Marine Microbe Celebrates 20th Anniversary

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About 20 years ago, researchers discovered Prochlorococcus, a photosynthetic marine microorganism. Penny Chisholm, a biologist at MIT whose work led to the discovery of the microbe, talks about its role in producing 20 percent of the oxygen in the Earth's air.

JOE PALCA, host:

From NPR News, this is Talk of The Nation: Science Friday. I'm Joe Palca. Imagine an organism that's so abundant that it's responsible for 50 percent of the photosynthesis in the ocean, and what if that same organism played a huge role in the ocean ecosystem, affecting everything from food for larger marine life, to the workings of climate change?

Well, there is such an organism, and it's called Prochlorococcus, and you can practice saying that later, because it's hard to say Prochlorococcus. But what's really amazing about this tiny marine bacterium, is that 20 years ago, it was completely unknown. Penny Chisholm wasn't looking for Prochlorococcus when she went on a cruise from Cape Cod to Bermuda two decades ago, but she and Rob Olson were the first to find this microorganism. Penny Chisholm is the Lee and Geraldine Martin Professor of Environmental Studies in the Department of Civil and Environmental Engineering at the Department of Biology at MIT. And she joins me from a studio at the MIT campus. Welcome.

Dr. PENNY CHISHOLM ( Lee and Geraldine Martin Professor of Environmental Studies, Department of Civil and Environmental Engineering, Department of Biology, MIT): Thank you. Thank you for having me.

PALCA: Oh, it's a pleasure, and if you want to ask questions about Prochlorococcus, and I hope you do, because it's a fascinating little critter. Give us a call, the number is 800-989-8255. That's 1-800-989-TALK. So, Penny Chisholm, what makes Prochlorococcus so special? Why are we excited about this organism?

Dr. CHISHOLM: Well, it's filled with superlatives.

(Soundbite of laughter)

Dr. CHISHOLM: It's the smallest and the most abundant photosynthetic cell on the planet. I mean, it's one of man - thousands of different species of phytoplankton in the oceans. And those are the small microscopic plants that are the base of the food chain in the oceans, but because it's so small and incredibly abundant and seems to be very versatile in the regions that it can occupy, and because it's a so recently discovered, it gets - it's starting to get a fair amount of attention.

PALCA: Mm, mm. And how did you discover it? I mean, it's a fascinating story. It wasn't something that you were saying, oh, let's go out and discover an incredibly unknown organism at this point. What do you want to do today? I don't know, let's go out and discover something.

(Soundbite of laughter)

Dr. CHISHOLM: No - yeah, indeed that wasn't how it happened. And in fact, Rob Olson gets the lion share of his credit for this discovery. I always say, Rob discovered it. I popularized it.

PALCA: Got it.

Dr. CHISHOLM: But, we were working with an instrument that was designed for biomedical research, called the Flow Cytometer, and we were using that to study cell-cycle progression in phytoplankton - in phytoplankton cells. And after we worked with that for a while, we realized that it was ideally suited for studying plankton at sea, because they have a fluorescence signal that's natural.

They autofluoresce. So, we managed to get one of these instruments, and Rob had the courage to put it on a ship. It's meant to be in a pristine laboratory in a hospital. And we started studying a cousin of Prochlorococcus, which is called Synecococcus. Equally hard to say.

PALCA: Uh-huh.

Dr. CHISHOLM: And that was pretty clear. It fluoresced red - I mean, orange, and was bigger, and so it was when way we were studying that, that these other tiny cells started to become apparent, and Rob noticed them coming out of the noise of the instrument.

PALCAL: Mm. I should remind people, or tell them that if you go to our website, sciencefriday.com, you can actually see some micro - some images of the phytoplankton, and if you're curious, you can see images of Penny Chisholm on our website. Although they look a little different. You'll have to - I hope you can tell one from the other. I want to...

Dr. CHISHOLM: The plankton are handsomer.

PALCA: Aww, don't say that. Anyway, we have a bunch of people who have questions. So why don't we go ahead and take a call now from, is it Eric, in Modesto. Eric, welcome to Science Friday.

