Abstract
Black band disease (BBD) is a complex, polymicrobial disease that consists of cyanobacteria, sulfide-oxidizing and sulfate-reducing bacteria (SRB), and heterotrophic bacteria. The cyanobacterium Roseofilum reptotaenium has been implicated as the primary pathogen of BBD, but other consortium members may be secondary pathogens that are necessary to the development of the disease. It is known that populations of the sulfate-reducing bacterium Desulfovibrio are present in BBD and that these populations generate sulfide within the band as a byproduct of dissimilatory sulfate reduction. It is also known that exposure of healthy corals to sulfide leads to cell lysis and coral tissue death. Previous work showed that when freshly collected BBD, which easily infects healthy corals, is exposed to sodium molybdate, a specific inhibitor of sulfate reduction, infection does not occur. In this study we examined the effect of sodium molybdate on infection of corals by a unialgal culture of R. reptotaenium. Coral fragments of Montastraea cavernosa and Siderastrea siderea were transferred into two experimental aquaria, one a control with only artificial seawater (ASW) and the second containing ASW and 2mM sodium molybdate. Small mats of cultured R. reptotaenium were inoculated onto the surface of experimental coral fragments. Both M. cavernosa (n = 6) and S. siderea (n=4) became infected and developed BBD-like infections in the control tank, while there were temporary attachments to, but no successful infection of M. cavernosa (n=3) or S. siderea (n=2) in the experimental tank containing sodium molybdate. The results of this study reveal that a secondary pathogen is essential to the infection process and development of BBD in scleractinian corals. Specifically, SRB such as Desulfovibrio are required for the development of BBD on the coral host. This is the first step in understanding the roles of secondary pathogens in a complex, polymicrobial coral disease. Rev. Biol. Trop. 62 (Suppl. 3): 1-9. Epub 2014 September 01.
Highlights
Coral diseases were first reported in the 1970s on reefs of the Caribbean (Antonius, 1976; Garret & Ducklow, 1975; Dustan, 1977; Gladfelter, Gladfelter, Monahan, Ogden & Dill, 1977)
The inoculum of R. reptotaenium filaments was observed to attach to the surface of the coral tissue within seven days (Fig. 2)
The results of this study revealed that blocking the growth of sulfate reducing bacteria (SRB) using the metabolic inhibitor Na molybdate prevented the development of black band disease (BBD) when using unialgal cultures of the BBD cyanobacterium R. reptotaenium as the inoculum
Summary
Coral diseases were first reported in the 1970s on reefs of the Caribbean (Antonius, 1976; Garret & Ducklow, 1975; Dustan, 1977; Gladfelter, Gladfelter, Monahan, Ogden & Dill, 1977). The inoculum can be observed to attach to the coral surface, penetrate and invade tissue, and develop into a BBD lesion that migrates across the coral surface, lysing coral tissue and eventually killing the coral, leaving a bare coral skeleton In this model the inoculum contains a fully developed BBD-consortium: cyanobacteria, sulfide-oxidizing and sulfate-reducing bacteria, as well as associated heterotrophic bacteria. Roseofilum reptotaenium is a gliding, filamentous, phycoerythrin-rich cyanobacterium detected in BBD mats of tropical and sub-tropical coral reefs around the world (Miller & Richardson, 2011) This cyanobacterium forms the matrix of the mat and is adapted to the harsh BBD conditions because of its ability to conduct sulfide-insensitive oxygenic photosynthesis (Myers, Sekar & Richardson, 2007). Roseofilum reptotaenium can perform sulfide-insensitive oxygenic photosynthesis in the presence of 0.5mM sulfide (Myers & Richardson, 2009), comparable to sulfide levels measured in intact Caribbean BBD (Carlton & Richardson, 1995)
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