21 June 2002
Reef
Killer Unmasked
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 Cambridge, MA, 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).
