A Peek Into The Sex Lives Of Algae

12:20 minutes

Diatoms are a type of single-celled algae found all over the world. Considered a model organism, they’re a staple in labs studying ocean water quality. Indeed, they’re as ubiquitous in microbiology research as lab mice are in human-health studies.

A diatom under an electron microscope. Credit: Commonwealth Scientific and Industrial Research Organisation. CC 3.0

[This researcher uses a 3D microscope to make beautiful pictures of sand]

In fact, diatoms are so well studied that some facts about them seem irrefutable, like the way they reproduce. Scientists thought that they only reproduced asexually, by single-cell division. But by happy accident, researchers at Oregon State University recently discovered a group of centric diatoms that could reproduce sexually, and that ammonium puts them in the mood (so to speak). Kimberly Halsey, an assistant professor in the Department of Microbiology at Oregon State University, joins Ira to discuss how scientists overlooked something so fundamental about an organism they thought they knew so well.

Segment Guests

Kimberly Halsey

Kimberly Halsey is an assistant professor in the department of microbiology at Oregon State University in Corvallis, Oregon.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Diatoms, they are a type of single-celled algae found all over the world. They’re considered a model organism, a staple in labs studying ocean water quality. They’re like the little white mice used to study human health. Well, diatoms are that for marine microbiology research. In fact, they are so well studied that some facts about diatoms seem irrefutable.

For instance, we know that they only reproduce asexually. Of course, that means by a single cell division. Or so we thought, until one of those happy accidents we’re always hearing about in science. My next guest caught a subset of diatoms in the act of sexual reproduction. Kimberly Halsey is Assistant Professor in the Department of Microbiology at Oregon State University. Dr. Halsey, welcome to Science Friday.

KIMBERLY HALSEY: Thank you very much.

IRA FLATOW: Are you sort of the Masters and Johnson of diatom sex research?

KIMBERLY HALSEY: Well, by accident, that’s for sure. It definitely was a serendipitous discovery so we’ve been learning a lot fast.

IRA FLATOW: Tell us how you made that serendipitous discovery.

KIMBERLY HALSEY: Oh, well, a lot of these stories start with a graduate student. So Eric Moore, graduate student working in my lab, was studying something else. He was studying how diatoms maybe share information, share carbon, share maybe vitamins. He was interested in seeing how diatoms might share these compounds with bacteria and other organisms in the ocean. And so, to do that, he had to first get over a big hurdle, and that was that he needed to devise a new culturing medium. So this is the seawater-like stuff that we create a recipe for that we can grow these cells in.

So he had to make this new media that could grow both the diatom that we were studying and a bacterium at the same time. So he worked on this for a couple of months, and finally, he had this great concoction that he’d developed. And it seemed that both the diatom and the bacterium he was studying could grow in this medium, so that was a great thing. Except that after a couple of weeks, he came into my office. He had that look of concern, and he said, you know, the bacteria are growing great, but the diatoms just look really strange. So I said, well, what do you mean? What do you mean they look strange?

And he said, well, I don’t know. They just get really clumpy. They look bigger than they’re supposed to look. And I said, well, did look at them under the microscope? And he said, no, and he kind of looked sheepish. And I have to say, this is one of those things that I think the most under-utilized instrument in the laboratory in microbiology, unfortunately, is probably the microscope. So we ended up looking under the microscope.

So we saw what we expected to see– some of these cells that look like little miniature cylinders. They look like what I would call little, teeny-tiny old-fashioned hat boxes with a lid and a bottom. But we also saw these other shapes in this culture that looked like gigantic beach balls. I kind of looked at Eric, and I said, Eric, I don’t know what you’re growing, but you’re not growing the diatom you’re supposed to be growing. You’ve contaminated the culture, and you need to start over. And you need to clean everything and make new media and start from scratch.

This was sort of this long road to realizing that, in fact, after he repeated and repeated, it turned out that, eventually, you have to believe your data. And so we said, something else is going on here. And that’s when we realized that these organisms were undergoing the sexual pathway instead of just regular single cell division. They were undergoing sexual reproduction, and that was a shock.

IRA FLATOW: Wow. So why would they decide to do that, versus the asexual one you’ve always been familiar with?

KIMBERLY HALSEY: Well, that’s the big question, why were they undergoing sex? And it was great, because we were suddenly in this position where Eric had two types of culturing medium, one in which they were always just going through regular cell division. And then, this other medium that every time he grew them in this new medium, they would go into sex.

And so he took this laborious path to try to figure out what chemicals were inducing sex, because never before has anybody ever really been able to reliably induce sex in diatoms. They can sometimes capture it in the environment, like a freak event, but we can now reliably cause this to happen. So we can trigger sex every time. So what he found out was that this really common ingredient in the laboratory, ammonium, stimulates sex in these organisms.

