes. 00:51:16.440 --> 00:51:18.360 align:middle line:84% So they're not technically plants. 00:51:18.360 --> 00:51:21.620 align:middle line:90% 00:51:21.620 --> 00:51:24.180 align:middle line:84% So the first thing is you need to know how much phytoplankton 00:51:24.180 --> 00:51:25.800 align:middle line:90% biomass is there. 00:51:25.800 --> 00:51:30.640 align:middle line:84% And it's somewhat circular because the amount of biomass 00:51:30.640 --> 00:51:34.240 align:middle line:84% is going to depend upon photosynthesis in the past. 00:51:34.240 --> 00:51:35.780 align:middle line:84% If you've had more photosynthesis, 00:51:35.780 --> 00:51:38.800 align:middle line:90% you can build up more biomass. 00:51:38.800 --> 00:51:49.320 align:middle line:84% But it's also going to depend upon grazing, sinking. 00:51:49.320 --> 00:51:52.773 align:middle line:84% Some of the larger phytoplankton actually have a sinking rate, 00:51:52.773 --> 00:51:54.440 align:middle line:84% or they will get to an end of the bloom, 00:51:54.440 --> 00:51:56.480 align:middle line:90% and they will sink out. 00:51:56.480 --> 00:52:01.740 align:middle line:84% And mortality, and within mortality is natural mortality, 00:52:01.740 --> 00:52:08.040 align:middle line:84% but it also might be, for example, virus, lysis. 00:52:08.040 --> 00:52:10.560 align:middle line:84% And there's actually a fairly dynamic interaction 00:52:10.560 --> 00:52:12.900 align:middle line:84% between viruses and phytoplankton. 00:52:12.900 --> 00:52:17.720 align:middle line:90% 00:52:17.720 --> 00:52:20.620 align:middle line:84% Second thing you'd need to know is how much light there is. 00:52:20.620 --> 00:52:24.310 align:middle line:90% 00:52:24.310 --> 00:52:27.490 align:middle line:84% Obviously, more light, probably more photosynthesis. 00:52:27.490 --> 00:52:30.350 align:middle line:90% 00:52:30.350 --> 00:52:41.510 align:middle line:84% Nutrients, the amount of inorganic carbon or CO2. 00:52:41.510 --> 00:52:46.870 align:middle line:84% And we'll talk about that as well. 00:52:46.870 --> 00:52:50.530 align:middle line:90% And then finally temperature. 00:52:50.530 --> 00:52:57.670 align:middle line:90% 00:52:57.670 --> 00:53:01.730 align:middle line:84% So we're mostly, today-- we're going to start into light. 00:53:01.730 --> 00:53:04.470 align:middle line:84% I don't know if we'll get all the way to temperature. 00:53:04.470 --> 00:53:05.850 align:middle line:90% Depends on how things go. 00:53:05.850 --> 00:53:09.190 align:middle line:84% But we'll go through these individual controls. 00:53:09.190 --> 00:53:14.030 align:middle line:84% We'll talk more about phytoplankton biomass on Tuesday 00:53:14.030 --> 00:53:18.310 align:middle line:84% when we start looking more at the whole community. 00:53:18.310 --> 00:53:28.050 align:middle line:84% So phytoplankton can only grow in the upper ocean. 00:53:28.050 --> 00:53:35.130 align:middle line:84% And if you imagine that if we were to plot versus depth, 00:53:35.130 --> 00:53:38.670 align:middle line:84% you have some surface irradiance. 00:53:38.670 --> 00:53:45.090 align:middle line:90% 00:53:45.090 --> 00:53:49.370 align:middle line:84% As an approximation, that irradiance 00:53:49.370 --> 00:53:53.090 align:middle line:84% is going to drop off, approximately, exponentially, 00:53:53.090 --> 00:53:55.810 align:middle line:90% with depth. 00:53:55.810 --> 00:54:04.810 align:middle line:84% And at some point, in the water column, 00:54:04.810 --> 00:54:11.270 align:middle line:84% the irradiance is going to equal 1% of the surface irradiance. 00:54:11.270 --> 00:54:13.930 align:middle line:90% 00:54:13.930 --> 00:54:19.410 align:middle line:84% That was-- is typically called-- that depth, 00:54:19.410 --> 00:54:22.430 align:middle line:84% where you reach 1% of the surface radiance, 00:54:22.430 --> 00:54:24.050 align:middle line:90% is called the euphotic zone. 00:54:24.050 --> 00:54:26.710 align:middle line:90% 00:54:26.710 --> 00:54:32.870 align:middle line:84% And to first order, most of the photosynthesis, 00:54:32.870 --> 00:54:35.130 align:middle line:84% or the vast majority of photosynthesis, 00:54:35.130 --> 00:54:38.030 align:middle line:84% is going to occur within the euphotic zone. 00:54:38.030 --> 00:54:41.510 align:middle line:84% It's going to occur above this euphotic depth, the 1% light 00:54:41.510 --> 00:54:42.090 align:middle line:90% level. 