How Algae Survived A Mass Extinction

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, if you’ve decided to gather for holidays this year, how to maximize the safety of pandemic travel and when to actually cancel your plans. But first, 66 million years ago, when an asteroid slammed into what is now the Yucatan Peninsula, it set off a period of near global darkness for almost two years.

Scientists predict the majority of land species went extinct. But what was going on in the Earth’s oceans? And how are these ecosystems able to bounce back from it? What may have saved the Earth’s oceans may have been a rare type of algae. My guest to tell the tale is Dr. Andrew Ridgwell, professor of earth systems science at the University of California, Riverside. Dr. Ridgwell, welcome to Science Friday.

ANDREW RIDGWELL: Thank you.

IRA FLATOW: So your models help us understand what ocean environments were like right after that asteroid impact. What did the post-impact ocean look like?

ANDREW RIDGWELL: Well, we don’t have direct evidence. So scientists like to do a lot of speculation, but based on what evidence that you have, we’re pretty sure that there would have been a period of darkness from all the soot and aerosols thrown up by the impact. And so we wondered what the little algae in the ocean were doing in a long period of darkness, which maybe was a few years. We don’t actually know how long.

IRA FLATOW: Because that would be rare. Don’t algae usually need photosynthesis to survive? They need the sunlight.

ANDREW RIDGWELL: Exactly, they’re just little plants in the ocean. They’re sort of unusual and they’re different from the plants on land is that– and they have a very short generation time. So sort of every day, they’re looking to divide and split apart and make another algae. And so they really can’t normally go very long with a lot of darkness.

So trees have strategy of having seeds in the ground or maybe ferns have a lot of their biomass under the ground. And they can sprout back up again when conditions are good. But it’s not clear, at least it wasn’t clear before our study to us, what would happen to little algae in the ocean.

IRA FLATOW: Yeah, because two years is a long time for plants to survive without any sunlight. So what allowed the algae to survive?

ANDREW RIDGWELL: So we already knew that some algae like diatoms, they have a little resting stage. You can almost look at them– it’s not quite like a seed, but you can almost imagine them. Diatoms making a seed, and then if conditions are better in the ocean, they can sort of pop out again.

But the major player of the ocean at the time didn’t have a resting stage. And they all went extinct. And there was almost no evidence of them for about two million years after the impact. So the question to ask was, well, where did they go? Because after two million years, they sort of came back alive and kicking. And they’re still a dominant species in the ocean today.

So it turns out to be– I mean, a nice story would be is if they suddenly looked around. They couldn’t photosynthesize, and they started eating everyone else. It’s a little bit more evolutionarily complex than that. So the ones that could only photosynthesize went extinct. But there was some rare cousins of them who lived maybe near the coast and could both photosynthesize and eat other organisms. A little bit like Venus fly traps, both on land, the plants, they photosynthesize, and they also try and catch flies and digest the flies.

So these special shelf cousins took over. And they could do some photosynthesis, but they also did a lot of eating of bacteria. And who knows? Maybe in the aftermath of the impact, there were sort of dead dinosaur sludge coming down the rivers and all sorts of yucky stuff with lots of organic matter, that if you could consume organic matter in the ocean, you would be in good shape.

IRA FLATOW: So how long did they survive in this sort of environment?

ANDREW RIDGWELL: They clearly survived– these sort of specialists, they clearly survived the post-impact darkness by eating whatever they could eat. But the original photosynthesizing-only cousins of them who went extinct, of course, were not around then. So there was a very long and interesting period of about two million years where these sort of interlopers ruled the roost.

They would sort of periodically go extinct, and new variants of them would turn up. And they would go extinct and then be replaced by yet one more, almost as if Darwin was rolling the dice. This is not the best species we could have. Let’s try another one. This one’s no good. Let’s try another one.

And this went on for about two million years that these sort of repeated rolling of the evolutionary dice, until one day– they’re called mixotrophs, is the technical word for an algae that can both photosynthesize and eat someone else. But after about two years, they re-evolved to just eat plants. So they relearned to not have to also eat other people once the conditions were good and the sunlight was back.

IRA FLATOW: And we still don’t know why there was this two million years of instability.

ANDREW RIDGWELL: No, and this is a topic of ongoing research. And it’s something that I helped develop models to try to understand. It’s sort of really enigmatic because these algae are dividing. Their generation time is one day. So, in a few years or decades or hundreds of years, they should be evolving to some new state or to take advantage of some new environment that they could be successful in.

