Abstract
High biodiversity and productivity, complexity of biotic interactions and trophic networks, wealth and product source, coastal protection are some of the many emblematic characteristics of the coral reef ecosystem. This prodigious diversity is largely due to the particular biology of the principal reef bioconstructors, the hermatypic corals and associated zooxanthellae symbionts. However, over the last decades, most of the reefs have been critically affected by various natural and anthropogenic disturbances which are, at least for some of them, increasing in frequency and intensity in correlation with global climate changes. Among these disturbances, coral bleaching (the loose of zooxanthellae and/or their photosynthetic pigments) and coral diseases have caused high or even massive rates of coral mortality worldwide. Results of the last worldwide coral reefs monitoring programs are particularly alarming and established that 19% of the reefs are completely destroyed, 20% present all symptoms of an imminent threat of collapse, and 20% are at risk for the forthcoming decades. In this context, the objective of this work was to improve our knowledge of enhanced physiological and transcriptomic mechanisms of coral under thermal stress leading to bleaching and bacterial stress. The objective was also to provide strong bases for the use of relevant biomarkers for coral health monitoring and early detection of these disturbances. This work focused on the scleractinian species Pocillopora damicornis, the bacteria Vibrio coralliilyticus, and on the implementation of stress experiments in a controlled but ecologically realistic system. In the first study, P. damicornis colonies were confronted to a gradual water temperature increase (28°C to 32°C) over a 15 days period. mRNAs differentially expressed (between control and thermally stressed conditions) were isolated by subtractive hybridization and the transcription rate of the most promising genes were measured by quantitative-RT-PCR. These approaches revealed 2 candidates that displayed a drastic expression decrease 6 days before the occurrence of the first symptoms of bleaching. RACE-PCR experiments showed that one of them (PdC-Lectin) contains a C-type lectin domain presenting a specificity for recognition and binding of mannose. Immuno-localization experiments have demonstrated that this protein could be the molecular mediator of host/symbionts interaction, which suggest a putative role in the acquisition or the sequestration of symbionts. The second gene (Pdcyst-rich) codes for a protein potentially involved in calcification processes. Its down-regulation could picture a trade-off mechanism leading to calcification failure during thermal stress. In the second study, responses of P. damicornis confronted to its specific pathogen V. coralliilyticus in a virulent (temperature increased) or non-virulent (constant low temperature) state were studied. The infectious process was exained by the combination of electronic microscope observations and quantitative-RT-PCR experiments, and coral health status was evaluated by visual observations and zooxanthellae density measures. Results show that infection only occurs after water temperature was increased. Among several ESTs obtained by subtractive hybridization (between control, bacterially, and thermally plus bacterialy conditions), 6 gene candidates were selected for their putative involvement in the immune response, and their transcription rate was measured by quantitative-RT-PCR all long the kinetics of virulent and non-virulent experiments. Among these genes, 3 belong to the lectin family, 2 code for proteins involved in metal binding and the last one codes for a protease inhibitor. Their expression patterns allowed better understanding the immune response of scleractinian corals as well as the impact of Vibrio on its host. The third publication focus on the characterization of the first antimicrobial peptide (AMP) of scleractinian corals, the damicornin of P. damicornis. Results showed that the damicornin is constituvely transcribed and stocked in an inactive form in ectodermal granular cells. Under immune challenge, damicornin is secreted and activated. Results also showed that after bacterial internalization, Vibrio triggers a down-regulation of the damicornin. In our knowledge this is the first report of the down-regulation of an AMP in a host/Vibrio interaction. Finally, 9 biomarkers of environmental stress were identified and demonstrated to be early regulated, in response of thermal stress for 2 of them, and of bacterial stress for the 7 others. For the last ones, it is possible to differentiate between corals confronted to virulent or non-virulent interaction. After this identification step, a validation stage appears to be necessary. This should focus on the quantification of expression variability of each biomarker at different scales, but also on the definition of an expression baseline, which is essential for biomarkers use for coral health monitoring. This work is particularly promising, and opens perspectives in the field of epigenetic mechanisms involvement in functional adaptation of corals to environmental stress. Another interesting perspective would be the study of the links existing between immunity and symbiosis, which have been put to the fore in the present work by the different involvement of a same gene in both functions (PdC-Lectin). Finally, studying responses of corals to acidic stress would be particularly relevant in the actual context of ocean acidification