00:54:42.090 --> 00:54:47.950 align:middle line:90% 00:54:47.950 --> 00:54:52.470 align:middle line:84% This euphotic zone, in very clear waters, 00:54:52.470 --> 00:54:59.830 align:middle line:84% might be 125 meters, and in very turbid waters and very turbid 00:54:59.830 --> 00:55:02.310 align:middle line:84% coastal waters, with either a lot of phytoplankton 00:55:02.310 --> 00:55:06.230 align:middle line:84% or a lot of other sediments or detrital material, 00:55:06.230 --> 00:55:08.530 align:middle line:90% might be a few meters. 00:55:08.530 --> 00:55:13.390 align:middle line:90% 00:55:13.390 --> 00:55:16.130 align:middle line:90% So there's a large range. 00:55:16.130 --> 00:55:22.980 align:middle line:90% 00:55:22.980 --> 00:55:26.340 align:middle line:84% I mentioned that you can approximate this 00:55:26.340 --> 00:55:27.640 align:middle line:90% as an exponential. 00:55:27.640 --> 00:55:32.740 align:middle line:90% 00:55:32.740 --> 00:55:39.580 align:middle line:84% For a single wavelength, the irradiance, 00:55:39.580 --> 00:55:44.300 align:middle line:84% as a function of depth, can be approximated 00:55:44.300 --> 00:55:52.240 align:middle line:84% as the surface irradiance times e to the minus k-- 00:55:52.240 --> 00:55:55.980 align:middle line:90% 00:55:55.980 --> 00:55:58.500 align:middle line:90% oh, sorry. 00:55:58.500 --> 00:56:01.820 align:middle line:90% Let me start that over. 00:56:01.820 --> 00:56:06.500 align:middle line:84% e at a depth, z, for a particular wavelength, lambda, 00:56:06.500 --> 00:56:12.500 align:middle line:84% is going to equal e 0 for that particular lambda, 00:56:12.500 --> 00:56:16.840 align:middle line:90% e to the minus k lambda z. 00:56:16.840 --> 00:56:19.340 align:middle line:84% It's just going to be an exponential falloff. 00:56:19.340 --> 00:56:22.480 align:middle line:90% 00:56:22.480 --> 00:56:25.720 align:middle line:84% It's going to be different for different wavelengths. 00:56:25.720 --> 00:56:28.680 align:middle line:90% 00:56:28.680 --> 00:56:32.460 align:middle line:90% So k as a function of lambda-- 00:56:32.460 --> 00:56:35.040 align:middle line:90% 00:56:35.040 --> 00:56:38.080 align:middle line:84% typically, the things that are going to absorb light 00:56:38.080 --> 00:56:47.240 align:middle line:84% in seawater are water itself, chlorophyll, 00:56:47.240 --> 00:56:52.520 align:middle line:84% and what's called CDOM, which is chromophoric 00:56:52.520 --> 00:56:56.440 align:middle line:90% dissolved organic matter. 00:56:56.440 --> 00:57:04.000 align:middle line:84% Sometimes you'll hear of CDOM as gelbstoff, 00:57:04.000 --> 00:57:10.320 align:middle line:84% gelbstoff, which is basically yellow stuff, 00:57:10.320 --> 00:57:12.680 align:middle line:84% otherwise known as a bunch of organic crap 00:57:12.680 --> 00:57:15.160 align:middle line:84% that they didn't know exactly what it was, 00:57:15.160 --> 00:57:16.700 align:middle line:84% but it was absorbing a lot of light 00:57:16.700 --> 00:57:19.220 align:middle line:84% and making a mess out of their experiments. 00:57:19.220 --> 00:57:23.444 align:middle line:90% 00:57:23.444 --> 00:57:30.460 align:middle line:84% In most waters, the reds get absorbed early, 00:57:30.460 --> 00:57:34.300 align:middle line:84% so red light disappears rather quickly. 00:57:34.300 --> 00:57:39.300 align:middle line:84% So if you go out to the very, very clearest open ocean, 00:57:39.300 --> 00:57:42.660 align:middle line:84% if you go diving in the Sargasso Sea, 00:57:42.660 --> 00:57:45.780 align:middle line:84% the water looks very, very blue, and that's 00:57:45.780 --> 00:57:50.000 align:middle line:84% because you've absorbed most of the red-- most of the red light. 00:57:50.000 --> 00:57:53.180 align:middle line:90% 00:57:53.180 --> 00:57:56.580 align:middle line:84% If you have a lot of chlorophyll, 00:57:56.580 --> 00:58:00.160 align:middle line:84% you'll also be absorbing in the blue 00:58:00.160 --> 00:58:03.420 align:middle line:84% because the chlorophyll absorbs in both the red and the blue. 00:58:03.420 --> 00:58:05.580 align:middle line:84% So if you have a lot of biomass, you'll 00:58:05.580 --> 00:58:08.740 align:middle line:90% also start soaking up the blue. 00:58:08.740 --> 00:58:17.030 align:middle line:84% So there are spectral shifts with depth. 