WEBVTT 00:00:00.000 --> 00:00:05.000 So today we are going to continue where we left off last time talking 00:00:05.000 --> 00:00:11.000 more specifically about variations on the theme of life. 00:00:11.000 --> 00:00:16.000 And last year I tried to do this lecture using PowerPoint and it was 00:00:16.000 --> 00:00:22.000 a total disaster so I'm going back to the board. You will have the 00:00:22.000 --> 00:00:28.000 PowerPoint slides. They'll be on the Web to download 00:00:28.000 --> 00:00:34.000 to summarize basically what I'm drawing on the board. 00:00:34.000 --> 00:00:42.000 But it will be slightly different on the board. But I found that for 00:00:42.000 --> 00:00:50.000 this material it really doesn't work to exclusively use the PowerPoint. 00:00:50.000 --> 00:00:58.000 So last time we talked about, remember, my life on earth 00:00:58.000 --> 00:01:07.000 abridged where -- 00:01:07.000 --> 00:01:16.000 -- we had photosynthesis making glucose or organic carbon plus 00:01:16.000 --> 00:01:25.000 oxygen? And then the reverse of this was respiration. 00:01:25.000 --> 00:01:32.000 And then we had elements cycling in 00:01:32.000 --> 00:01:38.000 the middle. And I said this is very, very abbreviated of how all life on 00:01:38.000 --> 00:01:44.000 earth works. And so today what I'm going to do is tell you that that's 00:01:44.000 --> 00:01:50.000 not right. That's grossly oversimplified. 00:01:50.000 --> 00:01:56.000 And there are some really interesting variations on the theme 00:01:56.000 --> 00:02:02.000 of how to extract energy and carbon and reducing power and electrons 00:02:02.000 --> 00:02:08.000 from the earth's system to create life. 00:02:08.000 --> 00:02:13.000 And it's mostly microbes that have these diverse possibilities. 00:02:13.000 --> 00:02:18.000 And, again, even what I'm going to talk to you about today is 00:02:18.000 --> 00:02:23.000 oversimplified. If you go to a microbiology 00:02:23.000 --> 00:02:28.000 textbook you'll find just about every possible combination of energy 00:02:28.000 --> 00:02:33.000 sources, carbon sources and electron sources in some microorganisms 00:02:33.000 --> 00:02:39.000 somewhere to get through life. So I'm giving you, 00:02:39.000 --> 00:02:45.000 again, the simplified version because otherwise it gets way too 00:02:45.000 --> 00:02:51.000 complicated. So all of life needs carbon and energy, 00:02:51.000 --> 00:02:57.000 and a lot of other elements, too, but these are the main axis 00:02:57.000 --> 00:03:03.000 upon which we're going to order our universe today. 00:03:03.000 --> 00:03:13.000 So for carbon the choices are inorganic or organic. 00:03:13.000 --> 00:03:23.000 So this would be CO2 and this might be glucose or sugars, 00:03:23.000 --> 00:03:33.000 any sugars. And then on the energy axis they can use solar energy, 00:03:33.000 --> 00:03:43.000 as in photosynthesis, or they can use chemical energy. 00:03:43.000 --> 00:03:54.000 And within the chemical energy sources they can be inorganic or 00:03:54.000 --> 00:04:05.000 organic like sugars, etc. And often here you have reduced 00:04:05.000 --> 00:04:16.000 compounds such as hydrogen sulfide, ammonia, and we'll talk about these. 00:04:16.000 --> 00:04:27.000 So these are the ways we divide up the possibilities for carbon and 00:04:27.000 --> 00:04:36.000 energy sources to be alive. All organisms also need to have an 00:04:36.000 --> 00:04:42.000 energy currency in the cell. And you've talked about this a lot 00:04:42.000 --> 00:04:48.000 already in the biochemistry lectures so I'm, again, 00:04:48.000 --> 00:04:54.000 just giving you the impressionist view of this. You know the details. 00:04:54.000 --> 00:05:00.000 This is just to get you organized. And so all life uses 00:05:00.000 --> 00:05:06.000 redox reactions. And in your handouts for today 00:05:06.000 --> 00:05:14.000 there's a primer on redox reactions just in case you want to review that. 00:05:14.000 --> 00:05:22.000 And one of the key reactions we'll talk about today is the conversion 00:05:22.000 --> 00:05:30.000 of NADP. If you put energy in you can reduce it to NADPH. 