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By Erik Stokstad
A few degrees of heat can “bleach” corals, putting them on the path of 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 found that some corals can recover from bleaching even before a heat wave is over, suggesting they have the potential to survive long heat waves.
“It gives us a path to coral recovery that we might not have imagined before,” says Steve Palumbi, a marine ecologist at Stanford University who was not involved in the research. In addition, research suggests that reducing water pollution and other stresses may make reefs more resilient to the shocks of climate change, says Nancy Knowlton, coral reef biologist at the National Museum of Natural History of the Smithsonian Institution, which was also not involved.
Corals are colonies of filtering invertebrates of millimeter size. Each of these so-called polyps contributes to the skeleton and harbors photosynthetic algae. In return, the microorganisms provide most of the polyp’s nourishment. In times of stress, especially when the water gets too hot, the polyp ejects algae and the coral turns white, which is why the state of stress is called coral bleaching. Once the water temperature returns to normal, all polyps that are not starved to death will harbor another algae. It was believed that most corals would survive only if a heat wave lasted only 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, was at the forefront of a very serious and protracted bleaching event. A year earlier, she and her students had started a study of the 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 coral 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 hit and then subsided.
From May 2015, the temperature increased by about 1 ° C in 2 months. As expected, corals that harbored heat-sensitive algae bleached earlier than those that harbored the heat-tolerant kind of algae. And as the water continued to heat up, even heat-resistant algae was ejected. Also, no surprise.
The “dropper” on Kiritimati, Baum says, was that many brain and star corals were recovering from the bleaching while the water was still unusually warm. Until now, marine biologists have only seen bleached corals recover after the water has returned to its normal temperature. The unexpected recovery in Kiritimati offers new hope, Baum says, “because it means that even under prolonged heat waves, there is a way forward for some of them.”
An unusual feature of recovery is that brain coral that started with heat sensitive algae had a higher survival rate (82%) than coral that started with heat tolerant algae (25%), reports the team today in Nature communications.
This discovery is surprising and “super interesting,” says Madeleine Van Oppen, a coral geneticist at the University of Melbourne, who was not involved in the work. The hope was that heat-tolerant algae would be better suited to help corals survive a heat wave, Baum says. But during a longer heat wave, it might be more beneficial to start with heat-sensitive algae, says lead author Danielle Claar, now a post-doctoral fellow at the University of Washington in Seattle. This is because these algae provide the coral host with more food than heat tolerant algae, thus providing them with greater reserves to survive bleaching.
The quality of the water could influence the choice of the algal partner. Because heat-tolerant algae also tend to be generally more resistant to stress, they can help corals survive in polluted waters. Kiritimati corals with heat-tolerant algae tend to be closer to larger villages, where the water contains excess sediment, sewage, and other types of pollution. The more remote parts of the reef have cleaner water, and corals are more likely to live with heat-sensitive algae. In addition to more energy stores, it’s possible that corals living in cleaner water have a more robust immune system or other factors, the team notes.
There has been debate over whether local conditions, such as pollution and overfishing, impact a reef’s ability to survive heat waves, Baum says. Some researchers have concluded that local conditions do not matter. “This article makes it clear that the argument is wrong, at least for those corals in this location,” Knowlton says. “Healthy local conditions were actually very important for the survival of the corals.”
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