00:58:17.030 --> 00:58:20.850 align:middle line:90% 00:58:20.850 --> 00:58:23.770 align:middle line:84% And so this equation, while it might hold well 00:58:23.770 --> 00:58:27.630 align:middle line:84% for a particular wavelength, is only an approximation. 00:58:27.630 --> 00:58:31.050 align:middle line:90% 00:58:31.050 --> 00:58:39.050 align:middle line:84% You'll sometimes see it written as this, approximately, i0, e 00:58:39.050 --> 00:58:48.730 align:middle line:84% to the minus k PAR over z, where this is now integrated over PAR. 00:58:48.730 --> 00:58:51.810 align:middle line:84% And that's only an approximation because some of the wavelengths 00:58:51.810 --> 00:58:54.790 align:middle line:84% are being absorbed differentially. 00:58:54.790 --> 00:59:02.850 align:middle line:90% 00:59:02.850 --> 00:59:06.810 align:middle line:84% The last thing on this before we look 00:59:06.810 --> 00:59:10.190 align:middle line:84% at actual photosynthetic irradiance curves 00:59:10.190 --> 00:59:13.170 align:middle line:90% is that this k-- 00:59:13.170 --> 00:59:15.940 align:middle line:84% for example, if I were to write out k, 00:59:15.940 --> 00:59:21.070 align:middle line:84% you would have a term for k water, 00:59:21.070 --> 00:59:27.210 align:middle line:84% plus a term for k chlorophyll, plus a term for k CDOM. 00:59:27.210 --> 00:59:30.630 align:middle line:90% 00:59:30.630 --> 00:59:34.590 align:middle line:84% And often, it's linearly separated like this in two 00:59:34.590 --> 00:59:38.270 align:middle line:90% different absorption terms. 00:59:38.270 --> 00:59:42.430 align:middle line:84% If you actually get into this specific field, 00:59:42.430 --> 00:59:46.690 align:middle line:84% you need to worry about not only absorption but also scattering. 00:59:46.690 --> 00:59:49.190 align:middle line:84% A lot of the light gets scattered. 00:59:49.190 --> 00:59:51.910 align:middle line:90% And so you can have-- 00:59:51.910 --> 00:59:54.430 align:middle line:84% when light scattered, it comes at a different angle, 00:59:54.430 --> 00:59:55.910 align:middle line:90% and it can get absorbed. 00:59:55.910 --> 00:59:58.630 align:middle line:84% I mean you increase the path length over which 00:59:58.630 --> 01:00:00.510 align:middle line:90% absorption can occur. 01:00:00.510 --> 01:00:07.690 align:middle line:84% And so actually doing this can be a fairly complicated process 01:00:07.690 --> 01:00:10.090 align:middle line:84% if you want to do it from first principles. 01:00:10.090 --> 01:00:14.210 align:middle line:84% A lot of these are actually what are called effective absorption 01:00:14.210 --> 01:00:17.330 align:middle line:84% coefficients because they're not the real absorption if you were 01:00:17.330 --> 01:00:20.890 align:middle line:84% to put it in a cell and measure it over a very small cell 01:00:20.890 --> 01:00:24.130 align:middle line:84% length, but they're the effective absorption, including 01:00:24.130 --> 01:00:27.670 align:middle line:84% all the effects of scattering light within the water column. 01:00:27.670 --> 01:00:43.350 align:middle line:90% 01:00:43.350 --> 01:00:48.850 align:middle line:84% So how do phytoplankton respond to light, 01:00:48.850 --> 01:00:53.730 align:middle line:84% in terms of their photosynthesis rates? 01:00:53.730 --> 01:00:58.610 align:middle line:84% What people often do is they take a phytoplankton culture 01:00:58.610 --> 01:01:03.530 align:middle line:84% or a phytoplankton sample, and they expose it 01:01:03.530 --> 01:01:06.250 align:middle line:84% to different levels of irradiance. 01:01:06.250 --> 01:01:10.840 align:middle line:84% And then they measure the photosynthesis rate, 01:01:10.840 --> 01:01:18.300 align:middle line:84% and they make what's called a PI curve, for photosynthesis 01:01:18.300 --> 01:01:21.440 align:middle line:90% irradiance curve. 01:01:21.440 --> 01:01:22.780 align:middle line:90% You will go back and forth. 01:01:22.780 --> 01:01:25.600 align:middle line:84% Sometimes the literature uses I for irradiance. 01:01:25.600 --> 01:01:34.040 align:middle line:84% Sometimes it uses E. And there are very strong opinions 01:01:34.040 --> 01:01:38.920 align:middle line:84% about which you should use, and I'll leave it at that. 01:01:38.920 --> 01:01:40.600 align:middle line:84% And people have their good reasons 01:01:40.600 --> 01:01:41.940 align:middle line:90% for using different things. 