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

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

Chitin biodegradation in marine environments : an experimental approach

Chitin biomasses and production in marine environments are quite high. Planktonic biocenoses arte the main producers and one should expect that sediments, mainly organoclastic ones, will constitute some kind of reserve compartment for the biogeochemical cycle of this polymer. In fact, this is not the case. The low chitin biomass in most marine sediments can only be explained if chitin is weathered at the same rate as it is produced. In order to test this hypothesis, we developed an experimental approach to chitin biodegradation in marine environments. In open water conditions, zooplanktonic remains are first degraded by autolytic processes making most organic compounds readily susceptible for further hydrolysis by extrinsic decomposers. Different populations (with high densities and various hydrolytic potentials) follow each other. The sequence of hydrolyc activities optimizes the recycling of most detritic compounds including nearly 90% of the chitin produced. At sediment-water interface, the remaining material appears to be pulvirized and incorporated into the aerobic sedimentary layers while the decomposer community changes once again. Sediment chitinoclasts are opportunistic and densities react quickly to chitin input. In sediments, oxic and anoxic, chitin appears essentially present in the form of chitinoproteic matrices inside mineralized skeletons. A rich population of microborers develops on these matrices by secreting extracellular hydrolases. Densities of microborers of 250-450 * 10 3 cm-2 are currently encountered. Anaerobic decomposers are more adapted to refractory compounds than aerobic ones. This leads to a nearly complete mineralization of the chitinoproteic matrices embedded in the biotic sedimentary layers (more than 90% of the chitin weathered within less than two years)

Chitin biomass in marine sedimenis

One hundred marine sediments of various origins were screened in order to evaluate their chitin biomass. Our purpose was to assess the detritic chitin stocks in order to find some potential new source of chitin. The chitin biomass of marine sediments is very diversified, from 2 up to 2 800 ug g-1 decalcified sediment (DS). Most sediments have low or very low chitin biomass (67 % under 100 ug g-1 DS). Nosignificant difference related to depth nor climatic influence was found except that all sediments richer in chitin (above 300 ug g-1 DS) are on the continental shelf (above 200 m depth). Actually, the chitin content is higher in coarse, much calcified sediments of organoclastic origin ; bryozoa and shelly sands and gravels are the richest. Th