Penny Chisholm
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I'd like to introduce you to a tiny microorganism that you've probably never heard of: its name is Prochlorococcus, and it's really an amazing little being.

For one thing, its ancestors changed the earth in ways that made it possible for us to evolve, and hidden in its genetic code is a blueprint that may inspire ways to reduce our dependency on fossil fuel. But the most amazing thing is that there are three billion billion billion of these tiny cells on the planet, and we didn't know they existed until 35 years ago.

So to tell you their story, I need to first take you way back, four billion years ago, when the earth might have looked something like this. There was no life on the planet, there was no oxygen in the atmosphere. So what happened to change that planet into the one we enjoy today, teeming with life, teeming with plants and animals?

Well, in a word, photosynthesis. About two and a half billion years ago, some of these ancient ancestors of Prochlorococcus evolved so that they could use solar energy and absorb it and split water into its component parts of oxygen and hydrogen. And they used the chemical energy produced to draw CO2, carbon dioxide, out of the atmosphere and use it to build sugars and proteins and amino acids, all the things that life is made of. And as they evolved and grew more and more over millions and millions of years, that oxygen accumulated in the atmosphere. Until about 500 million years ago, there was enough in the atmosphere that larger organisms could evolve. There was an explosion of life-forms, and, ultimately, we appeared on the scene. While that was going on, some of those ancient photosynthesizers died and were compressed and buried, and became fossil fuel with sunlight buried in their carbon bonds. They're basically buried sunlight in the form of coal and oil. Today's photosynthesizers, their engines are descended from those ancient microbes, and they feed basically all of life on earth. Your heart is beating using the solar energy that some plant processed for you, and the stuff your body is made out of is made out of CO2 that some plant processed for you. Basically, we're all made out of sunlight and carbon dioxide. Fundamentally, we're just hot air.


So as terrestrial beings, we're very familiar with the plants on land: the trees, the grasses, the pastures, the crops. But the oceans are filled with billions of tons of animals. Do you ever wonder what's feeding them? Well there's an invisible pasture of microscopic photosynthesizers called phytoplankton that fill the upper 200 meters of the ocean, and they feed the entire open ocean ecosystem. Some of the animals live among them and eat them, and others swim up to feed on them at night, while others sit in the deep and wait for them to die and settle down and then they chow down on them.

So these tiny phytoplankton, collectively, weigh less than one percent of all the plants on land, but annually they photosynthesize as much as all of the plants on land, including the Amazon rainforest that we consider the lungs of the planet. Every year, they fix 50 billion tons of carbon in the form of carbon dioxide into their bodies that feeds the ocean ecosystem. How does this tiny amount of biomass produce as much as all the plants on land? Well, they don't have trunks and stems and flowers and fruits and all that to maintain. All they have to do is grow and divide and grow and divide. They're really lean little photosynthesis machines. They really crank.

So there are thousands of different species of phytoplankton, come in all different shapes and sizes, all roughly less than the width of a human hair. Here, I'm showing you some of the more beautiful ones, the textbook versions. I call them the charismatic species of phytoplankton.

And here is Prochlorococcus. I know, it just looks like a bunch of schmutz on a microscope slide.


But they're in there, and I'm going to reveal them to you in a minute. But first I want to tell you how they were discovered.

About 38 years ago, we were playing around with a technology in my lab called flow cytometry that was developed for biomedical research for studying cells like cancer cells, but it turns out we were using it for this off-label purpose which was to study phytoplankton, and it was beautifully suited to do that. And here's how it works: so you inject a sample in this tiny little capillary tube, and the cells go single file by a laser, and as they do, they scatter light according to their size and they emit light according to whatever pigments they might have, whether they're natural or whether you stain them. And the chlorophyl of phytoplankton, which is green, emits red light when you shine blue light on it. And so we used this instrument for several years to study our phytoplankton cultures, species like those charismatic ones that I showed you, just studying their basic cell biology. But all that time, we thought, well wouldn't it be really cool if we could take an instrument like this out on a ship and just squirt seawater through it and see what all those diversity of phytoplankton would look like. So I managed to get my hands on what we call a big rig in flow cytometry, a large, powerful laser with a money-back guarantee from the company that if it didn't work on a ship, they would take it back. And so a young scientist that I was working with at the time, Rob Olson, was able to take this thing apart, put it on a ship, put it back together and take it off to sea. And it worked like a charm. We didn't think it would, because we thought the ship's vibrations would get in the way of the focusing of the laser, but it really worked like a charm. And so we mapped the phytoplankton distributions across the ocean. For the first time, you could look at them one cell at a time in real time and see what was going on — that was very exciting. But one day, Rob noticed some faint signals coming out of the instrument that we dismissed as electronic noise for probably a year before we realized that it wasn't really behaving like noise. It had some regular patterns to it. To make a long story short, it was tiny, tiny little cells, less than one-one hundredth the width of a human hair that contain chlorophyl. That was Prochlorococcus.

So remember this slide that I showed you? If you shine blue light on that same sample, this is what you see: two tiny little red light-emitting cells. Those are Prochlorococcus. They are the smallest and most abundant photosynthetic cell on the planet. At first, we didn't know what they were, so we called the "little greens." It was a very affectionate name for them. Ultimately, we knew enough about them to give them the name Prochlorococcus, which means "primitive green berry."

