1 00:00:00 --> 00:00:05 So today we are going to continue where we left off last time talking 2 00:00:05 --> 00:00:11 more specifically about variations on the theme of life. 3 00:00:11 --> 00:00:16 And last year I tried to do this lecture using PowerPoint and it was 4 00:00:16 --> 00:00:22 a total disaster so I'm going back to the board. You will have the 5 00:00:22 --> 00:00:28 PowerPoint slides. They'll be on the Web to download 6 00:00:28 --> 00:00:34 to summarize basically what I'm drawing on the board. 7 00:00:34 --> 00:00:42 But it will be slightly different on the board. But I found that for 8 00:00:42 --> 00:00:50 this material it really doesn't work to exclusively use the PowerPoint. 9 00:00:50 --> 00:00:58 So last time we talked about, remember, my life on earth 10 00:00:58 --> 00:01:07 abridged where -- 11 00:01:07 --> 00:01:16 -- we had photosynthesis making glucose or organic carbon plus 12 00:01:16 --> 00:01:25 oxygen? And then the reverse of this was respiration. 13 00:01:25 --> 00:01:32 And then we had elements cycling in 14 00:01:32 --> 00:01:38 the middle. And I said this is very, very abbreviated of how all life on 15 00:01:38 --> 00:01:44 earth works. And so today what I'm going to do is tell you that that's 16 00:01:44 --> 00:01:50 not right. That's grossly oversimplified. 17 00:01:50 --> 00:01:56 And there are some really interesting variations on the theme 18 00:01:56 --> 00:02:02 of how to extract energy and carbon and reducing power and electrons 19 00:02:02 --> 00:02:08 from the earth's system to create life. 20 00:02:08 --> 00:02:13 And it's mostly microbes that have these diverse possibilities. 21 00:02:13 --> 00:02:18 And, again, even what I'm going to talk to you about today is 22 00:02:18 --> 00:02:23 oversimplified. If you go to a microbiology 23 00:02:23 --> 00:02:28 textbook you'll find just about every possible combination of energy 24 00:02:28 --> 00:02:33 sources, carbon sources and electron sources in some microorganisms 25 00:02:33 --> 00:02:39 somewhere to get through life. So I'm giving you, 26 00:02:39 --> 00:02:45 again, the simplified version because otherwise it gets way too 27 00:02:45 --> 00:02:51 complicated. So all of life needs carbon and energy, 28 00:02:51 --> 00:02:57 and a lot of other elements, too, but these are the main axis 29 00:02:57 --> 00:03:03 upon which we're going to order our universe today. 30 00:03:03 --> 00:03:13 So for carbon the choices are inorganic or organic. 31 00:03:13 --> 00:03:23 So this would be CO2 and this might be glucose or sugars, 32 00:03:23 --> 00:03:33 any sugars. And then on the energy axis they can use solar energy, 33 00:03:33 --> 00:03:43 as in photosynthesis, or they can use chemical energy. 34 00:03:43 --> 00:03:54 And within the chemical energy sources they can be inorganic or 35 00:03:54 --> 00:04:05 organic like sugars, etc. And often here you have reduced 36 00:04:05 --> 00:04:16 compounds such as hydrogen sulfide, ammonia, and we'll talk about these. 37 00:04:16 --> 00:04:27 So these are the ways we divide up the possibilities for carbon and 38 00:04:27 --> 00:04:36 energy sources to be alive. All organisms also need to have an 39 00:04:36 --> 00:04:42 energy currency in the cell. And you've talked about this a lot 40 00:04:42 --> 00:04:48 already in the biochemistry lectures so I'm, again, 41 00:04:48 --> 00:04:54 just giving you the impressionist view of this. You know the details. 42 00:04:54 --> 00:05:00 This is just to get you organized. And so all life uses 43 00:05:00 --> 00:05:06 redox reactions. And in your handouts for today 44 00:05:06 --> 00:05:14 there's a primer on redox reactions just in case you want to review that. 45 00:05:14 --> 00:05:22 And one of the key reactions we'll talk about today is the conversion 46 00:05:22 --> 00:05:30 of NADP. If you put energy in you can reduce it to NADPH. 