01:01:41.940 --> 01:01:48.520 align:middle line:84% But in this class, I'll just talk about P-- use it as PI. 01:01:48.520 --> 01:01:55.800 align:middle line:84% Typically, the curves look something like this, where 01:01:55.800 --> 01:01:58.480 align:middle line:90% you have a region right near-- 01:01:58.480 --> 01:02:01.560 align:middle line:84% in very low light, right near the intercept, where 01:02:01.560 --> 01:02:04.520 align:middle line:90% it's approximately linear. 01:02:04.520 --> 01:02:08.280 align:middle line:84% So photosynthesis scales linearly with irradiance. 01:02:08.280 --> 01:02:11.420 align:middle line:90% 01:02:11.420 --> 01:02:14.360 align:middle line:84% You have some region that highlight where it saturates. 01:02:14.360 --> 01:02:17.820 align:middle line:90% 01:02:17.820 --> 01:02:23.500 align:middle line:84% And then you have, potentially, a region out at very high light, 01:02:23.500 --> 01:02:27.455 align:middle line:90% where you have photoinhibition. 01:02:27.455 --> 01:02:30.080 align:middle line:84% Essentially, you're shining so much light on the phytoplankton, 01:02:30.080 --> 01:02:31.820 align:middle line:84% and the phytoplankton is not happy, 01:02:31.820 --> 01:02:35.680 align:middle line:84% and it shuts down its photosynthetic apparatus. 01:02:35.680 --> 01:02:40.220 align:middle line:90% 01:02:40.220 --> 01:02:51.500 align:middle line:84% There's a whole series of theoretical/empirical curves 01:02:51.500 --> 01:02:53.840 align:middle line:90% for representing PI curves. 01:02:53.840 --> 01:02:56.940 align:middle line:90% 01:02:56.940 --> 01:03:00.420 align:middle line:84% One simple one that I like because it's 01:03:00.420 --> 01:03:04.840 align:middle line:84% fairly simple to conceptualize is that you have-- 01:03:04.840 --> 01:03:09.180 align:middle line:84% photosynthesis is going to be some function of Pmax. 01:03:09.180 --> 01:03:11.740 align:middle line:90% There's some at high saturation. 01:03:11.740 --> 01:03:16.380 align:middle line:84% There's some Pmax value that the cell will basically-- 01:03:16.380 --> 01:03:19.520 align:middle line:84% it has sufficient light, and that's as much 01:03:19.520 --> 01:03:22.560 align:middle line:90% photosynthesis as it can do. 01:03:22.560 --> 01:03:27.760 align:middle line:84% And then you have, basically, a correction term 01:03:27.760 --> 01:03:40.960 align:middle line:84% for light, where if you look at this term, 01:03:40.960 --> 01:03:50.486 align:middle line:84% at very low light, if i goes to 0, e to the 0 goes to what? 01:03:50.486 --> 01:03:50.986 align:middle line:90% AUDIENCE: 1. 01:03:50.986 --> 01:03:53.500 align:middle line:90% SCOTT DONEY: Right 1 minus 1. 01:03:53.500 --> 01:03:54.620 align:middle line:90% So this goes to 0. 01:03:54.620 --> 01:03:56.960 align:middle line:84% So it has the correct behavior at low light. 01:03:56.960 --> 01:04:03.510 align:middle line:84% And if i goes to infinity, e to the minus infinity goes to 0. 01:04:03.510 --> 01:04:06.350 align:middle line:84% So this drops out, and you're left at Pmax. 01:04:06.350 --> 01:04:09.090 align:middle line:84% So this does-- this saturating curve-- 01:04:09.090 --> 01:04:10.850 align:middle line:84% it doesn't do the photoinhibition part. 01:04:10.850 --> 01:04:12.590 align:middle line:84% You would have to add an additional term 01:04:12.590 --> 01:04:16.070 align:middle line:90% on for photoinhibition. 01:04:16.070 --> 01:04:19.310 align:middle line:84% One little clever trick that I like 01:04:19.310 --> 01:04:24.910 align:middle line:84% is if you have an equation, e to the b, and if you look 01:04:24.910 --> 01:04:34.510 align:middle line:84% at the limit of that as b goes to 0, e to the b goes to 1 plus 01:04:34.510 --> 01:04:35.050 align:middle line:90% b. 01:04:35.050 --> 01:04:38.230 align:middle line:90% 01:04:38.230 --> 01:04:42.070 align:middle line:84% That's an approximation for the exponential. 