Coral-bacteria associations-determining the role of Chemotaxis

Bacteria play a fundamental role in determining the health and ecology of corals. The important functions of the bacterial consortia inhabiting the coral holobiont include chemical cycling, provision of a food resource for the host, as well as a potential role in coral disease. Indeed, pathogenic bacteria have been implicated as an important cause of the world-wide decline in coral health. Currently, however, little is known about the mechanisms involved in establishing an association between corals and bacteria.
The relative abundance of bacterial communities associated with the branching coral Pocillopora damicornis was examined under laboratory conditions in aquaria and on Heron Island, where an additional species Montipora digitata was also monitored. Bacterial concentrations associated with the surface of corals were compared to the surrounding water using flow cytometry. On Heron Island, there was no significant difference observed in bacterial abundances between the healthy Coral Surface Microlayer (CSM) and those of the surrounding seawater for both P. damicornis and M. digitata. However, there were significantly more bacteria associated with the CSM of diseased specimens of both P. damicornis and M. Digitata.
To examine the potential mechanism behind this coral-bacteria association, we performed chemotaxis experiments where the extent of bacterial chemotaxis towards chemicals expected to be associated with the coral holobiont were tested using a modified capillary assay. Chemoattractants tested included zooxanthellae extracellular excretions, as well as organic compounds including a variety of amino acids and sugars, dimethyl sulphide (DMS) and dimethylsulfonopropionate (DMSP). In all cases, coral associated bacteria from Heron Island exhibited significantly higher levels of chemotaxis than non-coral associated bacteria, from both the laboratory and the field. Bacterial attraction towards the sugars and amino acids tested led to bacterial concentrations that reached 176 and 186 times higher within chemoattractant chambers respectively, in comparison to background concentrations in the seawater. Zooxanthellae exudates invoked strong chemotactic responses, leading to bacterial concentrations within chemoattractant chambers of up to 51 times higher than background. These data indicate that coral-bacterial associations may be maintained by chemotactic responses by bacteria towards several organic compounds that are likely to be released from coral surfaces. The high levels of chemotaxis of coral-associated bacteria, relative to those found in the surrounding water column, points to a potentially important role of chemotaxis in the establishment and maintenance of coral-bacteria associations in the environment.