47 00:05:30 --> 00:05:37 So that's a reduction. And the reverse you get energy out 48 00:05:37 --> 00:05:45 when it's oxidized. Now, we're going to be talking 49 00:05:45 --> 00:05:53 about oxidation and reduction today. And then they all use ATP which 50 00:05:53 --> 00:06:01 you've talked a lot about here. And the couple here is ADP. Put 51 00:06:01 --> 00:06:10 energy in. 52 00:06:10 --> 00:06:16 You make ATP which is a high energy intermediate. And in converting it 53 00:06:16 --> 00:06:23 back to ADP that energy can be released. And this is used in the 54 00:06:23 --> 00:06:30 biochemistry of the cell. So all cells have these two energy 55 00:06:30 --> 00:06:37 conversion processes in common. OK, so let's look at just 56 00:06:37 --> 00:06:44 summarizing what we're going to go over today. This is a summary of 57 00:06:44 --> 00:06:51 options for life. See also Freeman, 58 00:06:51 --> 00:06:58 Chapter 25. There is some discussion of this. 59 00:06:58 --> 00:07:03 And we can divide life here between what we call autotrophs. 60 00:07:03 --> 00:07:08 These are organisms that can make their own organic carbon. 61 00:07:08 --> 00:07:13 In other words, they can convert carbon dioxide to organic carbon. 62 00:07:13 --> 00:07:19 Heterotrophs are organisms that can only use organic carbon. 63 00:07:19 --> 00:07:24 They rely on the guts of other organisms in order to 64 00:07:24 --> 00:07:29 get through life. And so now we're going to 65 00:07:29 --> 00:07:35 systematically go through these processes that fall under each one 66 00:07:35 --> 00:07:40 of these. Oxygenic photosynthesis is the one we've been talking about 67 00:07:40 --> 00:07:45 last time and in my abbreviated version of life on earth. 68 00:07:45 --> 00:07:51 And this is carried out by eukaryotic organisms, 69 00:07:51 --> 00:07:56 plants, trees, etc., and also by prokaryotic organisms. 70 00:07:56 --> 00:08:02 Those are the cyanobacteria, microscopic photosynthetic plants. 71 00:08:02 --> 00:08:06 They use CO2 and sunlight. So our first variant on this theme 72 00:08:06 --> 00:08:10 we'll get into is a group of bacteria that do anoxygenic 73 00:08:10 --> 00:08:15 photosynthesis. Oxygenic means they evolve oxygen. 74 00:08:15 --> 00:08:19 These guys use solar energy but they don't evolve oxygen. 75 00:08:19 --> 00:08:24 And we'll get into how that works. And then there's a group of 76 00:08:24 --> 00:08:28 organisms that still use CO2. And in the very similar pathway the 77 00:08:28 --> 00:08:34 Calvin Cycle is photosynthesis. But they use chemical energy in 78 00:08:34 --> 00:08:42 order to make these intermediates to fix CO2. OK, so let's talk about 79 00:08:42 --> 00:08:50 those first. And so we're going to talk about the autotrophs. 80 00:08:50 --> 00:09:01 And all of them share this 81 00:09:01 --> 00:09:11 pathway, CO2 to C6H12. This would be glucose. 82 00:09:11 --> 00:09:21 And it takes ATP to run this reaction and it also takes 83 00:09:21 --> 00:09:34 reduced NADPH -- 84 00:09:34 --> 00:09:40 -- to run this reaction. It also takes this enzyme ribisco 85 00:09:40 --> 00:09:47 which you've talked about I'm sure, ribulose bisphosphate carboxylase. 86 00:09:47 --> 00:09:53 And this is the enzyme that initially takes the CO2 from the 87 00:09:53 --> 00:10:00 atmosphere and binds it to an organic carbon. 88 00:10:00 --> 00:10:04 Now, in a detailed version of this is what's called the Calvin Cycle or 89 00:10:04 --> 00:10:08 the Calvin/Benson Cycle. I don't know which one your book 90 00:10:08 --> 00:10:13 calls it. Calvin got the Nobel Prize but Benson was the graduate 91 00:10:13 --> 00:10:17 student that did all the work, so you should recognize that. 92 00:10:17 --> 00:10:22 Anyway, you studied this in great deal. But an interesting factoid is 93 00:10:22 --> 00:10:26 that ribisco is the most abundant protein on earth. 