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.

Coral reefs are among the most biodiverse ecosystems in the world, being essential for marine life. The engineers of these ecosystems, reef-building corals, live in association with a great diversity of microorganisms, which can affect their host’s health in beneficial or detrimental manners. Corals are currently threatened by climate change and other environmental stressors, that lead to the phenomenon of coral bleaching, in which these animals lose their endosymbiotic algae. Even though the stressors that cause coral bleaching are known, the exact cellular and molecular mechanisms that provoke this process are still undiscovered. The lack of information regarding micro-scale processes that happen in unhealthy corals could be resolved with more efforts in developing micro-scale studying models. The use of individual polyps that bail-out of the coral skeleton induced by acute stress has been suggested as a model to study these processes. However, little is known about how these polyps change after bailing-out of a colony, which could become a problem once reliable models should be consistent and well understood. Thus, investigating these changes and optimizing a methodology to minimize them is crucial to establish these polyps as models to study corals. Herein, we investigated microbiological changes of isolated polyps by performing an experiment to study shifts in their microbiome after the separation from the colony. Before the experiment, different methods to isolate polyps were tested to find the one that granted the highest survival. After finding that salinity-induced separation was the most efficient, this method was used to study the microbiome of coral polyps. We found that while no significant changes in the microbiome could be observed immediately after the separation of polyps from their colony compared to coral fragments, this pattern changed after two weeks. We propose that the maintenance of polyps without fixation to a substrate might be the cause for such changes, and that polyps able to attach to a substrate and regrow as a colony might still recover a microbiome composition closer to coral fragments. Finally, a new microfluidic device for fixation and maintenance of coral polyps was developed and tested for use in future experiments.

The black band disease (BBD) microbial consortium often causes mortality of reef-building corals. Microbial chemical interactions (i.e., quorum sensing (QS) and antimicrobial production) may be involved in the BBD disease process. Culture filtrates (CFs) from over 150 bacterial isolates from BBD and the surface mucopolysaccharide layer (SML) of healthy and diseased corals were screened for acyl homoserine lactone (AHL) and Autoinducer-2 (AI-2) QS signals using bacterial reporter strains. AHLs were detected in all BBD mat samples and nine CFs. More than half of the CFs (~55%) tested positive for AI-2. Approximately 27% of growth challenges conducted among 19 isolates showed significant growth inhibition. These findings demonstrate that QS is actively occurring within the BBD microbial mat and that culturable bacteria from BBD and the coral SML are able to produce QS signals and antimicrobial compounds. This is the first study to identify AHL production in association with active coral disease.