ScienceMatters - Season 2, episode 5: Hope for Coral Reefs

February 26, 2019
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In Part Two of our podcast with School of Biological Sciences Professor and Chair Mark Hay, the scientists says some coral reefs damaged by climate change could heal themselves – if given the chance. 

(Upbeat music)

Hello, and welcome to ScienceMatters, the podcast of the Georgia Tech College of Sciences. I’m Renay San Miguel.

We continue our conversation with Mark Hay, award-winning Regents Professor, and Harry and Linda Teasley Chair of the School of Biological Sciences. Hay is one of the world’s leading experts on algae and ocean bio-systems.

In Part One of this podcast, we detailed the symbiotic relationship between algae and corals. Algae, like seaweed and kelp, are photosynthetic organisms; they serve a key role in ocean health. They provide nutrients to corals, which in turn provide homes for algae and other sea life when they excrete exoskeletons that become coral reefs.

This delicate balance has been thrown out of whack by climate change. In Part One, Hay explained that after more than 30 years of diving in the world’s oceans, he wasn’t researching in the Caribbean Ocean anymore because most of the reefs there are dead.

Hay continues to dive and study in the Pacific. He describes how he and his team discovered other threats to coral reefs brought about by overfishing – threats in the form of certain hungry species of starfish and snails.

Mark Hay: The two things we have looked at: One of them, there's something called crown-of-thorns starfish that, back in ‘60s and ‘70s, people started noticing there were outbreaks of these and they eat coral. And if they're at high abundance, they eat most of the coral, and then they just line up in a mass and they walk down the reef like a lawn mower just devastating everything.

It’s really quite impressive. I mean, in Guam, there was like, I can't remember, 60 miles of reef was just eaten, and these guys sort of marched down over here, too. And so we started thinking about this. Well, you know, once you get these protected areas that’s got a lot of coral, where are these coral-eating things going to go? You know, is this like pouring a pile of candy in the middle of the daycare center and thinking it's going to persist?

Nobody really understands, even to today, where the babies come from; there's like almost none around, and then, all of a sudden, there's a big bloom of these things. And what we think is going on is that the babies are settling under rubble in the really damaged areas—these algal-overgrown things. Staying down in the rubble until they get big enough to ward off predators, then they emerge, and then they smell all this coral that’s 100 meters away, and they all head over there.

We found that that these predators were attracted to the good areas. And so once you succeed and rebuild one of these small reefs well, you may be at risk of the starfish coming in and eating the corals.

Now the other part of this is, there were some gastropod snails that eat corals. This was a species that—we called it “the tick” or “the vampire snail”—it sort of sits at the base of the coral and it sucks out tissue, and the coral tries to recover that tissue just like if you get a wound, you know, you start reallocating resources to that wound.

And so these guys just start sucking at the base and they sit there, and so you don't see a trail, you don't see damaged stuff, you just see, you know, a few snails sitting around. And so we planted out some of these corals and put different sized snails on them, and showed that they were actually slowing the growth of the coral by 20 to 40 percent, which is a big deal at the densities we were using. And we saw them then at huge densities in the overfished areas. And so what was happening, many of the susceptible corals are already gone. There's a few tough corals that are kind of the last man standing in these damaged areas. Those were the ones these guys were eating, and so they're killing off your last chance. And we were finding just dramatically high densities of these things in the damaged area on the little bit of coral that was left.

Renay San Miguel: Lest you think the news is too grim, Hay has seen how coral reefs may be trying to heal themselves. His research suggests that coral larvae – what he calls “coral babies” – steer away from bad places, such as reefs that have too much algae.

Mark Hay: It’s true that if we back off, most of the time nature will fix itself. Yeah, 2.7 times as many people on earth as when I was born. We're all using more stuff, you know, I got motorcycles, I've got cars, we're eating our food, we're all getting fatter. And, you know, we're not backing off. There's nothing—I can't find any data that says we're backing off.

I think we need to proactively go in and treat the damage.

But what we're trying to do, in part, is say what are the really critical interactions so we can go in and tweak those and help nature recover more rapidly. So we know, for these reefs that we worked on, that it's been effective to stop fishing; the corals came back. Now, 100 years from now, can they do that with the warmed, acidified oceans? We don't know. We're not sure. But it did it over the last 10 or 12 years; those reefs got a lot better. Once they were better, so that saved that part of the reef. One of the functions of that is to have enough babies coming out of there to save other parts of the reef. However, there’s been strong evolutionary pressure for these babies to go to a good place, not a bad place. And so we learned that. We looked at 15 different species of fish in six different families. We looked at herbivores, carnivores, coral-eaters, all the different groups of fish. The babies could smell the marine-protected areas that were covered by coral and go there. And they could smell the seaweed-dominated areas and avoid them. Coral larvae which, I mean, baby fish have fins and a nose so, I mean, I was surprised they could swim against currents and do this, but they did it. Coral larvae are like a bag of snot with some cilia on the outside. You would think they can’t swim against—they can't swim against current, OK? But, they did the same thing: They were attracted to the smell of a healthy reef. They avoided the smell of these algal-covered reefs.