01:04:42.070 --> 01:04:44.470 align:middle line:84% The nice thing about that is if you look up 01:04:44.470 --> 01:04:48.950 align:middle line:84% at this term, that would say that the term e to the minus 01:04:48.950 --> 01:05:00.390 align:middle line:84% alpha i Pmax would go to 1 minus alpha i Pmax. 01:05:00.390 --> 01:05:03.430 align:middle line:90% 01:05:03.430 --> 01:05:08.550 align:middle line:90% And then the 1s drop-- 01:05:08.550 --> 01:05:14.050 align:middle line:84% the 1s drop out the alpha i, because the Pmaxes also drop 01:05:14.050 --> 01:05:15.530 align:middle line:90% out. 01:05:15.530 --> 01:05:21.250 align:middle line:84% And so that's a way of thinking about what this alpha term is, 01:05:21.250 --> 01:05:23.210 align:middle line:90% is it's actually the slope. 01:05:23.210 --> 01:05:31.250 align:middle line:84% Alpha is the slope of the PI curve at low light. 01:05:31.250 --> 01:05:35.670 align:middle line:84% And sometimes you'll hear alpha described as the initial slope. 01:05:35.670 --> 01:05:58.955 align:middle line:90% 01:05:58.955 --> 01:06:08.520 align:middle line:84% So light is going to vary considerably over time, 01:06:08.520 --> 01:06:13.440 align:middle line:84% and it's going to vary for two reasons. 01:06:13.440 --> 01:06:20.720 align:middle line:84% Let's say we have our surface of the ocean, and we have depth. 01:06:20.720 --> 01:06:26.680 align:middle line:84% And we're going to plot time in this direction. 01:06:26.680 --> 01:06:31.800 align:middle line:84% So if I were to look at the surface irradiance, 01:06:31.800 --> 01:06:37.360 align:middle line:84% you're going to get a seasonal cycle in most places 01:06:37.360 --> 01:06:39.600 align:middle line:90% outside the tropics. 01:06:39.600 --> 01:06:41.800 align:middle line:84% In the extra-tropics, you'll get a seasonal cycle 01:06:41.800 --> 01:06:44.200 align:middle line:84% where you'll have low light in the winter 01:06:44.200 --> 01:06:46.240 align:middle line:84% and high light in the summer, and then 01:06:46.240 --> 01:06:49.560 align:middle line:90% low light again in the winter. 01:06:49.560 --> 01:06:52.800 align:middle line:84% At the same time, mixed-layer depths 01:06:52.800 --> 01:06:59.220 align:middle line:84% tend to be deep in the winter and shallow in the summer 01:06:59.220 --> 01:07:00.980 align:middle line:90% and deep in the winter. 01:07:00.980 --> 01:07:02.420 align:middle line:90% So this would be-- 01:07:02.420 --> 01:07:03.360 align:middle line:90% that's i0. 01:07:03.360 --> 01:07:04.960 align:middle line:90% This would be mixed-layer depth. 01:07:04.960 --> 01:07:09.700 align:middle line:90% 01:07:09.700 --> 01:07:14.260 align:middle line:84% What the phytoplankton sees when it's in the mixed-layer depth, 01:07:14.260 --> 01:07:18.700 align:middle line:84% or in the mixed layer itself, is an average light 01:07:18.700 --> 01:07:20.100 align:middle line:90% over the mixed layer. 01:07:20.100 --> 01:07:25.500 align:middle line:84% The overturning time for mixed layers are on the timescales 01:07:25.500 --> 01:07:29.140 align:middle line:84% of 10s of minutes to maybe an hour, 01:07:29.140 --> 01:07:31.860 align:middle line:84% much shorter than the generation time for an individual 01:07:31.860 --> 01:07:33.180 align:middle line:90% phytoplankton. 01:07:33.180 --> 01:07:35.900 align:middle line:84% So these phytoplankton are getting caught up 01:07:35.900 --> 01:07:39.100 align:middle line:84% in turbulent eddies, and they're going up and down 01:07:39.100 --> 01:07:41.900 align:middle line:84% and up and down and up and down within the mixed layer. 01:07:41.900 --> 01:07:48.780 align:middle line:84% And as an approximation, without going into a lot of details, 01:07:48.780 --> 01:07:51.860 align:middle line:84% you can say, well, they're seeing the average light 01:07:51.860 --> 01:07:54.000 align:middle line:90% of the mixed-layer depth. 01:07:54.000 --> 01:07:56.240 align:middle line:84% So when the mixed-layer depth is deep-- 01:07:56.240 --> 01:07:59.520 align:middle line:84% remember the light was dropping off exponentially? 01:07:59.520 --> 01:08:01.640 align:middle line:84% So when the mixed-layer depth is deep, 01:08:01.640 --> 01:08:04.140 align:middle line:84% even with the same surface irradiance, 01:08:04.140 --> 01:08:07.660 align:middle line:84% the average light experienced by the phytoplankton goes down. 01:08:07.660 --> 01:08:10.720 align:middle line:90% 01:08:10.720 --> 01:08:15.300 align:middle line:84% So during winter, you have lower effective light, 01:08:15.300 --> 01:08:17.880 align:middle line:84% both because you have lower light at the surface, 01:08:17.880 --> 01:08:20.760 align:middle line:84% and also you have lower light because 01:08:20.760 --> 01:08:22.300 align:middle line:90% of the deeper mixed layers. 