00:05:30.000 --> 00:05:37.000 So that's a reduction. And the reverse you get energy out 00:05:37.000 --> 00:05:45.000 when it's oxidized. Now, we're going to be talking 00:05:45.000 --> 00:05:53.000 about oxidation and reduction today. And then they all use ATP which 00:05:53.000 --> 00:06:01.000 you've talked a lot about here. And the couple here is ADP. Put 00:06:01.000 --> 00:06:10.000 energy in. 00:06:10.000 --> 00:06:16.000 You make ATP which is a high energy intermediate. And in converting it 00:06:16.000 --> 00:06:23.000 back to ADP that energy can be released. And this is used in the 00:06:23.000 --> 00:06:30.000 biochemistry of the cell. So all cells have these two energy 00:06:30.000 --> 00:06:37.000 conversion processes in common. OK, so let's look at just 00:06:37.000 --> 00:06:44.000 summarizing what we're going to go over today. This is a summary of 00:06:44.000 --> 00:06:51.000 options for life. See also Freeman, 00:06:51.000 --> 00:06:58.000 Chapter 25. There is some discussion of this. 00:06:58.000 --> 00:07:03.000 And we can divide life here between what we call autotrophs. 00:07:03.000 --> 00:07:08.000 These are organisms that can make their own organic carbon. 00:07:08.000 --> 00:07:13.000 In other words, they can convert carbon dioxide to organic carbon. 00:07:13.000 --> 00:07:19.000 Heterotrophs are organisms that can only use organic carbon. 00:07:19.000 --> 00:07:24.000 They rely on the guts of other organisms in order to 00:07:24.000 --> 00:07:29.000 get through life. And so now we're going to 00:07:29.000 --> 00:07:35.000 systematically go through these processes that fall under each one 00:07:35.000 --> 00:07:40.000 of these. Oxygenic photosynthesis is the one we've been talking about 00:07:40.000 --> 00:07:45.000 last time and in my abbreviated version of life on earth. 00:07:45.000 --> 00:07:51.000 And this is carried out by eukaryotic organisms, 00:07:51.000 --> 00:07:56.000 plants, trees, etc., and also by prokaryotic organisms. 00:07:56.000 --> 00:08:02.000 Those are the cyanobacteria, microscopic photosynthetic plants. 00:08:02.000 --> 00:08:06.000 They use CO2 and sunlight. So our first variant on this theme 00:08:06.000 --> 00:08:10.000 we'll get into is a group of bacteria that do anoxygenic 00:08:10.000 --> 00:08:15.000 photosynthesis. Oxygenic means they evolve oxygen. 00:08:15.000 --> 00:08:19.000 These guys use solar energy but they don't evolve oxygen. 00:08:19.000 --> 00:08:24.000 And we'll get into how that works. And then there's a group of 00:08:24.000 --> 00:08:28.000 organisms that still use CO2. And in the very similar pathway the 00:08:28.000 --> 00:08:34.000 Calvin Cycle is photosynthesis. But they use chemical energy in 00:08:34.000 --> 00:08:42.000 order to make these intermediates to fix CO2. OK, so let's talk about 00:08:42.000 --> 00:08:50.000 those first. And so we're going to talk about the autotrophs. 00:08:50.000 --> 00:09:01.000 And all of them share this 00:09:01.000 --> 00:09:11.000 pathway, CO2 to C6H12. This would be glucose. 00:09:11.000 --> 00:09:21.000 And it takes ATP to run this reaction and it also takes 00:09:21.000 --> 00:09:34.000 reduced NADPH -- 00:09:34.000 --> 00:09:40.000 -- to run this reaction. It also takes this enzyme ribisco 00:09:40.000 --> 00:09:47.000 which you've talked about I'm sure, ribulose bisphosphate carboxylase. 00:09:47.000 --> 00:09:53.000 And this is the enzyme that initially takes the CO2 from the 00:09:53.000 --> 00:10:00.000 atmosphere and binds it to an organic carbon. 00:10:00.000 --> 00:10:04.000 Now, in a detailed version of this is what's called the Calvin Cycle or 00:10:04.000 --> 00:10:08.000 the Calvin/Benson Cycle. I don't know which one your book 00:10:08.000 --> 00:10:13.000 calls it. Calvin got the Nobel Prize but Benson was the graduate 00:10:13.000 --> 00:10:17.000 student that did all the work, so you should recognize that. 00:10:17.000 --> 00:10:22.000 Anyway, you studied this in great deal. But an interesting factoid is 00:10:22.000 --> 00:10:26.000 that ribisco is the most abundant protein on earth. 