Mellow corals beat the heat

Species that overreact to distress signals from algae more likely to succumb to warming

If the symbiotic algae living inside some corals jumped off a bridge, the corals would follow. But other corals are able to resist such lemminglike behavior, and now scientists think a suicide protein may explain the difference between the two groups.

Some corals overreact to distress signals sent by resident algae when waters warm, researchers in New Jersey and Israel report online June 2 in the Proceedings of the National Academy of Sciences. The overreactors ramp up production of an executioner protein called caspase and eventually commit cellular suicide. Corals that survive warming start out with high levels of caspase but then quickly decrease the amount of the protein, the researchers found.

The study “adds critical data to help figure out how coral bleaching happens,” says Stephen Palumbi, a marine population biologist at Stanford University who was not involved in the research. Corals bleach when their algae become stressed by warming water, pollution or other factors and either leave, die or get eaten.

Scientists already knew that some corals stand up to bleaching better than others. Researchers even suspected that a violent form of cell suicide called apoptosis was involved. Palumbi likens those nuggets of knowledge to a disassembled car. “It’s as if the elements of the engine, the drivetrain and the wheels were all there, but there was nothing connecting them,”  he says. “They’ve filled in the details, connected the pieces.”

In the new study, a type of coral from the Red Sea called Seriatopora hystrix (known as bird’s nest or needle coral) bleached but stayed alive for six weeks in water that was warmed by 6 degrees Celsius, researchers led by Paul Falkowski of Rutgers University and Dan Tchernov of the University of Haifa in Israel found. In contrast, another Red Sea coral called Stylophora pistillata had a meltdown after only a week in the heat. Levels of caspase protein made by the melting coral shot up to six times normal levels, while levels of the protein dropped in the heat-resistant coral.

Using two other wilting coral species, the researchers found that adding a chemical that inhibits caspase activity could stop the corals from committing suicide.

 “It’s not practical for people to go out in their boats and sprinkle caspase inhibitors over coral reefs,” says Cliff Ross, a marine biologist at the University of North Florida in Jacksonville. But the findings are important for helping scientists better understand why some corals die when the heat is on, he says.

The results indicate that “potentially a single gene might control the fate of a coral,” Falkowski says. The coral’s survival depends upon whether its cells initiate a self-destruct program in response to its algae’s stress signals. Species that can stand the heat will likely be those that populate coral reefs of the future. “We’ll certainly lose the rest in the short term, and lose them in abundance.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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