01:08:22.300 --> 01:08:35.359 align:middle line:90% 01:08:35.359 --> 01:08:39.640 align:middle line:84% Often, what you'll see in temperate latitudes is 01:08:39.640 --> 01:08:45.020 align:middle line:84% during the winter, if I were to plot on the same time axis 01:08:45.020 --> 01:08:53.229 align:middle line:84% and then I were to plot phytoplankton, what I would see 01:08:53.229 --> 01:08:56.010 align:middle line:84% is low phytoplankton in the summer, 01:08:56.010 --> 01:08:59.310 align:middle line:84% and then I might see some bloom event, 01:08:59.310 --> 01:09:01.109 align:middle line:84% and then it might collapse down, and then 01:09:01.109 --> 01:09:03.890 align:middle line:84% I might see another bloom event in the fall. 01:09:03.890 --> 01:09:04.950 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:09:04.950 --> 01:09:05.700 align:middle line:90% SCOTT DONEY: Yeah. 01:09:05.700 --> 01:09:08.710 align:middle line:90% 01:09:08.710 --> 01:09:11.069 align:middle line:90% This is called the spring bloom. 01:09:11.069 --> 01:09:13.790 align:middle line:90% It's really original. 01:09:13.790 --> 01:09:17.470 align:middle line:90% And this is the fall bloom. 01:09:17.470 --> 01:09:19.189 align:middle line:84% This is not true everywhere, but it's 01:09:19.189 --> 01:09:27.710 align:middle line:84% canonical for, say, large parts of the temperate North Atlantic. 01:09:27.710 --> 01:09:30.870 align:middle line:84% The spring bloom is triggered by the fact 01:09:30.870 --> 01:09:35.029 align:middle line:84% that you have both rising surface light and shoaling 01:09:35.029 --> 01:09:38.350 align:middle line:84% mixed-layer depth, and you reach a point where you suddenly 01:09:38.350 --> 01:09:41.250 align:middle line:84% have enough light for the phytoplankton community to grow. 01:09:41.250 --> 01:09:53.410 align:middle line:90% 01:09:53.410 --> 01:10:01.210 align:middle line:84% And there's a nice theoretical discussion 01:10:01.210 --> 01:10:04.230 align:middle line:84% of this that was originally put forward by Sverdrup. 01:10:04.230 --> 01:10:07.610 align:middle line:90% 01:10:07.610 --> 01:10:09.710 align:middle line:90% Just go to lecture 13. 01:10:09.710 --> 01:10:14.170 align:middle line:90% 01:10:14.170 --> 01:10:17.810 align:middle line:84% And it's called Sverdrup's critical light 01:10:17.810 --> 01:10:27.250 align:middle line:84% theory, or critical depth theory-- critical depth theory. 01:10:27.250 --> 01:10:28.950 align:middle line:90% And it goes something like this. 01:10:28.950 --> 01:10:31.730 align:middle line:90% 01:10:31.730 --> 01:10:38.890 align:middle line:84% At the surface, photosynthesis rates are high. 01:10:38.890 --> 01:10:42.850 align:middle line:84% And they tend to drop off with depth because of light. 01:10:42.850 --> 01:10:46.330 align:middle line:84% Let's assume for a moment that the phytoplankton is well-mixed, 01:10:46.330 --> 01:10:51.310 align:middle line:84% and we're just worrying about- we're just 01:10:51.310 --> 01:10:55.250 align:middle line:84% worried about the effect of light on photosynthesis. 01:10:55.250 --> 01:10:58.550 align:middle line:90% So this would be depth. 01:10:58.550 --> 01:11:07.430 align:middle line:84% At some depth, you reach a depth- 01:11:07.430 --> 01:11:15.390 align:middle line:84% and we'll call this depth the compensation depth. 01:11:15.390 --> 01:11:24.030 align:middle line:84% The photosynthesis of the community 01:11:24.030 --> 01:11:26.890 align:middle line:84% is going to be balanced by the respiration. 01:11:26.890 --> 01:11:31.630 align:middle line:90% 01:11:31.630 --> 01:11:35.990 align:middle line:84% So this would be your community respiration. 01:11:35.990 --> 01:11:39.830 align:middle line:84% So above this depth, the bugs are growing, 01:11:39.830 --> 01:11:42.350 align:middle line:84% and they're producing a lot more organic matter 01:11:42.350 --> 01:11:46.820 align:middle line:84% than is being consumed by the community. 01:11:46.820 --> 01:11:49.260 align:middle line:84% Below this depth, the phytoplankton 01:11:49.260 --> 01:11:52.380 align:middle line:84% just are not growing as fast as the respiration 01:11:52.380 --> 01:11:53.460 align:middle line:90% of the community. 