Stress Induced Coral Mortality: The Role of Opportunistic Bacterial Infections

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Coral reefs have declined dramatically over the last three decades. Several factors may have contributed to this process, including over-fishing, increasing CO2 levels, changes in sea surface temperatures, eutrophication, sedimentation, and pollutants. However the causal relationship between most of these stressors and reef decline has yet to be established. There has also been an alarming increase in coral disease incidence during the last two decades. The causes of the majority of these diseases remain unknown yet the diseases continue to cause massive die offs of corals. Of the 29 coral diseases that have been reported, only two pathogens have been isolated which fulfill Koch's postulates. Are some of the undiagnosed diseases caused by opportunistic, rather than specific, pathogens? Coral disease incidences and human activity are correlated. Again, the causal connection, if any, between these two observations is not known. Dr. Rohwer and associates suggest that corals exposed to anthropogenic and environmental stressors become susceptible to opportunistic infection resulting in coral mortality and disease. In preliminary studies, they have found that the microbial community normally living on corals consists of specific bacteria growing at a controlled rate. For example, corals living on reefs as far apart as Bermuda and Panama have characteristic microbiotas, that are specific to individual coral species. They have also observed that bacterial numbers and growth rates are very similar between healthy corals. The relationship between the coral and its associated microbial community rapidly changes when the coral is exposed to stressful environmental or anthropogenic conditions. In experiments, additions of organic nutrients rapidly lead to coral death, which was prevented by adding antibiotics. This strongly suggests that bacterial overgrowth is responsible for the stressor-induced mortality. Other investigators have shown that coral mortality caused by stressors, including sedimentation, crude oil, and copper sulfate, could be prevented using antibiotics. The investigators have also documented that both bacterial numbers and production rates increase by over an order of magnitude on diseased, as compared to healthy, corals. In this research, they will characterize the normal range of microbial growth parameters and prokaryotic diversity on healthy, unstressed M. franksi. They will then treat corals with a number of potential stressors to determine which kill corals by causing microbial overgrowth. Finally, they will monitor changes in bacterial growth and community composition that occurs with opportunistic bacterial infection. This research will improve our understanding of the relationship between a coral and its associated bacterial community, and how this relationship changes with stress and disease. This research will also provide a better understanding of the connection between human activities and the incidence of coral disease, providing important data for the conservation and management of coral reefs.