Submitted to: Federation of European Microbiology Societies Microbiology Letters
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/15/2007
Publication Date: 5/16/2007
Citation: Richardson, L., Sekar, R., Meyers, J.L., Gantar, M., Voss, J.D., Kaczmarsky, L., Remily, E.R., Boyer, G.L., Zimba, P.V. 2007. The Presence of the Cyanobacterial Toxin Microcystin in Black Band Disease of Corals. Federation of European Microbiology Societies Microbiology Letters 272:182-187. . Interpretive Summary: Worldwide corals are declining due to nutrient enrichment and disease. Little is known of the biota inhabiting the exterior of corals suffering from black band disease. Black band samples were collected from sites in the Caribbean Sea and analyzed for biotic constituents. The presence of microcystin-like compounds was confirmed in black band samples. Large scale culturing experiments are underway to identify the exact toxin structure.
Technical Abstract: Black band disease (BBD) of corals consists of a pathogenic consortium of microorganisms of four physiological functional groups: phototrophs, heterotrophs, sulfate reducers, and sulfide oxidizers. Together, using a combination of behavioral and physiological strategies, the members of the BBD consortium interact synergistically to produce an extreme chemical microenvironment within the band, which includes two substances that are toxic to corals: sulfide and the cyanobacterial toxin microcystin. The band itself, normally less than 1 mm thick, consists of a dense matrix of filamentous and unicellular microorganisms that remain closely associated due to photophobic motility responses of the dominant cyanobacteria. As a result, high concentrations of chemicals produced physiologically within the band (such as oxygen and sulfide) accumulate due to a combination of diffusion kinetics and the presence of a diffusive boundary layer near the coral surface. The density of the band restricts light penetration to within the top 200-300 µm, allowing development of a permanent anoxic zone at the base of the band which results from both oxygen consumption and sulfate reduction. This zone is toxic to coral tissue, and the sulfide, in addition to poisoning corals, excludes other potentially competitive microorganisms from the organic and nutrient rich black band environment. Additionally, BBD cyanobacteria produce microcystins, a class of toxin that can kill coral tissue even when sulfide production is turned off using an inhibitor. The chemical environment of the band is dynamic, and contains a vertically migrating interface of sulfide and oxygen whose position depends on light intensity. With the exception of the bottom, anoxic/sulfide rich zone of the band, most of the band fluctuates between oxic and anaerobic/reducing conditions throughout the day and night. Thus, in addition to producing and tolerating a toxic environment, BBD microorganisms must be metabolically flexible and able to switch between metabolic, energy yielding pathways.