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Importance des assemblages interspécifiques face aux variations environnementales. Étude intégrée comparative de différentes associations corail-bénitier

Abstract : Interaction between organisms, direct or indirect, involved in the competition for food and space, or other mechanisms, are crucial to develop appropriate responses. For marine sessile organisms, interactions are mainly chemical. In addition to interactions, environmental pressures linked to global changes highly impact the physiology of organisms and their responses abilities, particularly in coral reef organisms that live in shallow waters strongly exposed to thermal stress. However, studies on marine sessile organisms have mainly focused on individual responses of organisms, using mono-specific experiment. During this thesis, the influence of neighboring species on the individual response of organisms was examined. Thus, various assemblages of corals (Pocillopora damicornis and Acropora cytherea) and giant clams (Tridacna maxima) were studied ex situ through a thermal stress experiment or in situ in Moorea lagoon (French Polynesia). Using an integrative approach, combining cytology, metabolomics and metabarcoding; the effect of assemblages on holobionts and their near environment was explored. This study showed that assemblages influence the biofouling development and composition but also coral recruitment, both qualitatively and quantitatively. The most significant impact was the antifouling effect observed when the three species were placed together. Holobionts analyses highlighted the impact of interaction on the organisms’ physiology, showing an increase of the dimethylsulfoniopropionate concentration, a key compound in the global sulfur cycle, for A. cytherea and T. maxima species according to multiple-species assemblages. Holobionts health status was also impacted according to the assemblages. Assemblages with corals, especially A. cytherea, leads to high giant clam mortality, particularly during thermal stress. Interestingly, it should be noted that holobiont composition (Symbiodiniaceae for the three species and bacteria for the giant clams) appears to depend on the nature and health status of the host, rather than the assemblages or the increase of seawater temperature. In conclusion, the overall results have clearly shown that marine sessile organisms differs in physiology and react differently to environmental pressures and neighboring species. This work highlights the importance of taking into account the surrounding diversity for predicting the future fate of coral reefs.
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Isis Guibert. Importance des assemblages interspécifiques face aux variations environnementales. Étude intégrée comparative de différentes associations corail-bénitier. Biochimie, Biologie Moléculaire. Sorbonne Université, 2018. Français. ⟨NNT : 2018SORUS337⟩. ⟨tel-02772634⟩

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