94 00:10:26 --> 00:10:30 That tells you how important this reaction is for sustaining 95 00:10:30 --> 00:10:36 life on earth. So notice that in order to drive 96 00:10:36 --> 00:10:43 this reaction, which is the Calvin Cycle, 97 00:10:43 --> 00:10:50 it requires energy and reducing power. So where do they get it? 98 00:10:50 --> 00:11:14 Well, there are three ways that 99 00:11:14 --> 00:11:20 autotrophs can get energy and reducing power to drive this 100 00:11:20 --> 00:11:27 reaction. And the first is oxygenic photosynthesis. And the 101 00:11:27 --> 00:11:34 second is anoxygenic. And the third is chemosynthesis. 102 00:11:34 --> 00:11:41 OK, those first three there. So now we're going to go through each 103 00:11:41 --> 00:11:49 of these and look at how they work remembering that all of them are 104 00:11:49 --> 00:11:56 generating ATP and NADPH in order to drive that. So all of the 105 00:11:56 --> 00:12:02 autotrophs have that in common. Well, oxygenic photosynthesis is the 106 00:12:02 --> 00:12:06 one that you know well already. You've studied it in great detail 107 00:12:06 --> 00:12:11 in biochemistry. So we're going to, 108 00:12:11 --> 00:12:15 again, give you the abbreviated version here just so you have a 109 00:12:15 --> 00:12:20 template to map these other ones onto. 110 00:12:20 --> 00:12:38 These are what are known as the 111 00:12:38 --> 00:12:45 light reactions of photosynthesis, the Z scheme taking solar energy, 112 00:12:45 --> 00:12:51 splitting water, evolving oxygen and synthesizing ATP 113 00:12:51 --> 00:12:58 and NADPH. This is all familiar, right? Very familiar. I'm just 114 00:12:58 --> 00:13:06 writing it in a cartoon version. OK, so this is the NADPH and ADP 115 00:13:06 --> 00:13:17 that goes to fuel that process. 116 00:13:17 --> 00:13:22 OK, so now, well, at least I can do it on that board. 117 00:13:22 --> 00:13:28 Let me do it on this board. Anoxygenic -- 118 00:13:28 --> 00:13:39 -- is almost exactly like this 119 00:13:39 --> 00:13:47 process, but instead of splitting water these guys oxidize hydrogen 120 00:13:47 --> 00:13:55 sulfide. So here's our ATP and NADPH. 121 00:13:55 --> 00:14:03 And they use sunlight to do this. 122 00:14:03 --> 00:14:09 So these are called photosynthetic bacteria. And they were around very 123 00:14:09 --> 00:14:15 early on the earth. Long before the earth's atmosphere 124 00:14:15 --> 00:14:21 was oxygenated these were the guys that were able to use solar energy 125 00:14:21 --> 00:14:27 and make organic carbon but without evolving oxygen. 126 00:14:27 --> 00:14:32 Then somewhere along the line some cell evolved, had some mutations and 127 00:14:32 --> 00:14:37 somehow figured out that water, this abundant source of water was a 128 00:14:37 --> 00:14:43 much better electron donor than hydrogen sulfide. 129 00:14:43 --> 00:14:48 And once the biochemistry figured this out, you can see the simple 130 00:14:48 --> 00:14:54 substitution here, the whole earth started going in a 131 00:14:54 --> 00:14:59 different direction. So this is an interesting example 132 00:14:59 --> 00:15:05 of how a small biochemical innovation can dramatically change 133 00:15:05 --> 00:15:10 the whole nature of the planet. Now, these guys are still around on 134 00:15:10 --> 00:15:14 earth. In fact, I'm going to show you some. 135 00:15:14 --> 00:15:19 I'll explain this at the end, but I have some captured in here. 136 00:15:19 --> 00:15:24 See that little purple band? Those are those guys. 137 00:15:24 --> 00:15:29 I've got other little tricks in here but I'll save those. 138 00:15:29 --> 00:15:34 Well, you cannot really see the purple band. But you can come up 139 00:15:34 --> 00:15:39 later and look at it. Those are photosynthetic bacteria. 