01:11:53.460 --> 01:11:56.940 align:middle line:84% And so up here, you have net organic matter generation. 01:11:56.940 --> 01:12:00.760 align:middle line:84% Down here, you have net organic matter loss. 01:12:00.760 --> 01:12:05.780 align:middle line:90% 01:12:05.780 --> 01:12:13.340 align:middle line:84% And this C is called the compensation depth, compensation 01:12:13.340 --> 01:12:14.260 align:middle line:90% depth. 01:12:14.260 --> 01:12:16.020 align:middle line:84% And it's essentially where respiration 01:12:16.020 --> 01:12:18.080 align:middle line:90% compensates for photosynthesis. 01:12:18.080 --> 01:12:21.540 align:middle line:90% 01:12:21.540 --> 01:12:23.900 align:middle line:84% Now, let's say this is all in a mixed layer. 01:12:23.900 --> 01:12:26.820 align:middle line:90% 01:12:26.820 --> 01:12:29.240 align:middle line:84% These are instantaneous photosynthesis rates, 01:12:29.240 --> 01:12:30.860 align:middle line:84% so this is the photosynthesis rate 01:12:30.860 --> 01:12:33.940 align:middle line:84% for that bug sitting there at that particular instance, 01:12:33.940 --> 01:12:35.740 align:middle line:90% at that depth. 01:12:35.740 --> 01:12:39.260 align:middle line:84% But if this is all in a mixed layer, 01:12:39.260 --> 01:12:43.540 align:middle line:84% the whole phytoplankton community is seeing this light-- 01:12:43.540 --> 01:12:49.040 align:middle line:84% the average light integrated over that depth-- 01:12:49.040 --> 01:12:53.680 align:middle line:84% we might want to know at what depth if we integrate 01:12:53.680 --> 01:13:02.440 align:middle line:84% the respiration, dz, would that equal the integrated 01:13:02.440 --> 01:13:03.620 align:middle line:90% photosynthesis? 01:13:03.620 --> 01:13:06.680 align:middle line:90% 01:13:06.680 --> 01:13:10.120 align:middle line:84% And the argument there is that if the mixed layer is really 01:13:10.120 --> 01:13:13.920 align:middle line:84% deep and you have a lot of the community that's 01:13:13.920 --> 01:13:17.320 align:middle line:84% down in this region where you have net organic loss, 01:13:17.320 --> 01:13:19.560 align:middle line:84% you can't have a phytoplankton bloom. 01:13:19.560 --> 01:13:21.760 align:middle line:84% You'll have guys are growing up here, 01:13:21.760 --> 01:13:23.960 align:middle line:84% but then they get mixed down, and then 01:13:23.960 --> 01:13:26.960 align:middle line:84% they're down here starving, or they're being getting eaten, 01:13:26.960 --> 01:13:29.840 align:middle line:84% and they're just-- all of that organic matter can't 01:13:29.840 --> 01:13:33.580 align:middle line:84% go into new biomass and the phytoplankton can't bloom. 01:13:33.580 --> 01:13:36.440 align:middle line:90% 01:13:36.440 --> 01:13:41.560 align:middle line:84% That depth is called a critical depth, 01:13:41.560 --> 01:13:45.020 align:middle line:84% where the total respiration integrated 01:13:45.020 --> 01:13:47.340 align:middle line:84% over the mix-- over this critical depth 01:13:47.340 --> 01:13:52.820 align:middle line:84% is equal to the total amount of photosynthesis. 01:13:52.820 --> 01:13:57.720 align:middle line:84% And you can solve for that if we assume that the-- 01:13:57.720 --> 01:14:02.140 align:middle line:90% 01:14:02.140 --> 01:14:11.140 align:middle line:84% if we assume for a moment that the irradiance will 01:14:11.140 --> 01:14:20.800 align:middle line:84% approximate as a surface irradiance and a single PAR. 01:14:20.800 --> 01:14:23.055 align:middle line:84% Now, as I said, that's not a perfect assumption, 01:14:23.055 --> 01:14:25.180 align:middle line:84% but we'll go with it for right now because it makes 01:14:25.180 --> 01:14:28.340 align:middle line:90% the math very straightforward. 01:14:28.340 --> 01:14:35.740 align:middle line:84% We'll also assume that P scales linearly with I. 01:14:35.740 --> 01:14:38.040 align:middle line:84% So we're down in that linear part of the curve, 01:14:38.040 --> 01:14:40.440 align:middle line:90% so we have no saturation yet. 01:14:40.440 --> 01:14:46.810 align:middle line:90% 01:14:46.810 --> 01:14:52.090 align:middle line:84% If you do that and you go back to these integrals, dr, 01:14:52.090 --> 01:14:58.770 align:middle line:84% dz is just going to equal whatever your background 01:14:58.770 --> 01:15:00.070 align:middle line:90% canonical respiration. 