ERIC (Caller): Hi. How are you guys doing today?

PALCA: Good.

ERIC: I just saw a report on (unintelligible) not too long ago, where a gentleman was refining pond scum, believe it or not, into usable fuel. Is this a growth that you guys have found, possibly able to be used for fuel in the near future? And I'll take my answer off the air.

PALCA: OK. Thanks, Eric. Penny, are we going to see Prochlorococcus power plants? Boy, that will be a hard sentence to say.

Dr. CHISHOLM: Yeah, that would. Probably not directly. But we think that studying Prochlorococcus, and how it works, could provide a interesting chassis for building synthetic, photosynthetic systems, because it con - it's the smallest amount of information that can convert solar energy into biomass.

It only has 2,000 genes and with that, it can make living biomass. But, it would not be the organism you would use directly to make fuels. It's finicky and it's growth. It grows slowly, and would be very difficult to harvest.

PALCA: You know that's one - you were mentioning the 2,000 genes, and one of the things that I find so interesting about Prochlorococcus, is the research that you've done has shown that of those 2,000, only about 1200 are shared by every Prochlorococci, and then the rest are sort of different, and it's hard to think of a species that, you know, only has, what is it, 60 percent of their genes shared and then everything else is different.

Dr. CHISHOLM: Yeah, that's in fact one of the things we think is the key to the - to how abundant Prochlorococcus is and how wide spread it is, because it is - it has this core of shared genes that makes it a Prochlorococcus, but these other genes are what make each different, what we call - we call them ecotypes(ph). And those genes are what make them slightly different.

So that there's this great diversity of Prochlorococcus out there, and so when the environment changes, one of the ecotypes gains a slight advantage over the others, and then if it changes another, another one gains an advantage, and that contributes to the overall stability of the group.

PALCA: But now, maybe I'm - is Prochlorococcus a species, or a genus, or how should we think about this?

Dr. CHISHOLM: Oh, that question. Ah.

PALCA: Mm-hmm.

Dr. CHISHOLM: I - in my lab, we've dispensed with the idea of bacterial species. And we think of them as - we think of them all as genomic variants. But, in the - traditionally, yes, Prochlorococcus as a collective, would have been considered a single species. And now, we know that within that group, whatever you name it, there are significant variants.

PALCA: Hm. It's just going to take a different way of thinking about things, I guess. But maybe the microbiologist are ahead of us on this one.

Dr. CHISHOLM: Yes, we are starting to think about it completely differently, than the old days.

PALCA: Cool. Let's take another call and go to Alvin (ph) in McLean, Virginia. Alvin, welcome to Science Friday.

ALVIN (Caller): Hi. I just had a question about the Prochlorovecaccus(ph) bacteria.

PALCA: Uh-huh.

ALVIN: Can we somewhat utilize it to reduce the greenhouse gas in the air right now?

PALCA: Reduce greenhouse gas. Interesting question, Alvin. What about that, Penny Chisholm.

Dr. CHISHOLM: Well, it is, as we speak doing it's job out in the oceans, because as it photosynthesizes, it draws CO2 out of the atmosphere, and plays a role in what we call the biological pump in the oceans, which is a steady moving of CO2 from the atmosphere, through the food web, and then small amounts of that carbon settles to the deep ocean.

And that's all part of the global-ocean-carbon cycle. And just to give you an example of how important this pump is, and all the phytoplankton are involved in that, not just Prochlorococcus, but if the oceans were dead, the carbon - and there was no photosynthesis, the carbon dioxide from the deep ocean would equilibrate with the atmosphere, and the atmosphere would have maybe double the CO2 it has now. So, that food web, that Prochloroca - Prochlorococcus is an integral part of, is playing a really important role maintaining the CO2 concentrations in the atmosphere, where they are.

PALCA: So, I just - just getting to definitions here for a second. Somebody earl - we had earlier call about pond scum, and you've been talking about phytoplankton. Are they the same thing or are these two different things?