140 00:15:39 --> 00:15:44 So they're still around on the earth but they're stuck in places 141 00:15:44 --> 00:15:49 where there's no oxygen. So they have a rather restricted 142 00:15:49 --> 00:15:54 niche on the planet now, but they're still extremely 143 00:15:54 --> 00:15:59 important. What did I do? Oh, here it is. 144 00:15:59 --> 00:16:03 So one of the places that they can be found, and if you're interested 145 00:16:03 --> 00:16:08 in them a great place to go find some is out at the Mystic Lakes in 146 00:16:08 --> 00:16:13 Arlington which is a permanently stratified lake so the bottom of the 147 00:16:13 --> 00:16:18 lake is always anaerobic. There's never oxygen there. 148 00:16:18 --> 00:16:23 In a typical lake like that you have a lot of mud on the bottom and 149 00:16:23 --> 00:16:28 you have a lot of hydrogen sulfide coming out of the mud from bacterial 150 00:16:28 --> 00:16:34 processes that we'll talk about. And you have light here. 151 00:16:34 --> 00:16:41 And so you have a gradient here of this is oxygen and this is H2S. 152 00:16:41 --> 00:16:49 And these photosynthetic bacteria have to life somewhere where there's 153 00:16:49 --> 00:16:56 enough light to photosynthesize and enough hydrogen sulfide to use in 154 00:16:56 --> 00:17:02 this part of the reaction. But they're very sensitive to oxygen 155 00:17:02 --> 00:17:06 so they cannot be in the oxygenated part of the lake. 156 00:17:06 --> 00:17:11 So you find them in a layer. It's called the squeeze. They have 157 00:17:11 --> 00:17:16 to have light so they have to be up, but they cannot have oxygen so they 158 00:17:16 --> 00:17:20 have to be down. And they need hydrogen sulfide so 159 00:17:20 --> 00:17:25 they have to be down. So they're layered in lakes. 160 00:17:25 --> 00:17:30 OK. So what about these guys, chemosynthesis? 161 00:17:30 --> 00:17:36 They don't rely on solar energy. Again, they're still driving the 162 00:17:36 --> 00:17:42 Calvin Cycle reducing CO2 from the air into organic carbon, 163 00:17:42 --> 00:17:48 but they're not using sunlight. So what do they do? They get their 164 00:17:48 --> 00:18:02 energy -- 165 00:18:02 --> 00:18:16 -- from redox reactions. And let's just show you an example. 166 00:18:16 --> 00:18:27 Redox reactions couple to the 167 00:18:27 --> 00:18:37 conversion of oxygen to H2O. So oxygen is involved in these 168 00:18:37 --> 00:18:47 reactions. And one organism, for example, can take ammonia and 169 00:18:47 --> 00:18:57 convert it to nitrite. Another type of organism can take 170 00:18:57 --> 00:19:07 nitrite and convert it to nitrate. And there are other organisms that 171 00:19:07 --> 00:19:17 can take hydrogen sulfide and convert it to sulfate. 172 00:19:17 --> 00:19:27 And some can take hydrogen sulfide, oh, no, take iron, ferrous iron, 173 00:19:27 --> 00:19:35 Fe2+ and convert it to Fe3+. So in all of these cases what is 174 00:19:35 --> 00:19:42 happening to these compounds? Are they being oxidized or reduced? 175 00:19:42 --> 00:19:49 I heard an oxidized. Yes, they're being oxidized. 176 00:19:49 --> 00:19:56 So these reduced compounds, relatively reduced compounds can be 177 00:19:56 --> 00:20:03 utilized by oxidizing them. The organism can release the energy 178 00:20:03 --> 00:20:10 that's needed. ATP is generated here. 179 00:20:10 --> 00:20:25 And NADPH is generated by any of 180 00:20:25 --> 00:20:31 these redox couples. So using this energy then the cell 181 00:20:31 --> 00:20:39 takes the reduced NADPH and the ATP and it runs the Calvin Cycle, 182 00:20:39 --> 00:20:46 chemosynthesis. OK. Now, you may think that these are kind of strange, 183 00:20:46 --> 00:20:54 weird bacteria that life in strange pockets of the earth where there's 184 00:20:54 --> 00:21:00 no oxygen. And who cares anyway? They're outdated. 