Black band disease



Black band disease is characterized by complete coral tissue degradation due to a pathogenic microbial consortium that appears as a dark red or black migrating microbial mat. The mat is present between apparently healthy coral tissue and freshly exposed coral skeleton.

Appearance
The band color may be blackish brown to red depending on the vertical position of a cyanobacterial population associated with the band. The vertical position is based on a light intensity-dependent photic response of the cyanobacterial filaments, and the color (due to the cyanobacterial pigment phycoerythrin) is dependent on the thickness of the band. The band is approximately 1 mm thick and ranges in width from 1 mm to 7 cm. White specks may be present on surface, at times forming dense white patches. The pathogenic microbial mat consortium moves across coral colonies at rates from 3 mm to 1 cm/day. Tissue death is caused by exposure to an anoxic, sulfide-rich microenvironment associated with the base of the band.

Composition
The black band microbial consortium consists of an assortment of photosynthetic and non-photosynthetic bacteria that co-exist synergistically. The consortium has three functionally and physically dominant members as well as numerous heterotrophic members whose role in the disease is as yet unknown. The three functionally dominant members are populations of cyanobacteria and sulfide-oxidizing and sulfate-reducing bacteria. The black band disease microbial consortium is structurally and functionally identical to cyanobacterial-dominated microbial mats found in other illuminated, sulfide-rich environments (Carlton and Richardson, 1995; Richardson et al., 1997).

Taxonomy
Several species of cyanobacteria have been found associated with black band disease (Frias-Lopez et al.,2002; Cooney et al., 2002), the most well-known of which is Phormidium corallyticum (Rützler and Santavy, 1983). Sulfide-oxidizing bacteria, dominated by Beggiatoa spp. (Ducklow and Mitchell, 1979), are present in well-developed bands and exhibit visible vertical migrations within the band matrix (Richardson, 1996; Viehman and Richardson, 2006). When present on the band surface Beggiatoa appears white due to intracellular inclusions of stored elemental sulfur. Sulfate-reducing bacteria dominated by Desulfovibrio spp. (Ducklow and Mitchell, 1979; Frias-Lopez et al.,2002; Cooney et al., 2002) are present at the base of the band and are responsible for producing high concentrations of sulfide within the band matrix (Carlton and Richardson, 1995). Light microscopic observation of black band reveals motile (gliding) filaments of P. corallyticum that are 4 mm wide, with one round end and one narrow (sharply tapering) end. Also present are gliding Beggiatoa filaments (1-4 mm wide) that are non-pigmented but contain highly refractive intracellular granules of elemental sulfur. Numerous gram-negative bacteria (small rods) are also present but not identifiable using light microscopy. The bacterial population has been characterized using molecular techniques and was found to contain over 500 species of bacteria that are different from bacterial communities found in the water column, healthy coral tissue, or dead coral skeleton (Frias-Lopez et al., 2002; Cooney et al., 2002). The functional role of this diverse population of bacteria is not known.

Black band disease affects 42 species of coral in a worldwide distribution. The only known reservoir is within cyanobacterial biofilms that are present on sediments in depressions of healthy black band disease susceptible corals (Richardson, 1997).