00:10:26.000 --> 00:10:30.000 That tells you how important this reaction is for sustaining 00:10:30.000 --> 00:10:36.000 life on earth. So notice that in order to drive 00:10:36.000 --> 00:10:43.000 this reaction, which is the Calvin Cycle, 00:10:43.000 --> 00:10:50.000 it requires energy and reducing power. So where do they get it? 00:10:50.000 --> 00:11:14.000 Well, there are three ways that 00:11:14.000 --> 00:11:20.000 autotrophs can get energy and reducing power to drive this 00:11:20.000 --> 00:11:27.000 reaction. And the first is oxygenic photosynthesis. And the 00:11:27.000 --> 00:11:34.000 second is anoxygenic. And the third is chemosynthesis. 00:11:34.000 --> 00:11:41.000 OK, those first three there. So now we're going to go through each 00:11:41.000 --> 00:11:49.000 of these and look at how they work remembering that all of them are 00:11:49.000 --> 00:11:56.000 generating ATP and NADPH in order to drive that. So all of the 00:11:56.000 --> 00:12:02.000 autotrophs have that in common. Well, oxygenic photosynthesis is the 00:12:02.000 --> 00:12:06.000 one that you know well already. You've studied it in great detail 00:12:06.000 --> 00:12:11.000 in biochemistry. So we're going to, 00:12:11.000 --> 00:12:15.000 again, give you the abbreviated version here just so you have a 00:12:15.000 --> 00:12:20.000 template to map these other ones onto. 00:12:20.000 --> 00:12:38.000 These are what are known as the 00:12:38.000 --> 00:12:45.000 light reactions of photosynthesis, the Z scheme taking solar energy, 00:12:45.000 --> 00:12:51.000 splitting water, evolving oxygen and synthesizing ATP 00:12:51.000 --> 00:12:58.000 and NADPH. This is all familiar, right? Very familiar. I'm just 00:12:58.000 --> 00:13:06.000 writing it in a cartoon version. OK, so this is the NADPH and ADP 00:13:06.000 --> 00:13:17.000 that goes to fuel that process. 00:13:17.000 --> 00:13:22.000 OK, so now, well, at least I can do it on that board. 00:13:22.000 --> 00:13:28.000 Let me do it on this board. Anoxygenic -- 00:13:28.000 --> 00:13:39.000 -- is almost exactly like this 00:13:39.000 --> 00:13:47.000 process, but instead of splitting water these guys oxidize hydrogen 00:13:47.000 --> 00:13:55.000 sulfide. So here's our ATP and NADPH. 00:13:55.000 --> 00:14:03.000 And they use sunlight to do this. 00:14:03.000 --> 00:14:09.000 So these are called photosynthetic bacteria. And they were around very 00:14:09.000 --> 00:14:15.000 early on the earth. Long before the earth's atmosphere 00:14:15.000 --> 00:14:21.000 was oxygenated these were the guys that were able to use solar energy 00:14:21.000 --> 00:14:27.000 and make organic carbon but without evolving oxygen. 00:14:27.000 --> 00:14:32.000 Then somewhere along the line some cell evolved, had some mutations and 00:14:32.000 --> 00:14:37.000 somehow figured out that water, this abundant source of water was a 00:14:37.000 --> 00:14:43.000 much better electron donor than hydrogen sulfide. 00:14:43.000 --> 00:14:48.000 And once the biochemistry figured this out, you can see the simple 00:14:48.000 --> 00:14:54.000 substitution here, the whole earth started going in a 00:14:54.000 --> 00:14:59.000 different direction. So this is an interesting example 00:14:59.000 --> 00:15:05.000 of how a small biochemical innovation can dramatically change 00:15:05.000 --> 00:15:10.000 the whole nature of the planet. Now, these guys are still around on 00:15:10.000 --> 00:15:14.000 earth. In fact, I'm going to show you some. 00:15:14.000 --> 00:15:19.000 I'll explain this at the end, but I have some captured in here. 00:15:19.000 --> 00:15:24.000 See that little purple band? Those are those guys. 00:15:24.000 --> 00:15:29.000 I've got other little tricks in here but I'll save those. 00:15:29.000 --> 00:15:34.000 Well, you cannot really see the purple band. But you can come up 00:15:34.000 --> 00:15:39.000 later and look at it. Those are photosynthetic bacteria. 