Scientists have solved the puzzle of a mysterious disease that 3 years ago wiped out a third of the coral at some reefs off the Florida Keys. The culprit, unmasked in the current Nature, is a previously unknown bacterium that--unlike other coral pathogens that operate as bacteria-algae teams--does its dirty work alone.
Florida's reefs have been hard hit by many new and virulent diseases over the last decade (ScienceNOW, 18 November 1997 ). When the elliptical star coral (Dichocoenia stokesi) began dying at an unprecedented rate of 2 centimeters per day in 1995, scientists could only name the condition "Common plague type 2." During the outbreak, the disease spread over a 400-kilometer stretch along Florida's coast and hit 16 other coral species. "In four areas that we repeatedly sampled, 38% of the [elliptical corals] were completely killed in 11 weeks or less," says lead author Laurie Richardson, a microbiologist at International University in Miami. Although the disease has not recurred in many areas initially hit, it has since spread to other regions off the Florida coast.
To figure out what causes the mystery disease, Richardson and her colleagues examined bacteria from both healthy and diseased elliptical corals. While healthy coral hosted a mix of bacteria, diseased coral was infected with a single species. When the researchers grew healthy coral together with that bacteria, it died of plague type 2. By comparing the bacteria's metabolic pathways and its RNA to that of other genera, the scientists classified it as a new species in the genus Sphingomonas.
"This is important research to identify the causative agents of these environmental scourges," says marine ecologist James Porter of the University of Georgia, Athens. Currently there's no treatment for the plague type 2, but Porter hopes the new results will give scientists the tools to investigate the prevention and treatment of this and other coral diseases.