Coral reefs are deteriorating at alarming rates around the world, and one of the most insidious causes is black band disease (BBD)--a fast-spreading and lethal bacterial infection. Once the coral succumbs, algae, sponges, and fish break down massive heads into sand. Now, for the first time, researchers have identified the cast of bacteria associated with the disease; they have met the enemy and found it may be us.
First spotted on reefs off Belize and Florida in 1972, BBD has since been observed worldwide. The disease takes its name from a black mat of bacteria that is colonized by dozens of other microbes. Previous studies have linked the disease to elevated water temperatures and runoff laden with sediments, toxins, or sewage. But scientists haven't been able to pin down exactly what causes the disease.
To search for suspects, geologist Bruce Fouke and microbiologists Jorge Frias-Lopez and George Bonheyo at the University of Illinois, Urbana-Champaign, collected more than 4700 samples of healthy, diseased, and dead coral, along with water samples, from reefs near the Caribbean port of St. Annabaai, Curaçoa, and pristine waters off Papua New Guinea in the Indo-Pacific. They took a microbial census by sequencing a gene that differs from species to species. The stringy bacteria that form the black band mat turned out to be several closely related species of cyanobacteria, the team reports in the May issue of Applied and Environmental Toxicology and in a presentation 21 May at the 2002 Meeting of the American Society for Microbiology.
They also found that coral samples with BBD included several species of pathogenic bacteria found in sewage. "The human-derived bacteria are present exclusively in the black band microbial mat, but we do not yet know their activity or potential role in the development of the disease," Fouke says. The diseased samples contained other microbes--ones associated with fish diseases, for example--that could also play a role in the disease.
The work is "ground-breaking," according to coral biologist Peter Edmunds of California State University, Northridge. The bacteria found in the dead coral samples may be the elusive pathogens causing the disease, he says. He cautions, however, that further research is needed to prove a cause-and-effect relationship. "It is premature to conclude that the 'smoking gun' for one coral disease has been found."