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Corals that stay cool beat the heat: species that overreact to loss of algae are more likely to die.

Some corals seem to have a suicide pact with the symbiotic algae living inside them. But others are able to survive the demise of their photosynthetic friends, and now scientists think a suicide protein may explain the difference between the two groups.

When waters warm, some corals overreact to distress signals sent by resident algae, researchers 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 are able to 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 then leave, die or get eaten.

In the new study, a type of coral from the Red Sea called Seriatopora bystrix (also 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 Stylophorapistillata had a meltdown and died after only a week in the heat. Levels of caspase protein made by the dying 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.

"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 probably be those that populate coral reefs in the future. "We'll certainly lose the rest in the short term," Falkowski says, "and lose them in abundance."

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Title Annotation:Environment
Author:Saey, Tina Hesman
Publication:Science News
Geographic Code:1USA
Date:Jul 2, 2011
Words:355
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