DNA testing identifies suspect bacteria in coral reef disease

May 21, 2002

SALT LAKE CITY -- Using molecular microbiology techniques, scientists are a significant step closer to understanding and identifying the deadly microbes responsible for the mysterious black band disease that is destroying the world's coral reef ecosystems.

One of the most destructive and widespread of the coral diseases, black band disease is characterized by a ring-shaped bacterial mat that rapidly migrates across a coral colony, leaving dead tissue and bare coral skeleton in its wake.

By sequencing the entire 16S rRNA gene - a genetic fingerprint found in all living organisms - geologist Bruce Fouke and postdoctoral microbiologist researchers Jorge

Frias-Lopez and George Bonheyo at the University of Illinois at Urbana-Champaign have now identified the main bacteria associated with the black band bacterial mat causing the disease.

"The black band microbial mat is dominated by large filamentous cyanobacteria that were previously optically identified as Phormidium corallyticum," Fouke said. "Based on the gene sequence analyses, however, we have identified at least three different closely related species of cyanobacteria associated with the bacterial black band mat in different ocean basins around the world."

In earlier work, Fouke and his colleagues showed that the bacteria inhabiting the

black-band disease microbial mat were different from those found either in healthy coral tissue or in the overlaying seawater. That work, which was based upon partial sequencing of bacterial 16S rRNA genes, appeared in the May 2002 issue of Applied and Environmental Microbiology.

To make a more accurate identification of the cyanobacterium inhabiting the black band biomat, the researchers recently collected samples from infected corals on the reef tracts of Papua New Guinea in the Indo-Pacific Ocean and the Netherlands Antilles in the Caribbean Sea. Then they extracted the microbes' 16S rRNA gene in their molecular geomicrobiology laboratory and completed the sequencing at the W.M. Keck Center for Comparative and Functional Genomics on the Urbana campus.

"Results from 57 sequences taken from 12 different bacterial mats show that, except in one case, a unique sequence was obtained from all infected coral species and in all locations," Fouke said. "Finding the same dominant organism in two widely separated ocean basins indicates that the pathogenic development of black band disease is a globally consistent phenomenon."

There has been considerable controversy as to whether black band disease is caused by environmental stress or is an infectious disease, or both, Fouke said. "Factors thought to contribute to the disease are increases in sea surface temperature and possibly the dumping of sewage and other pollutants onto reef systems."

In their earlier study of black band disease in corals off Curacao, the researchers found several organisms, including Arcobacter and Campylobacter, which are human pathogens that could be a direct link to raw human sewage.

"Although the health of the coral reef is directly correlated with the presence of pollution, we have not found a clear linkage between the frequency of black band infected corals and the geographic position of more polluted areas on the islands," Fouke said. "The human-derived bacteria are present in the black band microbial mat, but we do not yet know the activity or potential role of these bacteria in the development of the disease."

Cyanobacteria are a unique group of photosynthetic bacteria that cannot live without light. As they grow and multiply, these bacteria create a dense, ropy network - an infrastructure, of sorts - that other bacteria can invade and colonize.

"Black band disease is not simply one organism going in and doing dirty work," Fouke said. "The disease must be viewed as a whole consortium of bacteria that move in and create a unique physical and chemical microenvironment where the pathogens can take up residence. Then the wholesale destruction of coral tissue begins."

Before a disease can be successfully treated, the pathogens that are present must first be identified. "We're not saying that this ropy cyanobacteria is indeed the pathogen," Fouke said. "But we are saying that its colonization is a necessary first step for the bacterial infrastructure development of black band disease."
Frias-Lopez will present the team's findings at a meeting of the American Society for Microbiology, to be held in Salt Lake City May 19-23. The Office of Naval Research, the American Chemical Society and the Geological Society of America supported this work.

University of Illinois at Urbana-Champaign

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