A biological factor that is currently an area of outstanding research is the effect coral-specific microbial communities have on coral reefs and resilience. Coral-associated microbes are found in the mucus, tissue, and skeleton of corals. Microbial processes and metabolisms influence biogeochemical and ecological processes within the reef environment, such as food webs, organism life cycles and chemical and nutrient cycling. Studies also suggest that resident microbes may play a critical role in limiting the abundance of pathogenic microbes under normal conditions. They are also key drivers of the factors that influence the resilience of coral reef systems, like larval recruitment, colonization, and species diversity.1
An important correlate in bleaching and disease in reef-building corals is a shift in the makeup of the microbial community in the mucus layer surrounding the coral. When exposed to thermal stress, microbes on coral reefs show impairment of antibiotic activity, prominent shifts in diversity, and increases in disease prevalence [link to new disease section]. Rapid shifts to pathogen dominance have been observed in the microbial community following thermal stress and prior to bleaching. For example, studies provide support for a “microbial hypothesis of coral bleaching,” where bleaching is initiated by a shift to pathogen dominance in the microbial communities brought on by thermal stress, rather than by direct effects of thermal stress on the coral and its symbionts.2,3 An additional study4 demonstrated that changes in microbial communities are evident in bleached corals when compared to healthy corals, before bleaching and following recovery. As corals bleached, the microbial community shifted to the bacteria, Vibrio,- dominated community, and commenced prior to visual signs of bleaching. Shifts in coral-associated microbial populations may provide important indicators for environmental change and stress on reef organisms.
While uncertainty and many outstanding questions remain on the roles of microbes and host-microbe interactions, there is evidence that microbial communities are highly complex, environmentally and temporally dynamic and extremely significant in terms of the corals physiology and ecosystem function. Ultimately, the microbial perspective will improve the ability to accurately predict the resilience of specific reefs. Characterizing the linkages occurring between the microbial community and associated ecological changes will be fundamental to understanding resilience and will improve the capacity to predict changes.1
1 Ainsworth et al. 2010
2 Mao-Jones et al. 2010
3 Rosenberg et al. 2009
4 Borne et al. 2008