Elkhorn coral, once the most abundant coral species in the Caribbean, has recently been devastated by the white pox disease, one of the most destructive coral afflictions known. Now researchers have shown that a common human intestinal bacterium is behind the scourge.
Elkhorn coral, called the "redwood of the coral forest," is renowned for its massive, branching structure, which provides food and shelter for hundreds of reef species. In 1996, researchers discovered irregularly shaped white lesions on elkhorn corals off Key West, Florida, that were killing the thin layer of living tissue that sheaths the limestone skeleton of coral colonies. Since then, white pox has been found throughout the Caribbean.
To find the cause, a research team led by James Porter and Kathryn Patterson of the University of Georgia, Athens, collected tissue samples from healthy and pox-infected corals from the Florida Keys, the Bahamas, the U.S. Virgin Islands, and Caribbean Mexico. In the laboratory they isolated 221 strains of microbes from the diseased samples and picked out the four most prevalent organisms. Porter's team then exposed healthy elkhorn corals in waters in the Bahamas to each strain. Serratia marcescens, a bacterium found in the feces of humans and other animals, causes the lesions, the team reports in the 25 June issue of the Proceedings of the National Academy of Sciences. "When we started this research, we assumed we were dealing with an undescribed marine pathogen," Porter says. "We had no idea it would turn out to be one of the most common bacteria known to man." Although the source of the bacteria has not yet been determined, human waste is treated in septic tanks in the Florida Keys and elsewhere in the Caribbean--a method that does not wipe out all bacteria--and effluents may wash into the open sea.
Coral researcher Thomas Goreau of the Global Coral Reef Alliance in Chappaqua, New York, says the team has convincingly shown that Serratia causes white pox. This is the second coral disease shown to be linked to bacteria found in sewage, he notes; last month, researchers reported that sewage-associated bugs cause black band disease, another coral scourge on the rise (ScienceNOW, 22 May).