Dr. CHISHOLM: Pond scum is a derogatory term for phytoplankton. It's when...

PALCA: Oh, sorry. I apologize.

Dr. CHISHOLM: Yes. Please.

PALCA: We don't mean to cast aspersions here.

Dr. CHISHOLM: Does it - Yeah. Does - I bet. The same cells that become pond scum are really vital to the food web of that pond, and it becomes scum when a pond is enriched with nutrients by humans. Most of the time, that's the reason. But, yes. What we referr to as pond scum, is often relatives of Prochlorococcus.

PALCA: I got it. All right, let's take another call now, and go to Mark (ph) in Windsor, California. Mark, welcome to Science Friday.

MARK (Caller): Oh, hello. Thank you.

PALCA: You bet.

MARK: I - when you said the organism lit up red, it reminded me of another organism called serratia marcescens and I - and at one time that organism was used to - as a microorganism, because of its red color, and it turned out to be a a pathogen over so many years. Is there any possibility that this organism could be a pathogen in humans?

Dr. CHISHOLM: I think not. That - well, I think not. It fluoresces red, because it contains chlorophyll, which - so it fluoresces red, when you shine blue light on it, and that's from the chlorophyll. So, we never - we've never even thought about that question, but I would be - I would drink Prochlorococcus without worrying about it.

MARK (Caller): OK, thank you.

PALCA: But Mark, is that - Mark, can I ask you, is that other compound you were talking about, is that what sometimes is referred to as red tide, or is that something else?

Mark: No, no, it was an organism. It's a bacterial organism that was used by the Army many years ago as a marker organism. It's also used in Dentistry to - because it was red, it would - you could use it in a procedure, and it was thought to be non-pathogen, so you could put it on a tooth for example, operate on it, and then later look for the organism, see if that operation caused, you know, the organism to invade the tissue, and certainly it did, and after, you know, several years that we found out it was a pathogen. So, you know, I'd be careful if it was an organism that could grow in the blood stream.

PALCA: Ah, interesting.

Dr. CHISHOLM: Ah.

MARK: That's all I'm saying.

PALCA: Oh, well, well maybe something to pay attention too.

Dr. CHISHOLM: Yeah.

PALCA: Mark, thanks very much for the call. You know it does make me - I've heard people say that, you know, you could get a mouthful of this. Certainly, people have been - have swam in waters where Prochlorocos - Prochlorococcus is growing, and you don't hear reports about Prochlorococcus poisoning. (unintelligible), it's another sentence I shouldn't be having to say on the radio.

PALCA: So, maybe it's not a terrible pathogen, even if it's pathogenic to some people. Let's take another ca - go ahead.

Dr. CHISHOLM: No, I was just going to say, I'm fairly certain that it's not.

PALCA: OK, well, let's take another call now, and go to Michael. In - is it McCollumly(ph), California?

Michael: Yes, McCollumly Hill.

PALCA: OK.

MICHAEL (Caller): Yeah, I'm just questioning every organism has to eat something to survive. I know there's photosynthesis involved here, but what does this organism eat, and what eats it, and where does it stand on the food chain?

PALCA: Good question, Michael. Thanks. What does Prochlorococcus eat?

Dr. CHISHOLM: Ah, excellent question. It is as far as we know an obligate phototroph, which means it uses sun's energy and carbon dioxide to photosynthesize, and make its own organic carbon. So it doesn't eat any other organisms. But what eats it, is a big research challenge for the field.

We think microscopic protists - well, we know microscopic protists which are just larger single-celled microbes in the oceans, do eat Prochlorococcus, but we don't know particularly what kinds or how complicated that web is. The other thing that's really important in this food web are viruses. We study viruses that infect Prochlorococcus, and that's a source of mortality for them.

They're floating around the oceans, but they also carry genes from Prochlorococcus, and they move them around from one cell to another, and creating adversity that - and we think that's a really important process in maintaining the complexity, and stability, and diversity of the Prochlorococcus system.