185 00:21:00 --> 00:21:04 They dominated the earth way back in the early stages of the earth but 186 00:21:04 --> 00:21:09 they're not so important now. Well, that's not true. They're 187 00:21:09 --> 00:21:14 incredibly important. In some ecosystems they're the 188 00:21:14 --> 00:21:18 total base of the entire ecosystem. But also on a global scale, as 189 00:21:18 --> 00:21:23 you'll learn, you should have a feeling for this by the end of this 190 00:21:23 --> 00:21:28 lecture, but also when we talk about global biogeochemical cycles you 191 00:21:28 --> 00:21:32 will learn that these microbes are really messengers for electrons in 192 00:21:32 --> 00:21:37 the environment. Without them the redox balance of 193 00:21:37 --> 00:21:41 the earth would not be maintained, OK? You cannot have nothing but 194 00:21:41 --> 00:21:46 oxidizing reactions or nothing but reduction reactions and have a 195 00:21:46 --> 00:21:50 system sustain itself. So it's these microbes that are 196 00:21:50 --> 00:21:54 playing a really important role in maintaining the redox balance 197 00:21:54 --> 00:21:59 of the earth. OK. Now, one system that I'm going to 198 00:21:59 --> 00:22:04 show you in that DVD, that will do much better justice to 199 00:22:04 --> 00:22:09 it than my drawings here, that's a deep-sea volcano in case 200 00:22:09 --> 00:22:15 you didn't recognize it. And this is 2500 meters at the 201 00:22:15 --> 00:22:20 bottom of the ocean, very, very deep. And there is 202 00:22:20 --> 00:22:25 intense heat. I mean just think of a volcano on the surface 203 00:22:25 --> 00:22:31 of the earth. Intense heat and reduced compounds 204 00:22:31 --> 00:22:37 are found in the earth's mantle that are ready to erupt through this 205 00:22:37 --> 00:22:44 deep-sea volcano. And you have sulfate in the sea 206 00:22:44 --> 00:22:50 water that percolates through here. And as it percolates in and gets 207 00:22:50 --> 00:22:57 draw into the volcanic stuff that's coming out of here it's reduced to 208 00:22:57 --> 00:23:04 hydrogen sulfide coming out of the volcano. 209 00:23:04 --> 00:23:08 But you have oxygen in the water in the deep-sea. And we'll be talking 210 00:23:08 --> 00:23:12 about this when we talk about ocean circulation. But the oceans have a 211 00:23:12 --> 00:23:16 global ocean circulation where the surface water that's in equilibrium 212 00:23:16 --> 00:23:21 with the atmosphere actually sinks and travels along the bottom of the 213 00:23:21 --> 00:23:25 ocean. So there is oxygen in the bottom of the ocean, 214 00:23:25 --> 00:23:30 unlike many lakes where you don't have oxygen. 215 00:23:30 --> 00:23:37 And we'll talk about that difference. And in the hot vents the water 216 00:23:37 --> 00:23:44 coming out of here can be very, very hot, but there's a gradient 217 00:23:44 --> 00:23:51 right as it comes out meeting the colder sea water. 218 00:23:51 --> 00:23:58 And so what you have here is a perfect incubator for chemosynthetic 219 00:23:58 --> 00:24:07 bacteria -- 220 00:24:07 --> 00:24:12 -- that use the hydrogen sulfide in chemosynthesis to fix carbon dioxide 221 00:24:12 --> 00:24:18 using the oxygen here. And that forms the base of the 222 00:24:18 --> 00:24:23 entire food web in the deep ocean because there's no light down there. 223 00:24:23 --> 00:24:29 There's no photosynthesis. There's only chemosynthesis. 224 00:24:29 --> 00:24:35 And just a little story that goes back to when I first came to MIT as 225 00:24:35 --> 00:24:41 an assistant professor in 1976. You weren't even born. But when I 226 00:24:41 --> 00:24:47 was young we used to go the Muddy Charles Pub periodically after work 227 00:24:47 --> 00:24:53 and have beers. And there was a professor, 228 00:24:53 --> 00:24:59 in this department actually, John Edmond, who passed away several 229 00:24:59 --> 00:25:05 years ago but who used to be there. It was sort of like our Cheers. 230 00:25:05 --> 00:25:11 And I'll never forget the day he came back from a cruise. 