01:15:00.070 --> 01:15:01.445 align:middle line:84% We're going to assume that that's 01:15:01.445 --> 01:15:06.830 align:middle line:84% constant with depth, times whatever this critical depth is. 01:15:06.830 --> 01:15:09.530 align:middle line:90% 01:15:09.530 --> 01:15:12.230 align:middle line:84% If it's constant and you integrate it, 01:15:12.230 --> 01:15:16.550 align:middle line:84% it's just that value times the depth. 01:15:16.550 --> 01:15:20.650 align:middle line:90% 01:15:20.650 --> 01:15:25.330 align:middle line:84% The integral of pdz is then going to be the integral from 0 01:15:25.330 --> 01:15:39.870 align:middle line:84% to z CR of alpha times i, but i is then equal to i0, 01:15:39.870 --> 01:15:44.170 align:middle line:90% e to the minus kz, dz. 01:15:44.170 --> 01:15:49.550 align:middle line:90% 01:15:49.550 --> 01:15:51.130 align:middle line:90% And that's going to equal-- 01:15:51.130 --> 01:15:54.095 align:middle line:90% 01:15:54.095 --> 01:15:55.690 align:middle line:90% did I do this right? 01:15:55.690 --> 01:16:03.110 align:middle line:90% 01:16:03.110 --> 01:16:13.650 align:middle line:84% 1 over k, 1 minus e to the minus kz CR. 01:16:13.650 --> 01:16:21.070 align:middle line:90% 01:16:21.070 --> 01:16:33.750 align:middle line:84% And so if you then set that equal, you can find a depth. 01:16:33.750 --> 01:16:41.370 align:middle line:84% You can solve for z CR, where the respiration 01:16:41.370 --> 01:16:45.250 align:middle line:90% and photosynthesis are equal. 01:16:45.250 --> 01:16:54.070 align:middle line:84% And the argument is that if z mixed layer is greater than z 01:16:54.070 --> 01:16:59.210 align:middle line:90% CR, then you get no bloom. 01:16:59.210 --> 01:17:05.290 align:middle line:84% But when mixed-layer depth is less than 01:17:05.290 --> 01:17:08.250 align:middle line:84% or equal to the critical depth, that's 01:17:08.250 --> 01:17:11.550 align:middle line:90% when you trigger a spring bloom. 01:17:11.550 --> 01:17:16.010 align:middle line:90% 01:17:16.010 --> 01:17:18.770 align:middle line:84% And as long as you have some estimate of the respiration 01:17:18.770 --> 01:17:25.050 align:middle line:84% rate, the community respiration rate of the phytoplankton, 01:17:25.050 --> 01:17:27.570 align:middle line:90% you need to know the surface. 01:17:27.570 --> 01:17:30.690 align:middle line:84% If we go back up here-- you need to know the surface, P0, 01:17:30.690 --> 01:17:34.730 align:middle line:90% or some idea of alpha. 01:17:34.730 --> 01:17:35.640 align:middle line:90% Either will do. 01:17:35.640 --> 01:17:38.720 align:middle line:84% If you know alpha and you know your irradiance 01:17:38.720 --> 01:17:40.960 align:middle line:84% and your background respiration, you 01:17:40.960 --> 01:17:43.680 align:middle line:90% can then compute the critical-- 01:17:43.680 --> 01:17:46.040 align:middle line:84% you can then compute the critical depth 01:17:46.040 --> 01:17:48.100 align:middle line:84% as a function of those parameters, 01:17:48.100 --> 01:17:50.680 align:middle line:84% and then you can compare it with your mixed-layer depth. 01:17:50.680 --> 01:17:53.663 align:middle line:84% And so when the mixed-layer depth shoals, 01:17:53.663 --> 01:17:56.080 align:middle line:84% where we had that little the mixed-layer depth would go up 01:17:56.080 --> 01:18:05.400 align:middle line:84% like this, you'll reach a spot where-- 01:18:05.400 --> 01:18:09.840 align:middle line:84% let's say this was your z critical. 01:18:09.840 --> 01:18:12.380 align:middle line:84% You'll reach a spot where the mixed layer will shoal, 01:18:12.380 --> 01:18:14.700 align:middle line:84% and then you'll start to develop the bloom. 01:18:14.700 --> 01:18:17.200 align:middle line:84% And so then this all fills in with-- 01:18:17.200 --> 01:18:19.340 align:middle line:84% and you'll get this big spring bloom event. 01:18:19.340 --> 01:18:26.200 align:middle line:90% 01:18:26.200 --> 01:18:28.620 align:middle line:84% We've run out of time, so we got through light. 01:18:28.620 --> 01:18:31.600 align:middle line:84% We'll start on Tuesday with nutrients, 01:18:31.600 --> 01:18:35.390 align:middle line:84% and we'll do nutrients, temperature, and DIC.