Bacterial communities closely associated with coral tissues vary under experimental and natural reef conditions and thermal stress

The coral holobiont model highlights the integral role bacteria play in the health of reefbuilding corals. Documenting the natural diversity of bacterial communities within, and closely associated with, coral tissues provides information on the diversity, interaction and roles of bacteria to the function of reef-building corals. Fluorescence in situ hybridisation was used to visualise bacterial communities closely associated with the tissues of experimentally manipulated reef corals to determine how tissue-associated coral–bacterial interactions vary from normal associations in apparently healthy reef corals, to those occurring in controlled and thermally stressed experimental conditions. Branches of 2 coral species of the Great Barrier Reef, Acropora aspera and Stylophora pistillata, were collected from reefs adjacent to Heron Island and were maintained in controlled outdoor flowthrough aquaria conditions. Following acclimation, the branches were stressed using elevated temperatures to investigate the in situ (within-tissue) bacterial community changes. In situ bacterial community dynamics were found to vary not only due to maintenance within the aquaria conditions, but also following coral bleaching. An aggregation of rod-shaped γ-proteobacteria was evident within the gastrodermis of corals regardless of health or bleaching status, consistent with aggregations described within other coral species. However, bacterial colonisation of the tissues occurred only following the temperature-induced bleaching of the coral tissues. This study demonstrates that the natural bacterial communities of corals are severely altered during stress associated with experimental and field conditions, which suggests a potential mechanism for the link between disease and stresses arising from global warming.