And it was about that time that I became so smitten by these little cells that I redirected my entire lab to study them and nothing else, and my loyalty to them has really paid off. They've given me a tremendous amount, including bringing me here.


So over the years, we and others, many others, have studied Prochlorococcus across the oceans and found that they're very abundant over wide, wide ranges in the open ocean ecosystem. They're particularly abundant in what are called the open ocean gyres. These are sometimes referred to as the deserts of the oceans, but they're not deserts at all. Their deep blue water is teeming with a hundred million Prochlorococcus cells per liter. If you crowd them together like we do in our cultures, you can see their beautiful green chlorophyl. One of those test tubes has a billion Prochlorococcus in it, and as I told you earlier, there are three billion billion billion of them on the planet. That's three octillion, if you care to convert.


And collectively, they weigh more than the human population and they photosynthesize as much as all of the crops on land. They're incredibly important in the global ocean. So over the years, as we were studying them and found how abundant they were, we thought, hmm, this is really strange. How can a single species be so abundant across so many different habitats? And as we isolated more into culture, we learned that they are different ecotypes. There are some that are adapted to the high-light intensities in the surface water, and there are some that are adapted to the low light in the deep ocean. In fact, those cells that live in the bottom of the sunlit zone are the most efficient photosynthesizers of any known cell. And then we learned that there are some strains that grow optimally along the equator, where there are higher temperatures, and some that do better at the cooler temperatures as you go north and south.

So as we studied these more and more and kept finding more and more diversity, we thought, oh my God, how diverse are these things? And about that time, it became possible to sequence their genomes and really look under the hood and look at their genetic makeup. And we've been able to sequence the genomes of cultures that we have, but also recently, using flow cytometry, we can isolate individual cells from the wild and sequence their individual genomes, and now we've sequenced hundreds of Prochlorococcus. And although each cell has roughly 2,000 genes — that's one tenth the size of the human genome — as you sequence more and more, you find that they only have a thousand of those in common and the other thousand for each individual strain is drawn from an enormous gene pool, and it reflects the particular environment that the cell might have thrived in, not just high or low light or high or low temperature, but whether there are nutrients that limit them like nitrogen, phosphorus or iron. It reflects the habitat that they come from.

Think of it this way. If each cell is a smartphone and the apps are the genes, when you get your smartphone, it comes with these built-in apps. Those are the ones that you can't delete if you're an iPhone person. You press on them and they don't jiggle and they don't have x's. Even if you don't want them, you can't get rid of them.


Those are like the core genes of Prochlorococcus. They're the essence of the phone. But you have a huge pool of apps to draw upon to make your phone custom-designed for your particular lifestyle and habitat. If you travel a lot, you'll have a lot of travel apps, if you're into financial things, you might have a lot of financial apps, or if you're like me, you probably have a lot of weather apps, hoping one of them will tell you what you want to hear.


And I've learned the last couple days in Vancouver that you don't need a weather app — you just need an umbrella. So —



So just as your smartphone tells us something about how you live your life, your lifestyle, reading the genome of a Prochlorococcus cell tells us what the pressures are in its environment. It's like reading its diary, not only telling us how it got through its day or its week, but even its evolutionary history. As we studied — I said we've sequenced hundreds of these cells, and we can now project what is the total genetic size — gene pool — of the Prochlorococcus federation, as we call it. It's like a superorganism. And it turns out that projections are that the collective has 80,000 genes. That's four times the size of the human genome. And it's that diversity of gene pools that makes it possible for them to dominate these large regions of the oceans and maintain their stability year in and year out.

So when I daydream about Prochlorococcus, which I probably do more than is healthy —


I imagine them floating out there, doing their job, maintaining the planet, feeding the animals. But also I inevitably end up thinking about what a masterpiece they are, finely tuned by millions of years of evolution. With 2,000 genes, they can do what all of our human ingenuity has not figured out how to do yet. They can take solar energy, CO2 and turn it into chemical energy in the form of organic carbon, locking that sunlight in those carbon bonds.

If we could figure out exactly how they do this, it could inspire designs that could reduce our dependency on fossil fuels, which brings my story full circle.

The fossil fuels that are buried that we're burning took millions of years for the earth to bury those, including those ancestors of Prochlorococcus, and we're burning that now in the blink of an eye on geological timescales. Carbon dioxide is increasing in the atmosphere. It's a greenhouse gas. The oceans are starting to warm. So the question is, what is that going to do for my Prochlorococcus? And I'm sure you're expecting me to say that my beloved microbes are doomed, but in fact they're not. Projections are that their populations will expand as the ocean warms to 30 percent larger by the year 2100.

Does that make me happy? Well, it makes me happy for Prochlorococcus of course —


but not for the planet. There are winners and losers in this global experiment that we've undertaken, and it's projected that among the losers will be some of those larger phytoplankton, those charismatic ones which are expected to be reduced in numbers, and they're the ones that feed the zooplankton that feed the fish that we like to harvest.

So Prochlorococcus has been my muse for the past 35 years, but there are legions of other microbes out there maintaining our planet for us. They're out there ready and waiting for us to find them so they can tell their stories, too.

Thank you.