HETEROGENEITY OF ZOOXANTHELLAE DENSITY IN THE CORAL ACROPORA GLOBICEPS AROUND MOOREA (FRENCH POLYNESIA)

Understanding the natural heterogeneity of coral zooxanthellae density appears as very important to understand variations in coral survival due to bleaching (the loss of these micro-algal symbionts). Heterogeneity of coral bleaching at different observation scales (within a colony, among neighbouring colonies of the same species or on a wider scale) remains largely misunderstood. The present work explores intracolonial, spatial and temporal variations of zooxanthellae density in the coral Acropora globiceps Dana 1846, over a period of three months on the forereef of Moorea, French Polynesia. In our study, intracolonial zooxanthellae densities did not vary significantly. However, zooxanthellae densities differed slightly between inner and outer branches but this trend was not significant at 6, 12 and 18 meters depth. On a wider scale, zooxanthellae densities also vary spatially : a positive correlation was observed between depth and symbiont density (density increases when light intensity decreases, so according to depth). Moreover, the location of colonies exposed to different hydrodynamical conditions was not a parameter controling the concentration of zooxanthellae. Finally, the temporal variation of zooxanthellae densities did not show significant variations even if it decreased slightly over the study period. The results of this study highlight the importance of accounting for variations within and among colonies to determine zooxanthellae densities and to assess the evolution of zooxanthellae populations

Multidisciplinary approach of the bacterial implication in the bleaching phenomenon of Scleractinian corals (Indo-Pacific)

Vu le nombre croissant de maladies coralliennes lié à l'impact grandissant du réchauffement climatique et des activités anthropiques, le présent travail de recherche s'est intéressé au phénomène de blanchissement corallien. Les Cnidaires Scléractiniaires sont les éléments de base et essentiels de l'écosystème récifal; écosystème dont les intérêts écologiques, économiques et sociétaux ne sont plus à démontrer. Ce symptôme de blanchissement a des causes multiples et complexes qui sont encore mal connues. Il correspond à la rupture de la symbiose mutualiste existant entre le corail et des microalgues, appelées zooxanthelles, et peut engendrer la mort de nombreux coraux. Une hypothèse à la base de ce phénomène suggère que le corail blanchit suite à l'action de bactéries pathogènes, pénétrant à l'intérieur des tissus coralliens. Cependant, cette hypothèse semble assez controversée et le présent travail tente d'élucider un ensemble de questions liées à cette polémique. Pour ce faire, une approche pluridisciplinaire alliant l'écologie (générale et microbienne) et la biologie moléculaire a été mise en place lors d'études in situ et expérimentales. A l'aide de la microscopie électronique, des critères morphologiques ont été établis afin de caractériser l'état de blanchissement du corail. Par ailleurs, cette technique a également permis d'identifier différents types de bactéries au sein des tissus coralliens. Leur présence et leurs rôles au sein de l'holobionte corallien sont discutés. Plusieurs techniques de la biologie moléculaire (DGGE, séquençage) ont apporté des éléments sur la non-spécificité des communautés bactériennes associées à des coraux d'espèces différentes, provenant de sites de prélèvement différents et dans des états de blanchissement différents. Cette non-spécificité souligne le fait que les fonctions remplies par les bactéries au sein de l'holobionte corallien pourraient être le fait de bactéries opportunistes provenant de groupes taxonomiques variés. La présence de Cyanobactéries chez certains coraux, détectées à l'aide de la microscopie et confirmée par les techniques moléculaires, soulève la question de leur rôle au sein de l'holobionte et de l'alternative symbiotique qu'elles offrent à l'hôte par rapport aux zooxanthelles, microalgues symbiotiques "habituelles" des Scléractiniaires hermatypiques. Les questions de la compétition potentielle entre ces différents types de symbiotes ou de l'avantage de cette diversité symbiotique en cas de blanchissement restent ouvertes. Une approche transcriptomique a permis d'étudier la réponse du corail face à un stress bactérien, dans différentes conditions expérimentales de température déterminant l'expression de la virulence des bactéries. L'étude du profil d'expression de différents gènes impliqués dans l'immunité du corail a permis de comprendre la séquence de réactions et les modifications de l'allocation des ressources en réponse aux différentes étapes de l'infection bactérienne. A cette occasion, le premier peptide antimicrobien de Scléractiniaires, la damicornine, a pu être mis en évidence et caractérisé. L'ensemble de ces résultats mènent à penser que le contexte des modifications des paramètres environnementaux importe plus que la présence elle-même de pathogènes. Les bactéries agiraient alors comme facteur aggravant dans une situation où le corail, déjà stressé et affaibli, ne peut se défendre de manière adéquate. Finalement, les observations d'une augmentation du nombre de cnidocytes chez une espèce de corail ayant subi un blanchissement suggèrent que certaines espèces de corail seraient capables de s'adapter au manque d'apports occasionné par la perte significative de zooxanthelles, en passant à un régime nutritionnel plus hétérotrophe. Ceci favoriserait alors la résilience du corail.Given the increasing number of coral diseases linked to the growing impact of global warming and human activities, the present research focused on the phenomenon of coral bleaching. The Scleractinian Cnidaria are the essential building blocks of the coral reef ecosystem, whose ecological, economic and societal interests are demonstrated. The symptom of bleaching has multiple and complex causes that are still unclear. It corresponds to the rupture of the mutualistic symbiosis between the coral and microalgae, called zooxanthellae, and can causes death of many corals. An assumption underlying this phenomenon suggests that corals are bleaching by the action of pathogenic bacteria, penetrating inside the coral tissues. However, this hypothesis seems quite controversial and the present work attempts to elucidate a set of issues surrounding this controversy. To do this, a multidisciplinary approach combining ecology (general and microbial) and molecular biology has been established through field and experimental studies. Using electron microscopy, morphological criteria were established to characterize the status of coral bleaching. Furthermore, this technique was also used to identify different types of bacteria within coral tissues. Their presence and their role within the coral holobiont were discussed. Several molecular biology techniques (DGGE, sequencing) have provided evidences on the non-specificity of bacterial communities associated with corals species, sampling sites and different status of bleaching. This non-specificity shows that the functions performed by bacteria in the coral holobiont could be the result of opportunistic bacteria from various taxonomic groups. The presence of Cyanobacteria in some corals, detected by microscopy and confirmed by molecular techniques, raises the question about their role within the holobiont and the alternative pathway that they offer for the symbiotic host compared with zooxanthellae. The issues of potential competition between these different symbionts or the benefit of this symbiotic diversity, in case of bleaching event, remain open. Transcriptomic genetics was used to study the response of coral facing a bacterial stress in different experimental conditions of temperature determining the expression of bacterial virulence. Studying the expression profile of genes involved in immunity of the coral allowed to understand the sequence of reactions and changes in resource allocation in response to different stages of bacterial infection. On this experiment, the first antimicrobial peptide of Scleractinia, the damicornin, has been identified and characterized. All these results suggest that the context of changes in environmental parameters is more important than the presence of pathogens itself. The bacteria would then act as an aggravating factor in a situation where the coral, already weakened and stressed, cannot defend itself adequately. Finally, the observation of an increase in cnidocytes in a coral species that has undergone bleaching, suggests that some coral species are able to shift to a more heterotrophic nutritional regime adapting to the lack of inputs caused by the significant loss of zooxanthellae. This would favor the resilience of coral