231 00:25:11 --> 00:25:17 He came to the pub. He was a chemist and I'm a biologist. 232 00:25:17 --> 00:25:23 And he said you will not believe what we found on the bottom of the 233 00:25:23 --> 00:25:29 ocean. He had gone down in Alvin, this two-person submersible vehicle. 234 00:25:29 --> 00:25:33 And he started talking about these giant clams and these giant tube 235 00:25:33 --> 00:25:38 worms and all of these things, and I thought he had had one too 236 00:25:38 --> 00:25:43 many beers. I found it hard to believe. Well, 237 00:25:43 --> 00:25:48 it turned out that that was the first discovery of these deep-sea 238 00:25:48 --> 00:25:53 vents and he was on that expedition. And through that collegial 239 00:25:53 --> 00:25:58 relationship I actually ended up with one of the clam shells from the 240 00:25:58 --> 00:26:03 clams there, which is one of the giant clams. 241 00:26:03 --> 00:26:08 Their meat is blood red because they have a special kind of hemoglobin 242 00:26:08 --> 00:26:13 that they use to keep the oxygen tension perfect for these 243 00:26:13 --> 00:26:18 chemosynthetic bacteria. If the oxygen is too high they 244 00:26:18 --> 00:26:23 cannot do this because it will spontaneously oxidize the H2S. 245 00:26:23 --> 00:26:29 So the oxygen tension is very critical. 246 00:26:29 --> 00:26:32 And they have a special kind of hemoglobin that does that. 247 00:26:32 --> 00:26:36 So these claims had symbiotic chemosynthetic bacteria. 248 00:26:36 --> 00:26:40 Well, since then these vents have been discovered everywhere and 249 00:26:40 --> 00:26:44 ecosystems similar have been discovered on the surface. 250 00:26:44 --> 00:26:48 And there are all kinds of different vents. 251 00:26:48 --> 00:26:52 You're going to learn about not only hydrothermal vents, 252 00:26:52 --> 00:26:56 hot vents in this video, but also cold seeps they're called where you 253 00:26:56 --> 00:27:00 have methane bacteria that are really important. OK. 254 00:27:00 --> 00:27:05 So these are the main ways in which organisms can get energy to convert 255 00:27:05 --> 00:27:11 CO2 to organic carbon. Then you have all these 256 00:27:11 --> 00:27:17 heterotrophs, the ones that use the organic carbon, 257 00:27:17 --> 00:27:23 and they have various ways of doing that. You've learned in 258 00:27:23 --> 00:27:29 biochemistry the primary way, which is very powerful, and that is 259 00:27:29 --> 00:27:35 using aerobic respiration to do that. 260 00:27:35 --> 00:27:44 And so we are just going to abbreviate that here. 261 00:27:44 --> 00:27:54 That's our reverse of photosynthesis. So heterotrophs. 262 00:27:54 --> 00:28:03 So we have first aerobic. 263 00:28:03 --> 00:28:13 And let me jump ahead 264 00:28:13 --> 00:28:25 with the slides. 265 00:28:25 --> 00:28:32 OK, there you are. So this is a cartoon version of 266 00:28:32 --> 00:28:40 aerobic respiration. So we'll just put glucose, 267 00:28:40 --> 00:28:47 we'll come down to the Krebs' Cycle. And we are going to let electrons 268 00:28:47 --> 00:28:55 flow here and have oxygen be the final electron acceptor 269 00:28:55 --> 00:29:02 creating water. So we've really just accomplished 270 00:29:02 --> 00:29:09 the absolute reverse of photosynthesis and we've made NADH 271 00:29:09 --> 00:29:16 in doing this and we've made ATP. So these guys are getting the 272 00:29:16 --> 00:29:22 energy out of the glucose that all of the other organisms made. 273 00:29:22 --> 00:29:29 And oxygen is the terminal electron acceptor when there's 274 00:29:29 --> 00:29:35 oxygen around. But there are lots of environments, 275 00:29:35 --> 00:29:41 as we've talked about on earth, where there isn't oxygen. 276 00:29:41 --> 00:29:46 And there are bacteria that can take advantage of those environments. 