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Common bacteria kills elkhorn coral off Florida keys, says UGA research team

University of Georgia

Populations of the shallow-water Caribbean elkhorn coral, Acropora palmata, are being decimated by white pox disease. Losses of living elkhorn coral in the Florida Keys typically average 85 percent. A team of scientific investigators, led by researchers from the University of Georgia, has identified the common fecal enteric bacteria, Serratia marcescens, as the cause of white pox.

The source of the bacteria that is killing the coral is still under investigation, but it can be found in the intestines of humans and other animals. It can also survive as a free-living microbe in both water and soil. This is the first time this common bacterium has been shown to cause the death of marine invertebrates.

The research was just published in the Proceedings of the National Academy of Sciences as an outcome of the Coral Reef Monitoring Project being conducted in the Florida Keys National Marine Sanctuary. The project is supported by the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Environmental Protection Agency (EPA). The Coral Reef Monitoring Project began in 1995 as a component of the sanctuary's water quality protection program. The Florida Keys National Marine Sanctuary protects 2,896 square nautical miles of seagrass beds, mangroves, hardbottom areas, and coral reefs, including elkhorn coral.

Elkhorn coral is an important Caribbean shallow water species, providing both food and shelter for many animals on the reef. Its massive branching form produces the highly complex three-dimensional structure upon which many other reef organisms depend. This structure frequently supplies the foundation upon which other species in the reef live.

"It is very sad that the one coral species affected is the magnificent branching elkhorn coral. These are the giant redwoods of the reef," said James Porter, professor of Ecology and Marine Sciences at the University of Georgia and research team leader. "What used to be the most common coral in the Caribbean has now been recommended for inclusion on the endangered species list."

The disease was first documented in 1996 on Eastern Dry Rocks Reef off Key West, Fla. in the Florida Keys National Marine Sanctuary and exclusively affects the elkhorn coral. Coral colonies affected by white pox disease show irregularly shaped white lesions which eventually kill the coral by consuming the thin layer of living tissue that covers a coral's limestone skeleton. This study has shown lesions growing as fast as 10.5 cm2 per day with an average rate of tissue loss of 2.5 cm2 per day, making it one of the most destructive coral diseases known. Tissue loss was greatest during periods of seasonally elevated temperature.

"Identification of this common bacterium as the cause of white pox means we cannot blame global warming as the main problem on coral reefs, but it all adds up," said Kathryn Patterson, a UGA Marine Sciences doctoral student and principal investigator who conducted her research under a cooperative training agreement while at the EPA's Gulf Ecology Division in Gulf Breeze, Fla. "Warmer water depresses coral growth but increases bacterial growth. In combination, this domino effect could foretell a disaster. There appear to be environmental changes occurring that may be making this non-pathogenic bacterium pathogenic."

According to Porter and Patterson, this disease has already killed more than 98 percent of the elkhorn coral on some reefs near Key West. The disease effects, compounded with additional stressors such as recent hurricanes, coral bleaching and ship groundings, have caused elkhorn coral populations to crash.

"When we started this research, we assumed that we were dealing with an undescribed marine pathogen. We had no idea that the culprit would turn out to be one of the most common bacteria known to man," said Porter. White pox affected coral has been found all over the Caribbean including Florida, the Bahamas, the U.S. Virgin Islands, and Caribbean Mexico.

The disease is extremely contagious. Nearest elkhorn coral neighbors were most likely to become infected. Once white pox appeared on a reef, it spread to all areas on the reef within one year and also spread rapidly between reefs. Most of the loss found by the UGA team occurred between 1996 and 2000 when the average loss of elkhorn coral on the Florida reefs studied was 85 percent.

"These results suggest that we may be killing the goose that lays the golden egg," said Porter. "Despite the trends in our data, we still remain hopeful that the surviving coral will repopulate the reef. We must maintain the highest possible water quality standards in the Florida Keys. These coral reefs are so beautiful and so important. We must do our best to protect them."

The EPA and the State of Florida, in consultation with NOAA, have developed a Water Quality Protection Program (WQPP) for the Florida Keys. NOAA incorporated the WQPP into its Final Management Plan for the Florida Keys National Marine Sanctuary. The purpose of the WQPP is to recommend priority corrective actions and compliance schedules addressing sources of pollution and to restore and maintain the chemical, physical, and biological integrity of the Sanctuary. These findings will bolster ongoing efforts to implement the WQPP and provide improved treatment of wastewater and stormwater in the Florida Keys.

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