Under the intense heat of the southern sun, in a shallow lagoon where water temperatures can reach 95 degrees F, coral reefs are thriving. While corals elsewhere struggle when water temperatures reach the mid-80s, these equatorial corals can handle the extra degrees without breaking a sweat. Those are the kinds of water temperatures people expect to see by the end of this century, which means, in a sense, Ofu’s corals are already living in the future.

Marine biologist Steve Palumbi wants to find out how these corals are so chill about extreme heat. A researcher at Stanford University’s Hopkins Marine Station, Palumbi wanted to know if these outlying “supercorals” could hold the key to a secret resiliency available to all corals.

“Corals live in a very variable environment and they have for millions of years,” Palumbi tells me one afternoon, in his office in Monterey, Calif. “So it stands to reason that perhaps, in corals that live right now, there are already some that have adapted to live in future environments.”

Thickets of coral growing in the super-heated water off of Ofu, in American Samoa.Dan Griffin, Garthwaite Griffin Films

So Palumbi took a team of researchers and students to Ofu to put the supercorals through their paces. He chose to focus on a type of Acropora — the same genus that includes elkhorn and staghorn corals — called tabletop corals. As their name suggests, they grow in broad, table-like ledges, gathering as much solar energy as they can and providing shelter and shade for other species.

Fast-growing and generally susceptible to heat extremes, Acropora are both a fundamental builder of reef architecture and usually one of the more fragile species on the reef. If Ofu’s tabletops seemed hardier than usual, Palumbi was determined to find out why.

Using portable water coolers (the kind you’d use to stash a couple six-packs at a picnic) rigged with heating and refrigerating elements, the researchers took fragments of coral from the reef and subjected them to a uniform battery of stress tests. As the temperatures inched up, and the corals became more and more stressed, they started to expel the photosynthetic algae they need to live, and eventually bleached.

Here’s what’s interesting: The corals the researchers chose for testing came from different parts of the same reef, some of which experienced much higher temperatures on a regular basis. These “warm-adjusted” corals were much less likely to bleach under high temperatures than those that came from cooler parts of the reef.

There are two mechanisms by which these corals (or any organism really) can adjust to an extreme environment — increased temperatures, for example. The first, acclimation, is a short-term process whereby individual organisms change their physiology to become more heat tolerant. Then there’s adaptation, the longer-term process of natural selection whereby certain individuals pass on the genetic traits that make them more successful in warmer environments.

In other words: Acclimation is the tan you get on vacation, while adaptation is the skin tone you’re born with. In both cases, having more pigment in your skin reduces your risk of getting sunburned. So an organism’s life history might approximate some of the advantages of a genetic predisposition, through different mechanisms.

With corals, the combination of the two factors is more powerful than either on its own. To test this, Palumbi’s team transplanted corals from the cooler part of the reef to the warmest pool. They left them there for three years, after which the researchers subjected them to the same set of stress tests they’d undergone three years earlier, raising the temperature to an extreme level for several days at a time.

This time, the cool-adapted corals that had spent the last three years acclimating in warm pools did noticeably better than their cold-water clones. But the original warm-pool corals still outperformed their acclimated cousins, suggesting some older genetic advantage was also at work.

The next step, Palumbi says, is to start trying to use these supercorals to help corals in general. To that end, his PhD student, Megan Morikawa, is establishing a coral nursery off the coast of Ofu, where she hopes to cultivate the most resilient, heat-tolerant corals they find. Like Nedimyer, they want to replenish reefs with corals, and with the genetic diversity that will allow them to survive in a range of environments.

It’s too early to know if this will work. Palumbi is hopeful that something can be done, but aware of the challenges in scaling this kind of experiment up to a level where it can actually be effective.

“You can’t go to a replanted reef,” he says in his office, thousands of miles away from Ofu. “You can go to a replanted forest, but as of now, there is no replanted reef.”

Grist | Amelia Urry


When a coral bleaches, it doesn’t necessarily die — which means some interesting things can happen after a bleaching event sweeps a reef.

A coral polyp gathers some of its food from the water current, but it gets most of its energy by harvesting the sun’s energy, using tiny algae called zooxanthellae that live inside its cells. As the water heats up, those little algae start to produce chemicals that irritate the coral cells. As these so-called “free radicals” build-up, they start to damage the cell until the coral polyp gets so agitated it expels most or all of its zooxanthellae.

From then, the coral polyp is essentially on its own. It loses the pigment it gets from its symbiont algae, turning pale white, and loses the energy it gets from photosynthesis. Without that energy, the coral feeds on its reserves and strains what food it can get from the water column. It stops growing and is at risk of eventually starving to death.

If the temperature drops again, the polyp can take up new zooxanthellae from the water and keep growing as before. But the longer the coral stays bleached, the less likely it will be able to recover.

However, there is a secondary effect to all this bleaching and unbleaching, which Palumbi and others have seized on as a possible source of hope. After a coral has bleached once, the next time it is subjected to abnormally high temperatures, it is much less likely to succumb.

In the same El Niño season that wiped out Florida’s corals, the reefs around the tiny island nation of Palau bleached extensively. Today, nearly 10 years later, they have almost completely recovered, Palumbi says. In fact, at this point most reefs in the world have bleached, he says. And a good number of them have recovered.

So corals are resilient, maybe more than we thought, and can recover from near-fatal events. But how fast can they keep up? In all their history — and at 400 million years, it’s a long one — the organisms have never experienced a rate of change like what they are subjected to today.

What’s more, a coral’s vulnerability to and recovery from bleaching have a lot to do with the presence of other stressors, Palumbi points out. With pollution, habitat destruction, overfishing, sedimentation, and so on, what would be a tolerable amount of climate change can become an intolerable amount.

In this sense, Ofu is a lucky place to be a coral. Sure, the corals living in the sheltered lagoons of the remote island may have to put up with 95 degrees F — but they don’t have to deal with many humans.


Corals can adapt and acclimate to higher temperatures — but only to a point. Even Palumbi’s supercorals, subjected to monstrously high temperatures in the stress tanks, have a limit. Eventually, given enough global warming, the world’s reefs will find their limits, too.

“I can’t do everything,” Palumbi admits. “But if I can just help keep the ocean alive long enough for us to wean ourselves off of fossil fuels, we can make it.”

This summer, Palumbi brought 80 corals back from Ofu, small fragments from the different genotypes and environments he was hoping to test in the lab. Then all but two of the corals died when he got them to the lab, and they took their heat-resisting secrets with them. Palumbi will have to go back to the reef and start over. But he’s determined to find the corals of the world that possess the superpower to resist higher ocean temperatures, and protect them.

“Saving the ocean for the next 85 years — that’s a pretty good job,” he says. “I just need to show people that it is worth it.”

Continue to Part 3: Evolution