Microbial Responses and Coral Reef Resilience to Organic Matter Inputs

Little attention has been given to the small-scale mechanisms relevant to microbial processes that determine the resilience of individual corals to the stress of organic matter (OM) inputs. Such mechanisms may be a critical link for predicting larger scale patterns of reef resilience. The research presented here aims to develop methods necessary to elucidate these processes and to explore the in situ responses of microbial assemblages on coral reefs experiencing persistent OM enrichment. A new method using trypsinization of coral mucus before staining for epifluorescence microscopy is described. It is then applied to a coral reef ecosystem influenced by sewage effluent to discover that corals exposed to effluent had the same number of bacteria present as reference corals; however, the size structure of the community was significantly different. Investigating the responses of microbial communities (from both the water column and corals) to OM inputs from coastal milkfish (Chanos chanos ) pens, we found that the percentage of the water bacterial community attached to particles increased by more than 50-fold near the pens. This suggested a physiological or life-strategy change may be induced by such enrichment. A clonally replicated coral transplantation experiment examined the response of naïve coral-associated bacterial communities to high and low levels of pen effluent exposure. We found that the communities on corals exposed to high levels of effluent had drastically altered community compositions after five days and the abundance of bacteria in the coral mucus-tissue slurries were ~100-fold higher controls at low effluent and reference sites. We also observed a surprising resilience of these communities in that their composition and total abundance recovered by day 22. A combination of novel imaging technology (high speed laser scanning confocal microscopy on live coral) and controlled aquaria experiments were developed and used to investigate a microscale mechanism by which such resilience may occur: that corals may release ("shed") bacteria into the surrounding water as a mechanism for controlling bacterial abundance on their surface. We observed this phenomenon in real time, and quantified an increase in the rate at which corals shed bacteria as a response to OM enrichment