Is sewage destroying coral? (Marine Science).

About 38% of the coral reefs in the Florida Keys have died in the past five years, according to marine ecologists Kathryn Patterson and James W. Porter of the University of Georgia, Athens, and their colleagues. Some experts have blamed global warming or overfishing. Now Patterson and Porter have round that bacteria and viruses found in human sewage are responsible for some of the coral decline--and another team of scientists, led by environmental microbiologist Erin Lipp of the University of South Florida in St. Petersburg, says these microbes may be more widespread than thought. Although nonhuman sources of the microbes have not been ruled out, these findings raise the question of whether Florida needs to be doing more to contain its sewage.

Patterson and Porter have discovered that white pox disease, which targets elkhorn corals (Acropora palmata), is caused by the fecal bacterium Serratia marcescens. The magnificent branching elkhorn corals live close to shore and are "the giant redwoods of the reef," says Porter. Since white pox disease was first documented in the Florida Keys in 1996, it has killed 85% of the areas elkhorn corals. White pox disease also has killed elkhorn corals in Jamaica, Belize, the Bahamas, and other Caribbean locations. White pox disease is characterized by white lesions. The coral's tissue is lost, and its limestone skeleton is exposed. The disease is highly contagious, and lesions can grow as rapidly as 2.5 square centimeters a day.

Patterson and Porter's team collected mucus samples from the surface of corals infected with white pox disease. The mucus layer is composed largely of polysaccharides that slough off periodically. They isolated bacteria from the mucus and grew it in pure culture in the laboratory. Healthy elkhorn corals were then infected with bacterial isolates. One bacterium caused the corals to contract white pox disease. A combination of DNA sequencing and standard microbiological tests identified the bacterium as S. marcescens. The study appears in the 25 June 2002 issue of Proceedings of the National Academy of Sciences.

Lipp's study, published in the July 2002 issue of the Marine Pollution Bulletin, put a new twist on measuring fecal contamination in coastal waters. Traditionally, such contamination is monitored by obtaining seawater samples at different depths. Instead, in a proof-of-concept experiment, Lipp and her team collected samples not only of seawater, but also of the outer mucus layer from 13 coral colonies in the Florida Keys within a few miles of shore.

Common fecal bacteria such as enterococci were identified in two-thirds of the coral mucus samples, and 93% contained viruses found in sewage. Fecal microbes appear to "attach to the mucus surface layer and survive," says Lipp. She speculates that when corals become contaminated with fecal microbes, it may set up hospitable conditions that allow opportunistic infections to take hold.

However, the seawater samples did not contain detectable amounts of fecal microbes, partly because the size of the ocean allows for great dilution and partly because the microbes show a preference for attaching to coral mucus. This indicates that detecting microbes just by sampling the water is not as accurate as once believed. Lipp says she and her team will repeat the methods in waters progressively farther from shore to see how far out fecal microbes reach.

Whereas Patterson and Porter's findings are specific for one species of coral and one bacterium, Lipp's data "show without a shadow of a doubt that fecal microbes are resident on corals and are widespread," Porter says. Other tracer studies have confirmed that wastewater from septic systems and cesspools migrates to coral reefs near shore. According to county statistics, the Florida Keys are home to 24,000 septic systems and as many as 10,000 cesspools that rest on porous, leaky limestone. Building sewage treatment plants to prevent sewage leakage is a "local remedy that can and should be implemented," says Porter.