The world’s corals live on the edge. They need warm water and lots of sunlight to feed the symbiotic algae that live with them. But if it gets too hot, those algae start to produce toxins and the coral ejects them, thus killing themselves and producing what’s called “coral bleaching.” And as climate change heats the oceans and excess carbon dioxide gets absorbed into the water, making it more and more acidic and less friendly for coral, scientists worry that corals may not last long.
But a new study in Science provides a small ray of hope: One species of table top coral (Acropora hyacinthus) from the Pacific has shown a marked ability to adapt to warmer waters.
Organisms have a few ways of dealing with environmental changes like climate change. They can move to areas with conditions more like the ones they’re used to. Or they can acclimate to the new conditions at home, or genetically adapt to them. If a species is unable to use one or a combination of these options, they could go extinct. But climate change is happening so quickly that scientists worry that organisms, such as corals, won’t be able to respond fast enough to keep up.
The new study finds that at least one species of coral may be able to cope with the higher heat. “As the environment warms, [corals] will adjust and evolve to match it,” says the study’s lead author, Stanford University marine ecologist Stephen Palumbi. But the ability of corals to acclimate and adapt to warmer and warmer waters may stop at some point, he warns. “What we don’t know is how far that process can go and where it will max out.”
Palumbi and his colleagues studied A. hyacinthus corals that live off Ofu Island in the National Park of American Samoa. This species is one of the main builders of Ofu Island’s reefs, and it’s particularly sensitive to environmental stress, such as high heat. But some of these corals manage to thrive in spots where the water see-saws between 84° F (29°C) and 95°F (35°C), the latter when the tide is low and the sun is high. The corals don’t have to survive long in that high heat—just a few hours until the tide rises—but those conditions should be beyond the organism’s normal tolerance. The researchers wanted to find out how these corals manage to survive.
They began by taking corals from the areas where temperatures fluctuate greatly and transplanting them into waters where the temperature rarely got above 90°F (32°C). They also transplanted corals in the other direction, taking them from the less variable pool and letting them grow in the waters that got really hot every day. Twenty-seven months after the transplantations, the researchers tested the corals’ ability to tolerate heat.
Corals from the waters that stayed below 90°F acclimated somewhat to the higher, more-variable heat conditions, though they never were quite as heat-tolerant as the corals native to the fluctuating waters. Palumbi’s team then looked at the corals’ genetics—which genes they carried and the degree to whichcells were using those genes (gene expression).
The researchers found that when the corals were transplanted, there were some changes in their levels of gene expression. That’s because the corals were activating genes they had but didn’t need until then.
But, “we also find some genes that are different between corals from the two different habitats,” Palumbi says. “It has nothing to do with where the corals are living. It has only to do with where the corals are from.” Those differences are probably the result of genetic adaptation that happened a long time ago, he says.
Palumbi described these processes in his recent book The Extreme Life of the Sea, written with his son Anthony:
Three days of heating activates a battery of 250 different stress genes in the typical coral. In the Ofu lagoon, the corals keep about 60 of these “heat genes” operating at high capacity all the time. Some of these corals seem to be born with these guardian genes turned on, but others only turn them on when moved by scientists to the reef’s hottest region. Some never activate the crucial genes; these colonies simply die. The cumulative result is a small band of survivors thriving in a small backreef lagoon a quarter mile across, growing in the intense sun and heat.
The ability to cope with warmer water is therefore a combination of acclimation and genetic adaptation, Palumbi says. And it’s possible that other coral species off Samoa and perhaps around the world will be able to use a mix of those approaches to survive as temperatures rise and oceans become more acidic.
But such adaptation and acclimation are “not going to solve the entire problem,” Palumbi says. At some point, waters will get to hot or too acidic for even the hardiest corals to survive. Corals are “still going to be seriously affected by climate change in the future.” But their capacity for some tolerance of heat will “probably will give them more time,” he says.
And humans could use that time to make changes that would limit the worst of the potential effects of climate change, he says. “Right now we’re in a position where both for acidity and temperature corals are struggling, because [these conditions] have already changed. But they’re not to the point where it’s so devastatingly awful that [corals] can’t survive,” he notes. “The abilities of corals to withstand some degrees of warming gives us the time that we need to solve the problem” and limit the potential damage from pumping greenhouse gases into the atmosphere.