SYMBIODINIUM SP. CAN STAY ALIVE THROUGH THE GUT AND IN THE FAECES OF CNIDARIA. PREDATORS. THE CASE OF CORALLIOPHILLA MEYENDORFFI AND ANEMONIA VIRIDIS.

The gastropod Coralliophilla meyendorffi is a common predator of the zooxanthellate anemone Anemonia viridis. Zooxanthella from the anemones are an important constitutent of the mollusc faeces. Cell integrity, occurence of flagellated forms, live/dead proportions and mitotic index of Symbiodinium collected from the faeces and cultivated in vitro were examined and compared to that of algae in hospite. The results show that most algae withstand digestive processes of the predator, staying alive and dividing actively in the faeces. Motile (lagellated) algae arise from dividing cells and escape the fecal pellets. Impact on coral bleaching recovery is discussed

Functional diversity of microbial communities associated to the mucus of scleractinians around Moorea (French Polynesia)

Mucus production by scleractinians appears as an antifouling mechanism which prevents settlement of other organisms and accumulation of sediments on their surface. This Surface Muccopolysaccharide Layer (SML) harbours dense populations of bacteria which play a paramount role in scleractinians nutrition, metabolism and good health maintenance. However, environmental disturbances can alter these microbiocenoses. Characterization of bacterial communities was carried out using a set of simple techniques that enable us to describe the state and functions of whole microbial communities associated with different hard coral species. Multi-comparisons have been performed on bacterial communities from open water, interstitial water, sedimentary interface and macro algae as well as between healthy and bleached colonies, and patches associated or not with Pomacentridae fishes. The functional study included measurements of bacterial biomass, respiration, oxydative and hydrolytic metabolisms. Non-Fungiidae corals and sedimentary interface have a quite similar bacterial biomass but open water, interstitial water and macro-algae are characterized by higher bacterial biomass. Bacterial respiration potential is similar on corals and at the sedimentary interface, but it is higher in interstitial water and lower in open water and for bacterial community associated with macro-algae. Hydrolytic activities are highest in SML. Bleached corals and patches associated with Pomacentridae fishes show more abundant bacteria, with higher respiration rate and higher hydrolytic activity than corals without fishes and healthy ones. In addition, bacteria of bleached corals display a higher division percentage, a higher growth rate and a lower turn-over time We confirmed that bleaching events or the presence of sedentary fishes modify the bacterial communities structure and affect relationships between coral, endosymbiotic algae, SML-associated microbial community and associated organisms. Such results highlight that SML-bacterial communities are modified by bleaching and raise the question of a potential protection of fishes against pathogens