But it means that a lot of these damaged areas won't recover unless we give them the signals, unless we make them receptive to these larvae. And we can't just put out the right smell and have them come in because we've done transplants. If we transplant babies into the damaged area, they just die. If we transplant them in a good area, that do pretty well.

And so we've got to get rid of the seaweeds and make that more receptive. And we know that 29 species of important herbivorous fish in the reefs we've worked on, four of them are the ones that removed most of those seaweeds. So the villagers don't have to quit fishing, but if they quit fishing on those four, those four will remove the seaweeds; that’ll will make the area more attractive to corals and other fishes. You got that area back, they have coastal protection, they have more fish. And so, I think through those kind of tweaks and sort of understanding the chemical signals, because almost all these organisms are talking to each other, but they're doing that chemically, OK?

Renay San Miguel: I'm wondering if it's not too late, if the damage has already been done, because I'm hearing from other media and other reports about the health of coral reefs, some people being very, very pessimistic about it.

Mark Hay: It’s possible that coral reefs are on their way out and they'll never recover—that is not an impossibility. But what I'm supposed to do for society at some level is be sort of an encyclopedia that answers questions you haven't had yet. And so we're going to keep working at it and try to be—I mean, the data are not very promising; to the direction of change is not good. But I think some of us need to keep working at finding ways to improve that. And, you know, I'm willing to tilt at windmills if that's what it takes. 

Renay San Miguel: What parts of ocean health and marine ecology will have your attention and that of the researchers in Hay Lab?

Mark Hay:  We started working a little bit with this ocean acidification and temperature stuff. We've shifted over from looking at ocean acidification and temperature changes—both—to just looking at temperature now because, in the last couple of years, temperature alone has killed so much so quickly. Ocean acidification is a big deal, but the rise in temperature is going to kill off most stuff before acidification does.

Renay San Miguel:  That’s the more immediate threat?

Mark Hay: It seems so, it seems so. There's another interactor in all this that people haven't been able to address and didn't understand, and that was microbes. We’ve shown that seaweeds can poison and kill corals. It may be that what they're doing is poisoning and killing the beneficial microbes that are on the coral. And without those microbes, then the coral dies. And so there may be another step in here, and we're looking a lot at that. We are not seeing the big changes that many people have talked about, but most of those are correlative where they've gone to a good reef here and have reef a thousand miles away, and so the microbes are different, and so this bad reef must've been killed by the microbes. I'm a little worried that that might be like I looked at a live rabbit and I looked at a dead rabbit on the road, and the microbes were different, and therefore the microbes must've squished this rabbit on the road, OK?  And so could be a consequence instead of a cause. We're seeing differences in a few unusual microbes in these, not big, community changes. But it's entirely possible that those few unusual ones are making antibiotics that are protecting coral. And in fact, we have some data for that.

Renay San Miguel: Really?

Mark Hay: Yeah. So we can take one of the species of coral—so in the Pacific there's about 600 species of coral. A quarter of those are in one genus called Acropora, and it's the one that makes a lot of the structure in the shallow-water reefs.

The evolution of fish diversity in the Pacific correlates with the evolution of  Acropora diversity; it's a kingpin in what's going on out there. And if we take that coral from the seaweed-dominated area, and take the slime off of it, and test it against coral pathogens, it’s somewhat suppressive, OK, so it's somewhat antibiotic. If we take that same coral from the marine-protected area, which has high coral and almost no seaweed, it's very suppressive. OK, and so somehow the abundance of seaweeds is suppressing the ability of this coral to defend itself.

(Upbeat music)

Renay San Miguel: What was it that attracted you to this particular part of science?

Mark Hay: I always liked being in the water.

Renay San Miguel: Really?

Mark Hay: Yeah, I was in streams and lakes and stuff.  And, you know, they're nice—coral reefs are nicer. [Laughter] And so, you know, it was—I started working in the marine intertidal in California you doing some stuff, and then started working down in Mexico a little bit, and that was neater—there were more species and things—and then got the option of going to Panama for two years and working through the Smithsonian to do my dissertation. And I was like “Yes!” And I'd never seen—I mean I'd been in Florida and I'd snorkel around and stuff, but I was not—I didn't know squat about coral reefs. And so I sat there for three weeks reading everything I could find, and then realized people don't know enough, I'm just going to get in the water and do experiments. And I'm really good at it seeing questions and figuring out ways to do that in the wild as opposed to in the lab.

Mark Hay: Well, I don't want to do it in the lab where I can show it could happen; I want to do it in the field where I can show it does happen, you know? In other words, I’m not doing it with two species in an aquarium, I'm doing it with these two species that are surrounded by a thousand others, and that there’s storms, and there's heat waves, and it still was important enough to work. That's what excites me about doing ecology in the field.

(Upbeat music)

Renay San Miguel: In addition to the Smith Medal, Hay has won other major awards, including the Silver Medal, the highest honor from the International Society of Chemical Ecology.

My thanks to Mark Hay, professor and Harry and Linda Teasley Chair in the School of Biological Sciences.

Siyan Zhou, former research associate in the School of Psychology, composed our theme music.

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This is ScienceMatters, the podcast of the Georgia Tech College of Sciences. I’m Renay San Miguel. Thanks for listening.