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Temperature, Bacterial Adhesion, and Coral Bleaching
Toren, A., Landau, L., Kushmaro, A., Loya, Y. and Rosenberg, E. 1998. Effect of temperature on adhesion of Vibrio Strain AK-1 to Oculina patagonica and on coral bleaching. Applied and Environmental Microbiology 64: 1379-1384.

What was done
The authors of this study examined the effect of seawater temperature on the bleaching of the coral Oculina patagonica, induced by the bacterial agent Vibrio AK-1, in aquaria, as well as its effect on the adhesion of Vibrio strain AK-1 to its coral host.

What was learned
Coral bleaching was closely correlated with seawater temperature. Inoculation of 107 Vibrio strain AK-1 organisms into flasks containing 20ml of seawater at 25C and a fragment of Oculina patagonica resulted in net levels of bacterial adhesion to the coral of 45, 78, and 84% after 2, 6, and 8 hours, respectively. The adhesion was inhibited 65% by 0.001% D-galactose and 94% by 0.001% methyl--D-galacto-pyranoside. After the incubation of Vibrio strain AK-1 with the coral for 6 hours, 42% of the input bacteria were released from the coral with 0.01% methyl--D-galacto-pyranoside. Adhesion of the bacteria to the coral appeared to be temperature related, because bacteria grown at 16C did not adhere to corals grown at 25C, yet bacteria grown at 25C adhered to corals grown at 16C.

What it means
This study demonstrates the temperature dependence of a specific bacteria-induced bleaching of the coral Oculina patagonica. It also illustrates that adhesion of the bacteria is temperature related. However, because the bacteria grown at 25C adhered somewhat better to corals grown at 25C than those grown at 16C, the authors stated that "it is possible that the corals also play a part in the effect of temperature on adhesion."

The authors note that similar correlations between seawater temperature and coral bleaching have led many people to speculate that increased seawater temperature, resulting from global warming or El Nio events, is the direct cause of coral bleaching. However, they state that "there are a number of weak points in this argument," making reference to the fact that correlation does not imply causation. In support of their statement, the authors cite a number of studies of coral bleaching events not associated with any major sea surface temperature anomalies. In particular, they note that the "extensive bleaching in the Great Barrier Reef during the summer of 1982 was not associated with any major sea surface temperature increase." Finally, they point out that "several authors have reported on the patchy spatial distribution and spreading nature of coral bleaching" which is inconsistent with the seawater temperature-induced coral bleaching hypothesis. Instead, they state that "the progression of observable changes that take place during coral bleaching is reminiscent of that of developing microbial biofilms."

Reviewed 1 June 1999