Emma Ransome


The coral holobiont contains diverse communities of bacteria that play a role in the maintenance of coral ecosystems, however little is known about the structure and conservation of the host-bacterial relationship. Declines in coral ecosystems have been partly attributed to outbreaks of disease in tropical and sub-tropical regions, which have been linked to increasing temperatures. Bacteria are thought to play a role in some of these diseases, however little is understood about the mechanisms behind disease progression or the series of events involved in the shifts of coral-associated bacteria from conserved, potentially beneficial communities to those including potential pathogens. Investigations into a cold-water gorgonian coral, Eunicella verrucosa, have shown similar bacterial communities to those present in tropical and sub-tropical regions, with high proportions of Spongiobacter and Endozoicomonas genera, suggesting an important role for these associates in the coral holobiont irrespective of location or the presence of zooxanthellae. A shift in bacterial community with disease was also shown, with suggestions that sedimentation and depth may affect the extent of bacterial community alteration. With the increasing knowledge that bacteria exhibit elaborate systems of intercellular communication (quorum sensing; QS) to allow a population response and to control the expression of genes for pathogenesis, antibiotic production and biofilm formation, the present study showed the presence, stability and species-specific nature of N-acyl-homoserine lactones (AHLs; most prevalent type of QS) in situ in a number of coral species. This finding and a high proportion of coral-associated bacteria found producing AHLs suggests an important role for QS in the coral holobiont. Further, AHL signals have been shown to break down in Stylophora pistillata kept at 30 °C, which coincided with a drop in bacterial numbers and a changing bacterial community which included more quorum quenching (QQ; AHL-degrading) bacteria. Temperature was shown to affect AHL-QS in a strain-dependent manner in E. verrucosa isolates, suggesting that the decline seen in S. pistillata is not primarily an effect of temperature. Further experiments with three species of soft coral (Sinularia sp., Discosoma sp. and a gorgonian) showed no such decline in AHLs at 30 °C and instead show a coral-specific response to temperature, including the ability of coral extracts to inhibit putative pathogens. A decline in the ability of crude coral extract to degrade AHLs in the Discosoma sp. and the high QQ activity in crude extract from all three species suggests a role for QQ in the coral holobiont, confirmed by the high percentage of QQ found in coral-associated bacterial isolates; again suggesting a role in the maintenance of bacterial communities. Further investigations attempted to link QS and QQ to antagonism and susceptibility in coral associated bacteria; however these results were inconclusive. The thesis concludes that priority should be given to further research of QS and QQ in the coral holobiont, which will reveal important knowledge that may lead to future mitigation of some forms of coral disease.

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