Bleaching of the coral Oculina patagonica by Vibrio AK-1

Bleaching in stony corals is the result of a disruption of the symbiosis between the coral hosts and photosynthetic microalgal endosymbionts (zooxanthellae). Coral bleaching may be induced by a variety of environmental stimuli, including increased seawater temperature. Large-scale bleaching episodes have been suggested to be linked to global warming. We have discovered that coral bleaching, in this case, bleaching of the Mediterranean coral Oculina patagonica, is caused by a bacterial infection and that water temperature is a contributing factor. The causative agent, Vibrio AK-1, was present in 28 bleached O. patagonica examined, but absent from 24 healthy (unbleached) corals. The Vibrio sp. was isolated in pure culture, characterized microbiologically, and shown to cause bleaching when inoculated onto unbleached corals. An increase in seawater temperature may influence the outcome of bacterial infection by lowering the resistance of the coral to infection and/or increasing the virulence of the bacterium. When inoculated with 106 Vibrio AK-1 ml^-1 at 26°C, bleaching began at around 10 d and affected more than 80% of the corals after 44 d. Bleaching did not occur under the same conditions in the presence of antibiotics or if the temperature was lowered to 16°C.