00:15:39.000 --> 00:15:44.000 So they're still around on the earth but they're stuck in places 00:15:44.000 --> 00:15:49.000 where there's no oxygen. So they have a rather restricted 00:15:49.000 --> 00:15:54.000 niche on the planet now, but they're still extremely 00:15:54.000 --> 00:15:59.000 important. What did I do? Oh, here it is. 00:15:59.000 --> 00:16:03.000 So one of the places that they can be found, and if you're interested 00:16:03.000 --> 00:16:08.000 in them a great place to go find some is out at the Mystic Lakes in 00:16:08.000 --> 00:16:13.000 Arlington which is a permanently stratified lake so the bottom of the 00:16:13.000 --> 00:16:18.000 lake is always anaerobic. There's never oxygen there. 00:16:18.000 --> 00:16:23.000 In a typical lake like that you have a lot of mud on the bottom and 00:16:23.000 --> 00:16:28.000 you have a lot of hydrogen sulfide coming out of the mud from bacterial 00:16:28.000 --> 00:16:34.000 processes that we'll talk about. And you have light here. 00:16:34.000 --> 00:16:41.000 And so you have a gradient here of this is oxygen and this is H2S. 00:16:41.000 --> 00:16:49.000 And these photosynthetic bacteria have to life somewhere where there's 00:16:49.000 --> 00:16:56.000 enough light to photosynthesize and enough hydrogen sulfide to use in 00:16:56.000 --> 00:17:02.000 this part of the reaction. But they're very sensitive to oxygen 00:17:02.000 --> 00:17:06.000 so they cannot be in the oxygenated part of the lake. 00:17:06.000 --> 00:17:11.000 So you find them in a layer. It's called the squeeze. They have 00:17:11.000 --> 00:17:16.000 to have light so they have to be up, but they cannot have oxygen so they 00:17:16.000 --> 00:17:20.000 have to be down. And they need hydrogen sulfide so 00:17:20.000 --> 00:17:25.000 they have to be down. So they're layered in lakes. 00:17:25.000 --> 00:17:30.000 OK. So what about these guys, chemosynthesis? 00:17:30.000 --> 00:17:36.000 They don't rely on solar energy. Again, they're still driving the 00:17:36.000 --> 00:17:42.000 Calvin Cycle reducing CO2 from the air into organic carbon, 00:17:42.000 --> 00:17:48.000 but they're not using sunlight. So what do they do? They get their 00:17:48.000 --> 00:18:02.000 energy -- 00:18:02.000 --> 00:18:16.000 -- from redox reactions. And let's just show you an example. 00:18:16.000 --> 00:18:27.000 Redox reactions couple to the 00:18:27.000 --> 00:18:37.000 conversion of oxygen to H2O. So oxygen is involved in these 00:18:37.000 --> 00:18:47.000 reactions. And one organism, for example, can take ammonia and 00:18:47.000 --> 00:18:57.000 convert it to nitrite. Another type of organism can take 00:18:57.000 --> 00:19:07.000 nitrite and convert it to nitrate. And there are other organisms that 00:19:07.000 --> 00:19:17.000 can take hydrogen sulfide and convert it to sulfate. 00:19:17.000 --> 00:19:27.000 And some can take hydrogen sulfide, oh, no, take iron, ferrous iron, 00:19:27.000 --> 00:19:35.000 Fe2+ and convert it to Fe3+. So in all of these cases what is 00:19:35.000 --> 00:19:42.000 happening to these compounds? Are they being oxidized or reduced? 00:19:42.000 --> 00:19:49.000 I heard an oxidized. Yes, they're being oxidized. 00:19:49.000 --> 00:19:56.000 So these reduced compounds, relatively reduced compounds can be 00:19:56.000 --> 00:20:03.000 utilized by oxidizing them. The organism can release the energy 00:20:03.000 --> 00:20:10.000 that's needed. ATP is generated here. 00:20:10.000 --> 00:20:25.000 And NADPH is generated by any of 00:20:25.000 --> 00:20:31.000 these redox couples. So using this energy then the cell 00:20:31.000 --> 00:20:39.000 takes the reduced NADPH and the ATP and it runs the Calvin Cycle, 00:20:39.000 --> 00:20:46.000 chemosynthesis. OK. Now, you may think that these are kind of strange, 00:20:46.000 --> 00:20:54.000 weird bacteria that life in strange pockets of the earth where there's 00:20:54.000 --> 00:21:00.000 no oxygen. And who cares anyway? They're outdated. 