277 00:29:46 --> 00:29:52 And instead of having oxygen be the terminal electron acceptor there are 278 00:29:52 --> 00:29:57 a number of other elements that they can use, compounds that they can use. 279 00:29:57 --> 00:30:03 For example, there are some that use nitrate and they reduce 280 00:30:03 --> 00:30:10 it to nitrous oxide. N2. Ammonia. All the relatively 281 00:30:10 --> 00:30:19 reduced forms of nitrogen. And so this called anaerobic. 282 00:30:19 --> 00:30:29 And this process is called 283 00:30:29 --> 00:30:36 gentrification. And if it weren't for these bacteria, 284 00:30:36 --> 00:30:40 these anaerobic bacteria that can reduce nitrate, 285 00:30:40 --> 00:30:44 nitrogen would never return to the atmosphere. Remember last time we 286 00:30:44 --> 00:30:48 talked about nitrogen fixation, how specific types of microbes can 287 00:30:48 --> 00:30:52 take N2 from the atmosphere and pull it into the ecosystem? 288 00:30:52 --> 00:30:56 Well, if you didn't have these bacteria doing this process that 289 00:30:56 --> 00:31:00 nitrogen would never get back to the atmosphere. 290 00:31:00 --> 00:31:04 They're central to closing the nitrogen cycle. 291 00:31:04 --> 00:31:08 Then there are some that can use sulfate and reduce it to hydrogen 292 00:31:08 --> 00:31:13 sulfide. As you can imagine, these are critical to creating the 293 00:31:13 --> 00:31:17 hydrogen sulfide that's used in these other processes. 294 00:31:17 --> 00:31:22 There are some that use CO2 and convert to methane. 295 00:31:22 --> 00:31:26 These are methanogenic bacteria, and they're incredibly important in 296 00:31:26 --> 00:31:31 the global carbon cycle and in the methane cycle. 297 00:31:31 --> 00:31:36 Methane is a really powerful greenhouse gas, 298 00:31:36 --> 00:31:42 and we're going to talk about that later. And then there are some that 299 00:31:42 --> 00:31:48 can take Fe3+ and reduce it to Fe2+. And the same for manganese. 300 00:31:48 --> 00:31:55 So you should be starting to sense a 301 00:31:55 --> 00:32:00 sort of symmetry here, right, that these anaerobic bacteria 302 00:32:00 --> 00:32:05 are fulfilling functions on the earth. Let me write these down. 303 00:32:05 --> 00:32:17 These are sulfate reducers, these are methanogens, and these are 304 00:32:17 --> 00:32:30 iron reducers and manganese reducers. 305 00:32:30 --> 00:32:37 So these will all become extremely important when we talk about the 306 00:32:37 --> 00:32:44 global biogeochemical cycles of all of these elements. 307 00:32:44 --> 00:32:51 It's these microbes that make sure that the cycles can continue and 308 00:32:51 --> 00:32:58 don't run into a dead end of oxidation or reduction. 309 00:32:58 --> 00:33:05 OK. Before we go to the movie, I just want to say if you look at 310 00:33:05 --> 00:33:12 Table 25.2 in your textbook, I think it's that one. 311 00:33:12 --> 00:33:18 I'm assuming I'm using the most recent version. 312 00:33:18 --> 00:33:24 You'll see a variation of this theme in which there will be some 313 00:33:24 --> 00:33:30 entries of organisms that don't fall into these categories that 314 00:33:30 --> 00:33:36 I've just shown you. And that is to say that there are 315 00:33:36 --> 00:33:41 organisms that use light energy and organic carbon energy at the same 316 00:33:41 --> 00:33:46 time. For every variation that's possible there's an organism that's 317 00:33:46 --> 00:33:51 evolved to take advantage of it. I've just oversimplified it here, 318 00:33:51 --> 00:33:56 but you should know that. And the bottom line is if it's 319 00:33:56 --> 00:34:01 thermodynamically possible. And, again, this whole lecture could 320 00:34:01 --> 00:34:06 have been done in a thermodynamic mode. We could have looked at which 321 00:34:06 --> 00:34:12 redox couples were energetically possible and then assigned those to 322 00:34:12 --> 00:34:17 particular microbes. But for now I just want you to get 323 00:34:17 --> 00:34:22 the overview. But for anything that's thermodynamically feasible 324 00:34:22 --> 00:34:28 there's a microbe out there that's doing it. 325 00:34:28 --> 00:34:32 And, in fact, microbiologists actually comb through redox tables 326 00:34:32 --> 00:34:37 and put together different redox couples and hypothesize. 