Bacteria appear to play important roles in both causing and preventing the bleaching of the coral Oculina patagonica

Vibrio shiloi was reported in 1996 to be the causative agent of bleaching of the coral Oculina patagonica in the eastern Mediterranean Sea, and the mechanisms of infection and bleaching were studied intensively from 1996 to 2002. Sometime between 2002 and 2004, O. patagonica became resistant to V. shiloi yet continued to bleach seasonally. These findings raised 2 questions: (1) What is the current cause of the bleaching? (2) How did the coral become resistant to the pathogen V. shiloi? Data presented here indicate that a bacterium, or bacteria, other than V. shiloi is currently responsible for the bleaching, because the antibiotic nalidixic acid inhibited heat-induced bleaching in aquaria. Raising the temperature of O. patagonica in aquaria to 31°C caused 24 of 28 coral fragments to bleach. When the O. patagonica was exposed to the same temperature increase but treated with nalidixic acid for 24 h when the temperature reached 28°C, only 10 of 34 fragments bleached. Antibiotic-treated corals had 5.3-fold more zooxanthellae at the end of the heating experiment than the no-antibiotic control, supporting the visual bleaching observations. V. coralliilyticus strain EM1 is a candidate bleaching pathogen because it was isolated from bleached corals, caused bleaching in 5 of 6 antibiotic-treated corals and could be recovered from the experimentally bleached corals. In support of the coral probiotic hypothesis, anti biotictreated corals became sensitive to V. shiloi infection and bleaching (10 of 12 fragments), presumably by killing beneficial bacteria that inhibited V. shiloi growth. Bacterial strain EM3, isolated from O. patagonica fragments, may play a role in preventing V. shiloi infection of corals because it releases a diffusible material that specifically inhibited the growth of V. shiloi in vitro.