Great Barrier Reef at risk due to Black Band Disease

Wednesday, March 28th, 2012 6:33:00 by

Researchers are saying that the Great Barrier Reef is at risk due to the Black Band Disease which can destroy the entire coral colonies

Researchers of the Max Planck Institute for Marine Microbiology along with Australian colleagues, have examined corals from the Great Barrier Reef affected by the Black Band Disease and identified the critical parameters that allow this prevalent disease
to cause wide mortality of corals around the world. Corals infected with Black Band show a characteristic appearance of healthy tissue displaced by a dark front, the so called Black Band, which leaves the white limestone skeleton of the coral animal exposed.
The dark front is commonly one to two centimeters broad and consists of a complex microbial community among which there are phototrophic cyanobacteria, sulfur oxidizing bacteria and sulfate reducing microorganisms. The corals and their endosymbiotic algae
are struck by three stress factors at once: toxic sulfide, anoxia, and a low pH at the boundary of the bacterial mat and the coral tissue.

Tissue lesions were investigated by scientists for oxygen and sulphide pH.

“In diseased coral tissue two zones develop: A phototrophic zone at the top in which the cyanobacteria produce oxygen and a lower anoxic zone in which the bacteria degrade the necrotic coral tissue. Sulfide is formed in the degradation process”, Martin Glas
of the Max Planck Institute in Bremen explains the results. “In tissue that is only slightly infected the zonation is not nearly that strong. Usually we could not detect sulfide, and oxygen penetrated deep into the bacterial mat.”

At the front of the dark zone the conditions are particularly detrimental for the corals. The increased sulfide concentration around the necrosing tissue and the resulting decrease in oxygen leads to the spreading of the lesions to the surrounding, healthy
tissue; a positive feedback that causes rapid migration of the Black Band Disease.

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