IRA FLATOW: And that’s like fish waste, isn’t it? Isn’t that the waste that–

KIMBERLY HALSEY: It is. It is like fish waste, and it’s also released by the organisms that come along and eat these diatoms. The thought is perhaps that when these organisms are growing, possibly, their predators might come along and chew on them. What they do is they excrete ammonium, and maybe that will stimulate the sex response in the environment. This is a hypothesis. We don’t know yet.

IRA FLATOW: So, to continue this x-rated interview, could I, then, assume that any time you wanted a diatom to have sex, you could just put some ammonium in their media?

KIMBERLY HALSEY: Well, that’s what it looks like. For these particular diatoms, which are a large group of the overall diatom group, these diatoms that look like cylinders, it seems to be that if we add ammonium– In fact, the more ammonium you add, the more sex you get. So it looks like the most reliable method that anybody’s ever developed, and it’s been a long road to try to understand what might trigger sex. So this is pretty exciting.

IRA FLATOW: Just so we understand this, this is ammonium works in diatoms. You have no reference to people anywhere here in triggering sex.


IRA FLATOW: This is the internet, you know.

KIMBERLY HALSEY: There’s been a lot of fun in the lab after this discovery, that’s for sure.

IRA FLATOW: Do you think the finding influences how researchers study diatoms, then? What is their niche in the ecosystem?

KIMBERLY HALSEY: So, diatoms are these amazing organisms, and I would encourage anybody that wants to google diatoms and look at them really up close and personal. They are beautiful. One of the things that makes them super unique is they have these silica cell walls, which means that they really do live in glass houses. So they are surrounded in silica. And So their place in the environment– so remember these are plants– they’re really, really good at growing fast under low light conditions. So they’re unique in that regard, too.

So every year, predictably, when we’re coming out of the winter and just starting to get enough light in the springtime, you can reliably see really huge diatom blooms in the ocean. And that’s because they kind of get this head start, because they can grow really fast when the light is really low. Because of this ability, they end up making these huge blooms that contribute a massive amount of carbon to the oceanic systems. They have a huge role in the carbon cycle.

IRA FLATOW: Interesting. Will the warming of the oceans affect their cycle and how they live in the oceans?

KIMBERLY HALSEY: Well, there’s a lot of ideas about that. One hypothesis is that as the oceans warm, one possibility is that there will be less nutrients that move up towards the surface. And even more important, maybe, for diatoms is that even less silica might move into the surface waters. And so that limitation– Without that silica that they need for their cell walls, they’re going to have a really hard time, perhaps, growing to these massive blooms that we’re typically seeing on an annual basis. But that’s just a hypothesis right now, so we don’t know.

IRA FLATOW: All right. I’m going to give you the Science Friday $64 question. If you had unlimited budget and you wanted to study diatoms and you could spend any amount of money, what do you know and what would you need? What would you like to know?

KIMBERLY HALSEY: Wow. That’s a great question. What I would like to know is, really, I want to nail down what the ecology of sex is in the environment. So I want to know the answer to this question about whether or not other plankton like zooplankton, larger critters that come along and eat these diatoms, I want to know if that actually does trigger sex like in a stress response. Or maybe there’s another way that sex is triggered.

The next thing I want to know is what happens after you’ve got this new sexual reproduction. What’s the purpose in the environment? So they now you know diversify their genetics. They create these larger cells as a result of cell division and this whole sexual reproduction process. The new initial cell that’s formed in the process is really large compared to the original cell. And so one thought is that maybe, as a result of that, those large cells end up sinking deeper into the ocean. And maybe that’s a place where they can basically hide out and get redistributed into other areas in the ocean.

These are the questions I want to ask. Some of that is going to require just some hardcore lab experiments. Some of that would also require maybe a lot of genetic sequencing technologies or looking for gene expression changes. But those are the questions I’m excited about.

IRA FLATOW: You’re so passionate about this subject. Where does that come from?

KIMBERLY HALSEY: Well, it’s a lot of fun. Lab work is a ball. Science is great. I feel really privileged to get to do it. So, yeah, we have a lot of fun.

IRA FLATOW: Well, continue having fun. I think you’ve created some great conversation topic for tonight for people.

KIMBERLY HALSEY: I hope so. If people are talking about diatom sex on their Friday evenings over a beer, that’d be great.

IRA FLATOW: I’m sure they will now. I will. Kimberly Halsey is the Assistant Professor in the Department of Microbiology at Oregon State University. Thank you for taking time to be with us today, and keep us informed of your work.

KIMBERLY HALSEY: Well, thank you very much for having me.

IRA FLATOW: You’re welcome. After the break, we’re going to journey into the move from diatoms into ants, another small creature. And they build magnificent towers that you’ve probably never seen. They look just like the Eiffel Tower. We’re going to be talking about fire ants, and they do it with their own bodies. It’s another great story to talk about. We’ll talk about it after the break. Stay with us.

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About Katie Hiler

Katie Hiler is an assistant producer for Science Friday and the proud mother of two cats, Charleigh and Sadie.