A look into a mighty marine microbe

Prochlorococcus is a tiny plant like bacteria which is responsible for about 5 percent of all photosynthesis on the planet. Penny Chisholm is an institute professor at the Massachusetts Institute of Technology who was part of the group that discovered the marine microorganism almost 30 years ago and her lab is dedicated to studying the organism today.


Up next, it's one of the most abundant photosynthetic organisms on earth, but you may never have heard of it.

Prochlorococcus is a tiny plantlike bacteria, which is responsible for about 5% of all photosynthesis on the planet.

My next guest was part of the group that discovered the marine microorganism almost 30 years ago, and her lab is dedicated to studying the organism today.

Penny Chisholm is an institute professor at the Massachusetts Institute of Technology.

Thanks for joining me via Google Hangouts.

So, first of all, how is it possible that a tiny microbe like this could be responsible for 5% of an enormous process on the planet?

It's very small, but it's incredibly abundant.

Throughout the oceans, the mid-latitude oceans, it's the most abundant photosynthetic cell in the ocean.

So it contributes a significant fraction of ocean photosynthesis.

And this is in the ecosystem regardless of which part of the mid-ocean that you go to?


We pretty much find it everywhere except in some places along the coast.

It doesn't... It likes to grow in the open ocean, very pristine open ocean waters that are relatively warm.

We don't find them in the high latitudes.

So this is both key in what?

The food chain as well as oxygen production?


It's the base of the food web in these regions, and it's producing significant quantities of oxygen and taking up carbon dioxide in photosynthesis.

How old do we think this is?

Not just the... The process is forever, but how old do we know, do we have any kind of carbon dating that puts this particular microbe in history?

Its ancestors are incredibly ancient.

Its early ancestors were among the first photosynthetic cells on the planet.

After life evolved, they were the ones that actually put oxygen in our atmosphere.

But Prochlorococcus itself is actually very recent in terms of the evolution of microbes.

So if there is so much of this, is there a risk ever of too much?

I mean, in terms of, you know, people are familiar with algae blooms.

It's totally different, but are there particular areas of the ocean where there's an abundance of them versus other areas where they're more necessary?

That's what's so interesting about Prochlorococcus.

Where it is, it's fairly stable in abundance.

It grows.

It doubles about once every day.

The cells divide in half.

That's how it multiplies.

But it gets eaten just as fast as it grows.

So the actual numbers stay fairly steady wherever it is.

So it isn't one of these bloom-forming species like we see in coastal waters.

It's very finely tuned for the niche where it lives, and it stays at these very stable numbers.

And what's it like when it comes to genetic variation?

And how does that play into the success of its species?

That's one of the most amazing things that we've learned over the last 20 years since genomics has become a tool that we could use.

Each strain, or each individual cell, has about 2,000 genes.

But every time we look at another strain or another lineage, we find that it has maybe 100 or 200 genes that haven't seen before.

So there's extraordinary diversity in the, we call it the Prochlorococcus collective, such that if you... so far the estimates are if you took all of the Prochlorococcus genes in the oceans, there's probably up to 80,000 unique genes in that collective -- whereas our human genome is only 20,000 genes.

So there's more diversity of function in all the Prochlorococcus together than there is in our human, our complex human metabolism.

So does that give us an indication that perhaps that these are adapted to their specific environment really well, meaning that could you take one and plant it in a different type of ocean?

Would it survive or thrive or would it say, 'Well, the conditions are a little different here,' and it would just go away?

Excellent question.

And that's basically what we've been studying all these years, is looking at how these different, we call them ecotypes, are distributed along different gradients in the ocean.

So there are some that are adapted to high light intensities and others that are adapted to low light intensities that you'd find deep in the water column.

And they're so different that light intensities that are optimal for one strain would actually kill a different one.

And we see the same thing for temperature.

There are some that are much better adapted to the lower temperatures at the high latitudes than along the equator and others that are adapted to the equatorial regions.

So the range of environments that the collective can thrive in is much, much broader than any individual strain.

Could we use them in any sort of an intervention?

Meaning if there are spaces in the ocean which are becoming hypoxic or in low oxygen levels, could you take some of these, put them in there, and get them to be producing oxygen?

We don't think of it in that way because they are so finely tuned for their particular environment, although it's interesting you should ask that question because there are some regions in the oceans that are very low oxygen that are dominated by Prochlorococcus.

And we don't know why they're in the deep, deep layers of the low-light layers of the oceans, and we're actively trying to understand what it is.

The trouble is these environments are complex, so a cell could be dominant because its predators aren't there or it could be dominant because it's perfectly adapted to the chemistry or temperature of that water.

And so you have to study the food web in order to really get to the bottom of what's causing those patterns.

All right.

Penny Chisholm of MIT.

Thanks so much for joining us.

Thank you very much.