Black band disease is acoral disease in whichcorals develop a black band. It is characterized by complete tissue degradation due to a pathogenicmicrobial consortium.[1] The mat is present between apparently healthy coral tissue and freshly exposed coral skeleton.
Black band disease was first observed on reefs in Belize in 1973 byA. Antonius, who described the pathogen he found infecting corals asOscillatoria membranacea, one of thecyanobacteria.[2] 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 thecyanobacterial filaments, and the color (due to the cyanobacterial pigmentphycoerythrin) is dependent on the thickness of the band. The band is approximately 1 millimeter (0.04 in) thick and ranges in width from 1 millimeter (0.04 in) to 7 centimeters (2.8 in). White specks may be present on surface, at times forming dense white patches. The pathogenicmicrobial mat moves across coral colonies at rates from 3 millimeters (0.12 in) to 1 centimeter (0.4 in) a day. Tissue death is caused by exposure to ahypoxic,sulfide-rich microenvironment associated with the base of the band.
The black band microbial consortium consists of an assortment ofphotosynthetic and non-photosyntheticbacteria that co-existsynergistically. The consortium has three functionally and physically dominant members as well as numerousheterotrophic members whose role in the disease is as yet unknown. The three functionally dominant members are populations of cyanobacteria and sulfide-oxidizing andsulfate-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[3][4]
Several species of cyanobacteria have been found associated with black band disease,[5][6] the most well-known of which isPhormidium corallyticum.[7] Sulfide-oxidizing bacteria, dominated byBeggiatoa spp.,[8] 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 surfaceBeggiatoa appears white due to intracellular inclusions of stored elementalsulfur. Sulfate-reducing bacteria dominated byDesulfovibrio spp.[5][6][8] are present at the base of the band and are responsible for producing high concentrations of sulfide within the band matrix.[9]Light microscopic observation of black band reveals motile (gliding) filaments ofP. corallyticum that are 4 mm wide, with one round end and one narrow (sharply tapering) end. Also present are glidingBeggiatoa filaments (1–4 mm wide) that are non-pigmented but contain highly refractive intracellular granules of elemental sulfur. Numerousgram-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.[5][6] 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 cyanobacterialbiofilms that are present on sediments in depressions of healthy black band disease susceptible corals.[4]
Black band disease significantly affects boulder star coral in the reef ecosystem, allowing for more staghorn coral to grow.