PALCA: We're talking with Penny Chisholm, she's in the Department of Civil Environmental Engineering and the Department of Biology at MIT. And she's one of the people who have disc - who has discovered and popularized this tiny organism called Prochlorococcus. I'm Joe Palca and this is Talk of the Nation from NPR News.

And why don't we take another call now, and go to Richard in Springfield, Pennsylvania? Richard, welcome to Science Friday.

RICHARD (Caller): Hello. Yes, I'd - short question. Is Prochlorococcus...

PALCA: Very good.

RICHARD: Sensitive to temperature and does it appear all over the earth?

Dr. CHISHOLM: Oh, excellent question. The different ecotypes that I've referred to, or you can think of them as different strands, have different op - temperature optima. And so there are some that are dominant in the warm equatorial waters, and then there are some that take over as you go more pole ward.

And - but finally, you reach a point at about 40 degrees latitude, that they don't - they disappear. So, Prochlorococcus does not grow in the Polar regions. But their close cousin, Synecococcus, does and that's kind of a puzzle.

RICHARD: Interesting.

PALCA: Strange. Well, thanks for that question, Richard. You know, I'm wondering, you said that Prochlorococcus is beginning to get the respect that you think it deserves. Is it really still unknown or is it, I mean, is it now an established part of the ecosystem, and people are studying it just like they study anything else? Or is it neglect - is it a neglected marine microorganism?

Dr. CHISHOLM: Well, it's not neglected within our community...

PALCA: Uh-huh.

Dr. CHISHOLM: But I think all microorganisms are neglected. And we - in fact, we ca - we call - we sometimes refer to the phytoplankton in the oceans as the invisible forest, because they photosynthesize as much as all of the plants on land and yet, you know, we look at the ocean and we don't even think about that, and how profound that is.

So, microbes are really ruling this planet, and that's one of the reasons I've become an advocate of Prochlorcoccus, because it is something that's easy to advocate, because of all its superlatives and because of its global abundance. But all microbes, whether they're abundant or not, are playing a really important role in regulating our climate, and our atmosphere, and our soils.

And so, so I think they deserve a lot more attention. And in fact, one of the reasons that we've been able to make the advances we've made in the last four, five years, is because of accelerated funding from the Gordon and Betty Moore Foundation, who - and that foundation decided that microbes were under appreciated, and has - is funding a lot of basic research in marine microbiology.

PALCA: I got it. So, what is next? I mean, we only have a few seconds left. But is there a particular - if you had one question to ask about Prochlorcoccus you'd like the answer to, what would you ask?

Dr. CHISHOLM: The frontiers are, who's eating you? What role are the viruses actually playing?

PALCA: Ooh, viruses. Have we talked about those yet?

Dr. CHISHOLM: Little bit.

PALCA: OK.

Dr. CHISHOLM: But, they're really important.

PALCA: OK.

Dr. CHISHOLM: And I think that they're actually - even though they kill Prochlorococcus cells, they're very important in maintaining the diversity of the global population. So, those are two big questions. And also, how Prochlorcoccus interacts with all of the other bacteria out there that are growing on the products of Prochlorococcus' photosynthesis.

So that's where their organic carbon comes from, and we know that there are probably synergies.

PALCA: OK. Penny Chisholm, we have to leave it right there. I apologize. Penny Chisholm is in the Department of Biology at MIT and thanks for talking to me today.

Dr. CHISHOLM: Thank you.

PALCA: OK, when we come back, some people can't carry a tune and why some scientists care about them? Stay with us.

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The Most Important Microbe You've Never Heard Of

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'Science Friday' Video

By some estimates, the oxygen in one out of every five breaths you take comes from a bacterium called Prochlorococcus. Numbering in the trillion trillions, this tiny microbe is one of the most abundant organisms on Earth.

So when Prochlorococcus perform photosynthesis — using energy from the sun to release oxygen from a water molecule — that adds up to a lot of oxygen.

And yet this mass producer of an essential ingredient for life was unheard of until 20 years ago. Recently, scientists got together in Cambridge, Mass., to celebrate the discovery of the bacterium.

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