Bacterial enhancement of bleaching and physiological impacts on the coral Montipora digitata

Bleaching affects corals worldwide. We studied the effects of high seawater temperature and bacterial challenges on bleaching, metabolism, and antioxidant defenses of coral. The coral Montipora digitata and five bacterial strains (Vibrio coralliilyticus, Vibrio harveyi, Paracoccus carotinifaciens, Pseudoalteromonas sp., and Sulfitobacter sp.) were used in an inoculation experiment. The bacteria were isolated from the bleached corals and their surrounding seawater. At normal temperatures, the bacteria did not cause coral bleaching and did not affect metabolism or antioxidant enzyme activities. However, bacterial challenges in addition to high-temperature stress resulted in coral bleaching, with a 70% decrease in zooxanthellae density compared with the control, a 25% decrease in photosynthetic efficiency (Fv/Fm), a 66% decrease in photosynthesis, and a 101% reduction in calcification activity. Tissue necrosis was observed in the most compromised branches. Respiration and activity levels of antioxidant enzymes were not affected. Corals under thermal stress alone showed signs of bleaching, but the changes in zooxanthellae density and metabolic rates were less severe than those under synergistic thermal stress and bacterial challenge. Among the bacteria examined, Sulfitobacter sp. had a greater capacity to enhance and accelerate the bleaching process under thermal stress. The mechanisms by which bacteria affect corals are not yet understood. Direct actions include infection and production of toxic substances by bacteria; indirect effects due to changes in the bacterial community under the influence of added bacteria must also be considered. Our results suggest that bacterial challenges combined with thermal stress can synergistically lead to negative outcomes in corals.

► We studied the multiple effects of high temperature and bacteria on coral Montipora.
► Under normal temperatures, bacteria did not affect coral bleaching and metabolisms.
► Under high temperature, bacteria caused severe bleaching and decreased metabolisms.
► Bacteria Sulfitobacter sp., greatly enhanced and accelerated the bleaching process.