6 research outputs found

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

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    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

    Impact of pCO2 on the energy, reproduction and growth of the shell of the pearl oyster Pinctada margaritifera

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    International audienceThe possible consequences of acidification on pearl farming are disruption of oyster metabolism and change in growth. In the laboratory, we studied the impact of pCO2 (3540, 1338 and 541μatm) on the physiology of pearl oysters exposed for 100 days. This experiment was repeated after an interval of one year. Several physiological compartments were examined in pearl oysters: the scope for growth by measuring ingestion, assimilation and oxygen consumption, gametogenesis by means of histological observations, shell growth by measurement and observation by optical and electronic microscopy, and at molecular level by measuring the expression of nine genes of mantle cells implied in the biomineralisation process. Results from both experiments showed that high pCO2 had no effect on scope for growth and gametogenesis. High pCO2 (3540 μatm) significantly slowed down the shell deposit rate at the ventral side and SEM observations of the inside of the shell found signs of chemical dissolution. Of the nine examined genes high pCO2significantly decreased the expression level of one gene (Pmarg-PUSP 6). This study showed that shell growth of the pearl oyster would be slowed down without threatening the species since the management of energy and reproduction functions appeared to be preserved. Further investigations should be conducted on the response of offspring to acidification

    Sub-micrometric spatial distribution of amorphous and crystalline carbonates in biogenic crystals using coherent Raman microscopy

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    International audienceIn living organisms, calcium carbonate biomineralization combines complex bio-controlled physical and chemical processes to produce crystalline hierarchical hard tissues (usually calcite or aragonite) typically from an amorphous precursor phase. Understanding the nature of the successive transient amorphous phases potentially involved in the amorphous-to-crystalline transition requires characterization tools, which are able to provide a spatial and spectroscopic analysis of the biomineral structure. In this work, we present a highly sensitive coherent Raman microscopy approach, which allows one to image molecular bond concentrations in post mortem shells and living animals, by exploiting the vibrational signature of the different carbonates compounds. To this end, we target the nu_1 calcium carbonate vibration mode and produce spatially and spectroscopically resolved images of the shell border of a mollusk shell, the Pinctada margaritifera pearl oyster. A novel approach is further presented to efficiently compare the amount of amorphous carbonate with respect to its crystalline counterpart. Finally, the whole microscopy method is used to image in vivo the shell border and demonstrate the feasibility and the reproducibility of the technique. These findings open chemical imaging perspectives for the study of biogenic and bio-inspired crystals

    Dynamique de l’holobionte corallien et plasticité transcriptomique : variabilité interspécifique, interpopulationnelle et interindividuelle

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    International audienceLe but du projet de thèse est de tester l'hypothèse théorique : Plus un individu est soumis à un environnement fluctuant plus sa plasticité est grand

    Potential implication of host/symbiont recognition mechanisms in coral bleaching

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    Bleaching in corals can be attributed to loss of endosymbiotic zooxanthellae and/or loss of photosynthetic pigments within zooxanthellae. This major disturbance of the reef ecosystem is principally induced by increases in water temperature. Since the beginning of the 80ís and the onset of global climate change, this phenomenon has been occurring at increasing rates and scales, and with increasing severity. In this study, we focused on finding early regulated genes involved in bleaching. In aquaria, one set of Pocillopora damicornis nubbins was subjected to a gradual seawater temperature increase from 28°C to 32°C over 15 days, and a second control set remained at constant temperature (28°C). Bleaching was monitored by measuring zooxanthellae density. The mRNA differentially expressed between the stressed state (sampled just before the onset of bleaching) and the non stressed state (control) were isolated from the nubbins by Suppression Subtractive Hybridization. The corresponding cDNA were sequenced and confronted to sequence databases to obtain gene similarities. Finally, transcription rates of the most interesting genes were conducted by Q-PCR. Two particularly interesting candidate genes showed an important decrease in their transcription rates following thermal stress and before zooxanthellae loss. These two genes show similarities with genes involved in host/symbiont and host/parasite models. The implication of these molecular actors suggests a possible role of recognition mechanisms between the host and its symbiont, in the breakdown of the symbiosis during the bleaching phenomenon. Experiments such as RACE-PCR, in situ hybridization and immunohistochemistry are currently underway to confirm our hypotheses
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