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Coral
Disease Identification and Information Main | Ecology | Microbiology of Pathogens | Susceptible Species | Microbial Consortium | Cause of Coral Death | Verification | Management Disease Overview 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. 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. 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, sulfate-reducing, and sulfide-oxidizing 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) . 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, in
press). 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)
.
Black Band Disease Black band disease is characterized by a dark ring, or band, that separates apparently healthy coral tissue from freshly exposed coral skeleton. It migrates across coral colonies completely degrading coral tissue. A closeup view reveals that the band is composed of numerous microorganisms, here revealed as a dark community of phosynthetic cyanobacteria (“bluegreen algae”) and white specks of sulfur bacteria. *** Click images to view full sized high resolution image ***
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