BOSTON--A hot bath may be just the remedy for a cold or the flu, but for corals, at least, it could mean death. Researchers have linked the summertime warm-up in the Mediterranean to coral bleaching, a whitening that occurs when stressed corals expel the algae they depend on for food. The finding, announced here yesterday at the annual meeting of the Society for Integrative and Comparative Biology, fuels concern that global warming will step up coral bleaching.
Two years ago, Ariel Kushmaro, Yosi Loya, and their colleagues at Tel Aviv University in Israel demonstrated that the stress from infection by the so-called Vibrio bacterium, a relative of the cholera pathogen, triggered bleaching in a Mediterranean coral. During their work, the researchers noticed that a type of Mediterranean coral turns white during the hottest stretch of summer and tends to recover its greenish tint during cooler times of the year. When they checked, Vibrio was present in all the summertime samples of the bleached coral, but in none of the unbleached coral. Indeed, after sampling the now-healthy reef again last winter, the researchers found no Vibrio.
To confirm that Vibrio attacks only at higher temperatures, the researchers tried to infect coral in the lab. They succeeded easily when the water was 29 degrees Celsius--the average summer temperature in the Mediterranean. At 20 degrees C, only 30% of the coral had turned white, and at the winter temperature of about 16 degrees C, there were no signs of infection at all, says Kushmaro--even when flooded with high doses of bacteria. At that temperature, they found, the Vibrio failed to stick to the coral surface.
The findings could help explain the apparent rise in coral disease in the Caribbean, which has been warmer than usual in the last decade, says Howard Lasker, a coral reef scientist at the State University of New York, Buffalo. But because the Caribbean doesn't cool off as much as the Mediterranean during winter, Kushmaro adds, "there's no chance for the [Caribbean] corals to recover."