327 00:34:37 --> 00:34:41 I ought to be able to find an organism that does this in that 328 00:34:41 --> 00:34:46 environment. And then they go out. And they can almost always actually 329 00:34:46 --> 00:34:50 find it. So they're incredibly versatile. And it gives you a 330 00:34:50 --> 00:34:55 really good strong feeling for the power of thermodynamics in driving 331 00:34:55 --> 00:35:00 the evolution of these biochemical processes. 332 00:35:00 --> 00:35:07 Finally, before we show you the movie I want to show you what this 333 00:35:07 --> 00:35:15 thing is all about. There was a Russian microbiologist 334 00:35:15 --> 00:35:23 back in the previous century named Winogradsky -- 335 00:35:23 --> 00:35:33 -- who wanted to isolate some of 336 00:35:33 --> 00:35:39 these photosynthetic bacteria. And knowing what their 337 00:35:39 --> 00:35:45 characteristics were he went out and got himself some mud and some pond 338 00:35:45 --> 00:35:52 water. And he set up what we've come to call a Winogradsky column. 339 00:35:52 --> 00:35:58 This is a Winogradsky juice bottle, but it works the same. And what you 340 00:35:58 --> 00:36:05 do is you put mud in the bottom and you put pond water here. 341 00:36:05 --> 00:36:09 And the pond water has basically an inoculum. It has representatives of 342 00:36:09 --> 00:36:13 all different types of bacteria. They might be spores. If they 343 00:36:13 --> 00:36:17 don't like the environment they're in they sporulates and then they 344 00:36:17 --> 00:36:21 just don't germinate. But presumably in pond water you 345 00:36:21 --> 00:36:25 have everything that could possibly grow in here. And in the mud you 346 00:36:25 --> 00:36:29 add a source of sulfate. And so you might add calcium 347 00:36:29 --> 00:36:33 sulfate and you might add a little organic matter, 348 00:36:33 --> 00:36:38 you know, plant parts or something just to jumpstart it. 349 00:36:38 --> 00:36:45 And eventually you set up a gradient here of hydrogen sulfide and oxygen. 350 00:36:45 --> 00:36:53 And over time the organisms grow along that gradient. 351 00:36:53 --> 00:37:01 So you'll end up down here with the anaerobic respiration. 352 00:37:01 --> 00:37:08 In fact, the organisms generate this 353 00:37:08 --> 00:37:12 gradient. When you start out the whole thing is oxygenated. 354 00:37:12 --> 00:37:16 And what you should think about in this context is what happens. 355 00:37:16 --> 00:37:20 How do these gradients get generated when you start out with a 356 00:37:20 --> 00:37:24 completely mixed system, everything in there, everything 357 00:37:24 --> 00:37:29 oxygenated? Eventually you have anaerobic -- 358 00:37:29 --> 00:37:33 First you'll just have anaerobic respiration, right? 359 00:37:33 --> 00:37:38 Anything that can use organic carbon and oxygen is going to go 360 00:37:38 --> 00:37:42 like mad, and that's what's going to draw the oxygen down. 361 00:37:42 --> 00:37:47 Then you'll have anaerobic respiration here. 362 00:37:47 --> 00:37:51 You'll have photosynthesis up here, evolving oxygen. You'll have 363 00:37:51 --> 00:37:56 chemosynthetic bacteria here because they need a little bit of oxygen but 364 00:37:56 --> 00:38:00 they also need some of this hydrogen sulfide and photosynthetic 365 00:38:00 --> 00:38:07 bacteria here. 366 00:38:07 --> 00:38:11 Well, they're like down here. Because they need light but cannot 367 00:38:11 --> 00:38:16 have oxygen. And so you can set these up. And this purple band here 368 00:38:16 --> 00:38:19 tells you that you've got your photosynthetic bacteria.