A diptych showing a healthy coral reef and an unhealthy bleached coral reef

Many corals around Kiritimati, an island in the Pacific Ocean, bleached and died (right side) during a long heat wave, but others survived thanks to a surprising partner.

Danielle Claar and Kevin Bruce

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In surprising sign of resilience, some corals can survive long heat waves

Just a few degrees of heat can “bleach” corals, putting them on a path to starvation and death. Driven by climate change, marine heat waves are becoming one of the greatest threats to the existence of tropical reefs. But in some rare good news, researchers have discovered some corals can recover from bleaching even before a heat wave ends, suggesting they have the potential to survive long heat waves.

“That gives us a pathway to coral recovery that we wouldn’t necessarily have imagined before,” says Steve Palumbi, a marine ecologist at Stanford University who was not involved in the research. In addition, the research suggests reducing water pollution and other stresses can make reefs more resilient to the shocks of climate change, says Nancy Knowlton, a coral reef biologist with the Smithsonian Institution’s National Museum of Natural History, who was also not involved.

Corals are colonies of millimeter-size, filter-feeding invertebrates. Each of these so-called polyps contributes to the skeleton and houses photosynthetic algae. In return, the microorganisms provide most of the polyp’s food. In times of stress, particularly when the water gets too warm, the polyp ejects the algae, and the coral turns white, which is why the stressed state is called coral bleaching. Once the water temperature returns to normal, any polyps that haven’t starved to death will host another alga. Most corals were thought to survive only if a heat wave lasted just a few weeks.

But no one had studied this process during a much longer heat wave. Then in 2015 and 2016, Julia Baum, a marine ecologist at the University of Victoria, had a front row seat to a very severe and prolonged bleaching event. One year earlier, she and her students had begun a survey of reefs around Kiritimati in the central Pacific Ocean. To track the fate of individual corals, they attached metal tags to two common species, called brain and star coral. For these 141 corals, they identified the symbiotic algae by DNA sequencing. They checked the condition of the corals and resampled their algae symbionts six more times as the heat wave struck and then waned.

Starting in May 2015, the temperature rose about 1°C within 2 months. As expected, corals that housed heat-sensitive algae bleached sooner than those that housed the heat-tolerant genus of algae. And as the water continued to warm, even heat-tolerant algae were ejected. Also, no surprise.

The “jaw dropper” on Kiritimati, Baum says, was that many brain and star corals recovered from bleaching while the water was still unusually warm. Up until now, marine biologists had only seen bleached corals recover once water had cooled to its normal temperature. The unexpected recovery on Kiritimati provides new hope, Baum says, “because it means that even under prolonged heat waves, there’s a path forward for some of them.”

An unusual feature of the recovery is that brain coral that started out with heat-sensitive algae had a higher survival rate (82%) than coral that began with heat-tolerant algae (25%), the team reports today in Nature Communications.

That finding is surprising and “superinteresting,” says Madeleine Van Oppen, a coral geneticist at the University of Melbourne, who was not involved with the work. The expectation was that heat-tolerant algae would be better suited for helping coral survive a heat wave, Baum says. But during a longer heat wave, it might be more advantageous to start with a heat-sensitive alga, says lead author Danielle Claar, now a postdoc at the University of Washington, Seattle. That’s because these algae supply the coral host with more food than do heat-tolerant algae, thereby providing them with greater reserves to survive bleaching.

Water quality could influence the choice of algal partner. Because heat-tolerant algae also tend to be generally more stress resistant, they may help coral survive in polluted water. The corals on Kiritimati with heat-tolerant algae tend to be closer to large villages, where the water contains excess sediments, sewage, and other types of pollution. The more distant parts of the reef have cleaner water, and corals there are more likely to be living with heat-sensitive algae. In addition to more energy stores, it’s possible that coral living in cleaner water have more robust immune systems or other factors, the team notes.

There’s been some debate about whether local conditions, such as pollution and overfishing, impact the ability of a reef to survive heat waves, Baum says. Some researchers have concluded that local conditions don’t matter. “This paper clearly shows that argument is false, at least for these corals in this location,” Knowlton says. “Healthy local conditions were in fact very important for coral survival.”