Regulation of microbial populations by coral surface mucus and mucus-associated bacteria

Caribbean populations of the elkhorn coral Acropora palmata have declined due to environmental stress, bleaching, and disease. Potential sources of coral mortality include invasive microbes that become trapped in the surface mucus and thrive under conditions of increased coral stress. In this study, mucus from healthy A. palmata inhibited growth of potentially invasive microbes by up to 10-fold. Among cultured bacteria from the mucus of A. palmata, 20% displayed antibiotic activity against one or more tester strains, including the pathogen implicated in white pox disease. A novel mucus-mediated selection for coral symbionts revealed a discrete subset of bacteria and selected for isolates that produce antibiotics. This result suggests that coral mucus plays a role in the structuring of beneficial coral-associated microbial communities and implies a microbial contribution to the antibacterial activity described for coral mucus. Interestingly, antibiotic activity was lost when mucus was collected during a summer bleaching event. Isolates from apparently healthy A. palmata tissue during this event lacked antibiotic-producing bacteria and were dominated by members of the genus Vibrio, including species implicated in temperature-dependent bleaching of corals worldwide. This indicates an environmental shift from beneficial bacteria, and variability in the protective qualities of coral mucus, which may lead to an overgrowth of opportunistic microbes when temperatures increase. Finally, coral mucus inhibited antibiotic activity and pigment production in a potentially invasive bacterium, illustrating that coral mucus may inactivate mechanisms used for bacterial niche establishment.