Corals shed bacteria as a potential mechanism of resilience to organic matter enrichment

Understanding the mechanisms of resilience of coral reefs to anthropogenic stressors is a critical step toward mitigating their current global decline. Coral–bacteria associations are fundamental to reef health and disease, but direct observations of these interactions remain largely unexplored. Here, we use novel technology, high-speed laser scanning confocal microscopy on live coral (Pocillopora damicornis), to test the hypothesis that corals exert control over the abundance of their associated bacterial communities by releasing (‘shedding'') bacteria from their surface, and that this mechanism can counteract bacterial growth stimulated by organic inputs. We also test the hypothesis that the coral pathogen Vibrio coralliilyticus can evade such a defense mechanism. This first report of direct observation with high-speed confocal microscopy of living coral and its associated bacterial community revealed a layer (3.3–146.8 μm thick) on the coral surface where bacteria were concentrated. The results of two independent experiments showed that the bacterial abundance in this layer was not sensitive to enrichment (5 mg l⁻¹ peptone), and that coral fragments exposed to enrichment released significantly more bacteria from their surfaces than control corals (P<0.01; 35.9±1.4 × 10⁵ cells cm⁻² coral versus 1.3±0.5 × 10⁵ cells cm⁻² coral). Our results provide direct support to the hypothesis that shedding bacteria may be an important mechanism by which coral-associated bacterial abundances are regulated under organic matter stress. Additionally, the novel ability to watch this ecological behavior in real-time at the microscale opens an unexplored avenue for mechanistic studies of coral–microbe interactions.