00:21:00.000 --> 00:21:04.000 They dominated the earth way back in the early stages of the earth but 00:21:04.000 --> 00:21:09.000 they're not so important now. Well, that's not true. They're 00:21:09.000 --> 00:21:14.000 incredibly important. In some ecosystems they're the 00:21:14.000 --> 00:21:18.000 total base of the entire ecosystem. But also on a global scale, as 00:21:18.000 --> 00:21:23.000 you'll learn, you should have a feeling for this by the end of this 00:21:23.000 --> 00:21:28.000 lecture, but also when we talk about global biogeochemical cycles you 00:21:28.000 --> 00:21:32.000 will learn that these microbes are really messengers for electrons in 00:21:32.000 --> 00:21:37.000 the environment. Without them the redox balance of 00:21:37.000 --> 00:21:41.000 the earth would not be maintained, OK? You cannot have nothing but 00:21:41.000 --> 00:21:46.000 oxidizing reactions or nothing but reduction reactions and have a 00:21:46.000 --> 00:21:50.000 system sustain itself. So it's these microbes that are 00:21:50.000 --> 00:21:54.000 playing a really important role in maintaining the redox balance 00:21:54.000 --> 00:21:59.000 of the earth. OK. Now, one system that I'm going to 00:21:59.000 --> 00:22:04.000 show you in that DVD, that will do much better justice to 00:22:04.000 --> 00:22:09.000 it than my drawings here, that's a deep-sea volcano in case 00:22:09.000 --> 00:22:15.000 you didn't recognize it. And this is 2500 meters at the 00:22:15.000 --> 00:22:20.000 bottom of the ocean, very, very deep. And there is 00:22:20.000 --> 00:22:25.000 intense heat. I mean just think of a volcano on the surface 00:22:25.000 --> 00:22:31.000 of the earth. Intense heat and reduced compounds 00:22:31.000 --> 00:22:37.000 are found in the earth's mantle that are ready to erupt through this 00:22:37.000 --> 00:22:44.000 deep-sea volcano. And you have sulfate in the sea 00:22:44.000 --> 00:22:50.000 water that percolates through here. And as it percolates in and gets 00:22:50.000 --> 00:22:57.000 draw into the volcanic stuff that's coming out of here it's reduced to 00:22:57.000 --> 00:23:04.000 hydrogen sulfide coming out of the volcano. 00:23:04.000 --> 00:23:08.000 But you have oxygen in the water in the deep-sea. And we'll be talking 00:23:08.000 --> 00:23:12.000 about this when we talk about ocean circulation. But the oceans have a 00:23:12.000 --> 00:23:16.000 global ocean circulation where the surface water that's in equilibrium 00:23:16.000 --> 00:23:21.000 with the atmosphere actually sinks and travels along the bottom of the 00:23:21.000 --> 00:23:25.000 ocean. So there is oxygen in the bottom of the ocean, 00:23:25.000 --> 00:23:30.000 unlike many lakes where you don't have oxygen. 00:23:30.000 --> 00:23:37.000 And we'll talk about that difference. And in the hot vents the water 00:23:37.000 --> 00:23:44.000 coming out of here can be very, very hot, but there's a gradient 00:23:44.000 --> 00:23:51.000 right as it comes out meeting the colder sea water. 00:23:51.000 --> 00:23:58.000 And so what you have here is a perfect incubator for chemosynthetic 00:23:58.000 --> 00:24:07.000 bacteria -- 00:24:07.000 --> 00:24:12.000 -- that use the hydrogen sulfide in chemosynthesis to fix carbon dioxide 00:24:12.000 --> 00:24:18.000 using the oxygen here. And that forms the base of the 00:24:18.000 --> 00:24:23.000 entire food web in the deep ocean because there's no light down there. 00:24:23.000 --> 00:24:29.000 There's no photosynthesis. There's only chemosynthesis. 00:24:29.000 --> 00:24:35.000 And just a little story that goes back to when I first came to MIT as 00:24:35.000 --> 00:24:41.000 an assistant professor in 1976. You weren't even born. But when I 00:24:41.000 --> 00:24:47.000 was young we used to go the Muddy Charles Pub periodically after work 00:24:47.000 --> 00:24:53.000 and have beers. And there was a professor, 00:24:53.000 --> 00:24:59.000 in this department actually, John Edmond, who passed away several 00:24:59.000 --> 00:25:05.000 years ago but who used to be there. It was sort of like our Cheers. 00:25:05.000 --> 00:25:11.000 And I'll never forget the day he came back from a cruise. 