Since 1996, a disease known as whitepox has been decimating populations of coral, particularly in the Florida Keys. Now scientists writing in the current issue of the Proceedings of the National Academy of Sciences report that a bacterium commonly found in human feces may be to blame. Although the source of the coral-killing bacteria remains unconfirmed, the findings suggest that maintaining high standards of water quality is crucial to saving the reefs' remaining coral.

As one of the fastest-spreading coral diseases, whitepox can devour between two and 10 square centimeters of living reef tissue every day (see image). It specifically targets the species Acropora palmata, or elkhorn coral, and some Florida reefs have lost as much as 85 percent of their A. palmata. "These are the giant redwoods of the reef," says study co-author James W. Porter of the University of Georgia. "What used to be the most common coral in the Caribbean has now been recommended for inclusion on the endangered-species list."

Porter and his colleagues thus set out to determine what was causing the coral-killing plague. Initially expecting to find a unique marine pathogen, the scientists were surprised to discover that the culprit was in fact one of the most common bacteria known. The researchers developed cultures from samples of healthy coral and colonies afflicted with whitepox. They identified 221 bacterial strains, four of which appeared more frequently on diseased coral. But only one of them—Serratia marcescens, which resides in the guts of humans and other animals as well as in soil and water—is capable of causing whitepox in healthy coral samples, the team reports.

Elevated water temperatures, perhaps the result of global warming, have been implicated in another blight against coral—bleaching. Higher temperatures also increase the rate whitepox-induced coral loss and could be a root cause. "Warmer water depresses coral growth but increases bacterial growth," study leader Kathryn Patterson of the University of Georgia explains. "In combination, this domino effect could foretell disaster. There appear to be environmental changes occurring that may be making this nonpathogenic bacterium pathogenic."