So why it took two million years of evolution– if you imagine 365 times 2 million, that’s a lot of generations. So there must be something else about how the environment of the Earth interacts with life.

So evolution doesn’t just proceed independent of climate and nutrient supply as life evolves. It modifies maybe the climate or the environment around it, which then also redirects the direction of evolution. So my suspicion and something that I want to develop some computer models to test is that somehow this interaction of normal rates of evolution with how it interacts with the Earth as a whole.

IRA FLATOW: And during those 2 million years, what was the rest of the ocean like? Were there higher forms of life that died out also? Or were they evolving ways to survive also?

ANDREW RIDGWELL: It partially recovered in terms of the production in the ocean. And to eat someone and someone else eats someone else, there was different layers to the ecosystems. But the various lines of sort of– some of the chemical evidence suggests that the sort of pumping of carbon around the ocean wasn’t operating as at its normal rate. So we certainly had production in the ocean and different trophic layers and fish.

But there was something still not right about the ocean for about 2 million years. And then once these mixotrophs reverted back into being just the harmless little plants again, this settled everything down. And sort of the carbon cycle reverted back to sort of a normal mode of operation. And over the following sort of 10 million years, then diversity gradually increased until we got back to roughly the state prior to the impact.

IRA FLATOW: I find it surprising– because I don’t know much about oceans and mass extinctions. But do you find it surprising how short of time it took for ocean environments to collapse after that mass extinction? What, we’re talking two years? Isn’t that basically nothing in geological time?

ANDREW RIDGWELL: I was surprised, thinking about it, how vulnerable the entire ocean ecosystem is. There have been many mass extinctions over Earth’s history. This one is super interesting because it seemed to strike at the very heart of the whole ocean ecosystem, which are the primary producers. If you don’t have algae photosynthesizing to make food for the zooplankton, for the fish, for the sharks, for everything else, everything would completely collapse.

So we lost most of those precious key photysnthesizing algae just after the impact. Probably what sustained us or sustained the ocean at the time was cyanobacteria, which are very, very ancient. They can photosynthesize. They go back more than 3 billion years to very early Earth. And I guess, evolutionarily, they’ve seen a few things in their time. So they’re a very, very, very hardy organism. And they sort of provided probably that tiding over of light-based primary production until these algae got their act back together.

IRA FLATOW: What do you need to know or what tools would you like to have so that you know what was happening during those 2 million years?

ANDREW RIDGWELL: Time machine.

IRA FLATOW: [LAUGHS] I got one right here, back of my office.

ANDREW RIDGWELL: I have a lot of colleagues who have very expensive, very complex instruments for measuring tiny ratios of atoms. And these machines seem to get bigger and more complex and measuring even tinier ratios of atoms to try and learn something about the past environment. And sometimes I just think, shouldn’t we spend the money building that time machine? Because then we can just go back. Go back with a beaker and sample.

What I would like to know– because it’s my colleagues who do all the hard work, getting samples of mud that are being drilled from maybe four or five kilometers down in the ocean from the ocean floor. And they wash the mud and they sift it. And they spend hours, weeks, months, years looking down microscopes, taking apart the remains of these tiny organisms to identify them. See who is there, see who disappeared, find out when people came back again.

I sit at home in front of a computer writing computer code. So it’s very easy for me to say what would I like to know. Sure, I’d like them to spend a lot longer looking down microscopes, finding these things.

IRA FLATOW: There’s some debate these days about whether it was the asteroid impact or the volcanic activity that caused the Cretaceous extinction or possibly some combination of the two. What does your gut tell you about all of this?

ANDREW RIDGWELL: I think it’s pretty well known that it’s that one shot, big rock slamming into the Earth that had this catastrophic, sudden, rapid extinction. There was certainly volcanism before, episodic. And there was volcanism after. And at other times of Earth, in Earth history, we think that it was volcanism driving extinctions.

But I would say, certainly from the perspective of why did most of the photosynthesizing algae in the ocean go extinct, you need this period of darkness. You need a lot of stuff, a lot of rubbish put up into the atmosphere that doesn’t come down for a year or so.

IRA FLATOW: That’s an interesting point of view, Dr. Ridgwell. Thank you for taking the time to be with us today.

ANDREW RIDGWELL: Thank you.

IRA FLATOW: Dr. Andrew Ridgwell, professor of earth systems science at the UC Riverside.