00:25:11.000 --> 00:25:17.000 He came to the pub. He was a chemist and I'm a biologist. 00:25:17.000 --> 00:25:23.000 And he said you will not believe what we found on the bottom of the 00:25:23.000 --> 00:25:29.000 ocean. He had gone down in Alvin, this two-person submersible vehicle. 00:25:29.000 --> 00:25:33.000 And he started talking about these giant clams and these giant tube 00:25:33.000 --> 00:25:38.000 worms and all of these things, and I thought he had had one too 00:25:38.000 --> 00:25:43.000 many beers. I found it hard to believe. Well, 00:25:43.000 --> 00:25:48.000 it turned out that that was the first discovery of these deep-sea 00:25:48.000 --> 00:25:53.000 vents and he was on that expedition. And through that collegial 00:25:53.000 --> 00:25:58.000 relationship I actually ended up with one of the clam shells from the 00:25:58.000 --> 00:26:03.000 clams there, which is one of the giant clams. 00:26:03.000 --> 00:26:08.000 Their meat is blood red because they have a special kind of hemoglobin 00:26:08.000 --> 00:26:13.000 that they use to keep the oxygen tension perfect for these 00:26:13.000 --> 00:26:18.000 chemosynthetic bacteria. If the oxygen is too high they 00:26:18.000 --> 00:26:23.000 cannot do this because it will spontaneously oxidize the H2S. 00:26:23.000 --> 00:26:29.000 So the oxygen tension is very critical. 00:26:29.000 --> 00:26:32.000 And they have a special kind of hemoglobin that does that. 00:26:32.000 --> 00:26:36.000 So these claims had symbiotic chemosynthetic bacteria. 00:26:36.000 --> 00:26:40.000 Well, since then these vents have been discovered everywhere and 00:26:40.000 --> 00:26:44.000 ecosystems similar have been discovered on the surface. 00:26:44.000 --> 00:26:48.000 And there are all kinds of different vents. 00:26:48.000 --> 00:26:52.000 You're going to learn about not only hydrothermal vents, 00:26:52.000 --> 00:26:56.000 hot vents in this video, but also cold seeps they're called where you 00:26:56.000 --> 00:27:00.000 have methane bacteria that are really important. OK. 00:27:00.000 --> 00:27:05.000 So these are the main ways in which organisms can get energy to convert 00:27:05.000 --> 00:27:11.000 CO2 to organic carbon. Then you have all these 00:27:11.000 --> 00:27:17.000 heterotrophs, the ones that use the organic carbon, 00:27:17.000 --> 00:27:23.000 and they have various ways of doing that. You've learned in 00:27:23.000 --> 00:27:29.000 biochemistry the primary way, which is very powerful, and that is 00:27:29.000 --> 00:27:35.000 using aerobic respiration to do that. 00:27:35.000 --> 00:27:44.000 And so we are just going to abbreviate that here. 00:27:44.000 --> 00:27:54.000 That's our reverse of photosynthesis. So heterotrophs. 00:27:54.000 --> 00:28:03.000 So we have first aerobic. 00:28:03.000 --> 00:28:13.000 And let me jump ahead 00:28:13.000 --> 00:28:25.000 with the slides. 00:28:25.000 --> 00:28:32.000 OK, there you are. So this is a cartoon version of 00:28:32.000 --> 00:28:40.000 aerobic respiration. So we'll just put glucose, 00:28:40.000 --> 00:28:47.000 we'll come down to the Krebs' Cycle. And we are going to let electrons 00:28:47.000 --> 00:28:55.000 flow here and have oxygen be the final electron acceptor 00:28:55.000 --> 00:29:02.000 creating water. So we've really just accomplished 00:29:02.000 --> 00:29:09.000 the absolute reverse of photosynthesis and we've made NADH 00:29:09.000 --> 00:29:16.000 in doing this and we've made ATP. So these guys are getting the 00:29:16.000 --> 00:29:22.000 energy out of the glucose that all of the other organisms made. 00:29:22.000 --> 00:29:29.000 And oxygen is the terminal electron acceptor when there's 00:29:29.000 --> 00:29:35.000 oxygen around. But there are lots of environments, 00:29:35.000 --> 00:29:41.000 as we've talked about on earth, where there isn't oxygen. 00:29:41.000 --> 00:29:46.000 And there are bacteria that can take advantage of those environments. 