Factors controlling the microbial community associated with reef building corals

Abstract

Several studies have shown that corals are associated with diverse, host speciesspecific bacterial communities and these have been proposed to be of primary importance for their health. Various factors have been suggested to influence the structure of these communities, including production of antimicrobial chemicals, the supply of microorganisms from the surrounding environment (e.g. sediments and water column), mucus composition and production rates by the coral. However, few studies have investigated the factors that control the development and maintenance of these communities. Describing the microbial communities of healthy corals and how they interact with their surrounding environment is imperative to understanding how environmental stress and health problems in corals are related. This study utilised a culture-independent 16S rRNA gene approach to investigate the structure of the bacterial community on corals, the factors that might control the development of these microbial communities and their organisation within the coral host. In addition, the study identified the role of cytophagous ciliates as a potential cause of White Syndrome in the GBR. Study of the bacterial (16S rRNA gene) community of the surrounding water column (the potential supply to the surface mucus layer of corals) revealed that changes in productivity and/or vertical diurnal migrations of plankton might have greater effects than large scale water movements effected by tidal flows. Results also showed that waterborne bacterial communities and their underlying benthos were not strongly linked, suggesting either that there is little benthic-pelagic coupling or that large-scale (island wide) water column mixing is rapid and highly efficient, resulting in homogeneous bacterial communities in the water column, independent of the underlying benthos. The bacterial communities forming on artificial surfaces and those associated with the mucus layer of corals were different from the water column as well as each other, with a variety of ribotypes of - proteobacteria favouring both the biofilms and those of the surface mucus layer, compared to a high dominance of -proteobacetria within the water column. This suggests that the coral actively controls the microbial community on its surface, rather than it being a result of passive settlement from the water column. Results also show that bacterial communities within the coral are highly compartmentalized with distinct assemblages inhabiting the mucus layer, tissue and skeleton, which indicates high levels of complexity in the coral-microbial associations. The use of a broad spectrum antibiotic treatment further allowed investigation of turnover rates of the microbiota associated with healthy corals. Re-establishment of the corals’ normal microflora was slower than anticipated, taking over 96 h to return to that of its original bacterial community once disturbed, however the fact that the community returned towards its original state indicates a high degree of resilience and strong controls on the microbial community structure Despite the undisputed importance the bacteria associated with corals have on the overall coral holobiont, other microorganisms such as ciliates were also important for coral health, particularly during thermal stress. Results indicated that coral White Syndromes, previously attributed to pathogenic bacteria such as Vibrio spp., may have different etiologies, with cytophagous ciliates playing an important role. These findings further complicate correct disease identification in the field and appropriate treatment and/or prevention methods for diseased corals.