00:29:46.000 --> 00:29:52.000 And instead of having oxygen be the terminal electron acceptor there are 00:29:52.000 --> 00:29:57.000 a number of other elements that they can use, compounds that they can use. 00:29:57.000 --> 00:30:03.000 For example, there are some that use nitrate and they reduce 00:30:03.000 --> 00:30:10.000 it to nitrous oxide. N2. Ammonia. All the relatively 00:30:10.000 --> 00:30:19.000 reduced forms of nitrogen. And so this called anaerobic. 00:30:19.000 --> 00:30:29.000 And this process is called 00:30:29.000 --> 00:30:36.000 gentrification. And if it weren't for these bacteria, 00:30:36.000 --> 00:30:40.000 these anaerobic bacteria that can reduce nitrate, 00:30:40.000 --> 00:30:44.000 nitrogen would never return to the atmosphere. Remember last time we 00:30:44.000 --> 00:30:48.000 talked about nitrogen fixation, how specific types of microbes can 00:30:48.000 --> 00:30:52.000 take N2 from the atmosphere and pull it into the ecosystem? 00:30:52.000 --> 00:30:56.000 Well, if you didn't have these bacteria doing this process that 00:30:56.000 --> 00:31:00.000 nitrogen would never get back to the atmosphere. 00:31:00.000 --> 00:31:04.000 They're central to closing the nitrogen cycle. 00:31:04.000 --> 00:31:08.000 Then there are some that can use sulfate and reduce it to hydrogen 00:31:08.000 --> 00:31:13.000 sulfide. As you can imagine, these are critical to creating the 00:31:13.000 --> 00:31:17.000 hydrogen sulfide that's used in these other processes. 00:31:17.000 --> 00:31:22.000 There are some that use CO2 and convert to methane. 00:31:22.000 --> 00:31:26.000 These are methanogenic bacteria, and they're incredibly important in 00:31:26.000 --> 00:31:31.000 the global carbon cycle and in the methane cycle. 00:31:31.000 --> 00:31:36.000 Methane is a really powerful greenhouse gas, 00:31:36.000 --> 00:31:42.000 and we're going to talk about that later. And then there are some that 00:31:42.000 --> 00:31:48.000 can take Fe3+ and reduce it to Fe2+. And the same for manganese. 00:31:48.000 --> 00:31:55.000 So you should be starting to sense a 00:31:55.000 --> 00:32:00.000 sort of symmetry here, right, that these anaerobic bacteria 00:32:00.000 --> 00:32:05.000 are fulfilling functions on the earth. Let me write these down. 00:32:05.000 --> 00:32:17.000 These are sulfate reducers, these are methanogens, and these are 00:32:17.000 --> 00:32:30.000 iron reducers and manganese reducers. 00:32:30.000 --> 00:32:37.000 So these will all become extremely important when we talk about the 00:32:37.000 --> 00:32:44.000 global biogeochemical cycles of all of these elements. 00:32:44.000 --> 00:32:51.000 It's these microbes that make sure that the cycles can continue and 00:32:51.000 --> 00:32:58.000 don't run into a dead end of oxidation or reduction. 00:32:58.000 --> 00:33:05.000 OK. Before we go to the movie, I just want to say if you look at 00:33:05.000 --> 00:33:12.000 Table 25.2 in your textbook, I think it's that one. 00:33:12.000 --> 00:33:18.000 I'm assuming I'm using the most recent version. 00:33:18.000 --> 00:33:24.000 You'll see a variation of this theme in which there will be some 00:33:24.000 --> 00:33:30.000 entries of organisms that don't fall into these categories that 00:33:30.000 --> 00:33:36.000 I've just shown you. And that is to say that there are 00:33:36.000 --> 00:33:41.000 organisms that use light energy and organic carbon energy at the same 00:33:41.000 --> 00:33:46.000 time. For every variation that's possible there's an organism that's 00:33:46.000 --> 00:33:51.000 evolved to take advantage of it. I've just oversimplified it here, 00:33:51.000 --> 00:33:56.000 but you should know that. And the bottom line is if it's 00:33:56.000 --> 00:34:01.000 thermodynamically possible. And, again, this whole lecture could 00:34:01.000 --> 00:34:06.000 have been done in a thermodynamic mode. We could have looked at which 00:34:06.000 --> 00:34:12.000 redox couples were energetically possible and then assigned those to 00:34:12.000 --> 00:34:17.000 particular microbes. But for now I just want you to get 00:34:17.000 --> 00:34:22.000 the overview. But for anything that's thermodynamically feasible 00:34:22.000 --> 00:34:28.000 there's a microbe out there that's doing it. 00:34:28.000 --> 00:34:32.000 And, in fact, microbiologists actually comb through redox tables 00:34:32.000 --> 00:34:37.000 and put together different redox couples and hypothesize. 00:34:37.000 --> 00:34:41.000 I ought to be able to find an organism that does this in that 00:34:41.000 --> 00:34:46.000 environment. And then they go out. And they can almost always actually 00:34:46.000 --> 00:34:50.000 find it. So they're incredibly versatile. And it gives you a 00:34:50.000 --> 00:34:55.000 really good strong feeling for the power of thermodynamics in driving 00:34:55.000 --> 00:35:00.000 the evolution of these biochemical processes. 00:35:00.000 --> 00:35:07.000 Finally, before we show you the movie I want to show you what this 00:35:07.000 --> 00:35:15.000 thing is all about. There was a Russian microbiologist 00:35:15.000 --> 00:35:23.000 back in the previous century named Winogradsky -- 00:35:23.000 --> 00:35:33.000 -- who wanted to isolate some of 00:35:33.000 --> 00:35:39.000 these photosynthetic bacteria. And knowing what their 00:35:39.000 --> 00:35:45.000 characteristics were he went out and got himself some mud and some pond 00:35:45.000 --> 00:35:52.000 water. And he set up what we've come to call a Winogradsky column. 00:35:52.000 --> 00:35:58.000 This is a Winogradsky juice bottle, but it works the same. And what you 00:35:58.000 --> 00:36:05.000 do is you put mud in the bottom and you put pond water here. 00:36:05.000 --> 00:36:09.000 And the pond water has basically an inoculum. It has representatives of 00:36:09.000 --> 00:36:13.000 all different types of bacteria. They might be spores. If they 00:36:13.000 --> 00:36:17.000 don't like the environment they're in they sporulates and then they 00:36:17.000 --> 00:36:21.000 just don't germinate. But presumably in pond water you 00:36:21.000 --> 00:36:25.000 have everything that could possibly grow in here. And in the mud you 00:36:25.000 --> 00:36:29.000 add a source of sulfate. And so you might add calcium 00:36:29.000 --> 00:36:33.000 sulfate and you might add a little organic matter, 00:36:33.000 --> 00:36:38.000 you know, plant parts or something just to jumpstart it. 00:36:38.000 --> 00:36:45.000 And eventually you set up a gradient here of hydrogen sulfide and oxygen. 00:36:45.000 --> 00:36:53.000 And over time the organisms grow along that gradient. 00:36:53.000 --> 00:37:01.000 So you'll end up down here with the anaerobic respiration. 00:37:01.000 --> 00:37:08.000 In fact, the organisms generate this 00:37:08.000 --> 00:37:12.000 gradient. When you start out the whole thing is oxygenated. 00:37:12.000 --> 00:37:16.000 And what you should think about in this context is what happens. 00:37:16.000 --> 00:37:20.000 How do these gradients get generated when you start out with a 00:37:20.000 --> 00:37:24.000 completely mixed system, everything in there, everything 00:37:24.000 --> 00:37:29.000 oxygenated? Eventually you have anaerobic -- 00:37:29.000 --> 00:37:33.000 First you'll just have anaerobic respiration, right? 00:37:33.000 --> 00:37:38.000 Anything that can use organic carbon and oxygen is going to go 00:37:38.000 --> 00:37:42.000 like mad, and that's what's going to draw the oxygen down. 00:37:42.000 --> 00:37:47.000 Then you'll have anaerobic respiration here. 00:37:47.000 --> 00:37:51.000 You'll have photosynthesis up here, evolving oxygen. You'll have 00:37:51.000 --> 00:37:56.000 chemosynthetic bacteria here because they need a little bit of oxygen but 00:37:56.000 --> 00:38:00.000 they also need some of this hydrogen sulfide and photosynthetic 00:38:00.000 --> 00:38:07.000 bacteria here. 00:38:07.000 --> 00:38:11.000 Well, they're like down here. Because they need light but cannot 00:38:11.000 --> 00:38:16.000 have oxygen. And so you can set these up. And this purple band here 00:38:16.000 --> 00:38:19.000 tells you that you've got your photosynthetic bacteria.