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Ecologie chimique du dinoflagellé toxique Ostreopsis cf. ovata en Méditerranée Nord Occidentale

Abstract : Toxic microalgae are known to produce a large panel of secondary metabolites whose effect on the environment remains so far poorly understood. To better understand this ecological problematic, this thesis aimed to decipher the chemical ecology of the benthic, toxic dinoflagellate Ostreopsis cf. ovata in the Northern Occidental Mediterranean Sea. First, we realized a monitoring of O. cf. ovata abundances on biotic substrates (macroalgae) and in the water column to confirme the tychoplanctonic lifestyle of O. cf. ovata, meaning that this dinoflagellate alternatively switches between benthic and planktonic phase within a single day. Because of these daily changes, it thus impacts both benthic and planktonic ecosystems. In order to study the interactions between O. cf. ovata and its environment during both benthic and planktonic phases, we evaluated its impact on survival, growth, and metabolome of various biologic models, including competitors (diatoms), several direct (benthic and planktonic copepods) and indirect (sea urchins) predators during in situ and laboratory assays. Both in situ and laboratory monitoring could not reveal any effect of O. cf. ovata on the physiology and behavior of sea urchins, suggesting that the invertebrate mass mortality episodes that have been previously documented rather result of the hypoxia linked to O. cf ovata blooms. By contrast, our results showed that O. cf. ovata inhibits the growth of competitor diatoms, while the later in return inhibits photosynthetic efficiency and growth of O. cf. ovata. Finally, our results exhibited species-related effects on copepods, with most importantly reprotoxic effects on benthic species. To further analyze toxic effects of O. cf. ovata strains during blooms in a faster, easier and steadier way, we then adapted to this dinoflagellate an Artemia fransciscana based toxicity assay. Finally, this work was complemented by a metabolomic approach, which allowed us to identify the nature of the secondary metabolites produced by O. cf. ovata. Among these metabolites, toxins are likely to take part in the biological interactions mentioned above. In addition, our results further suggested that other metabolites of unknown nature also contribute to the chemical ecology of this species. In sum, our work demonstrates that O. cf. ovata is a valuable biological model to study chemical ecology of toxic marine microalgae, because of its frequent blooms, its tychoplanctonic lifestyle as well as the great diversity of secondary metabolites it produces.
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Anne-Sophie Pavaux. Ecologie chimique du dinoflagellé toxique Ostreopsis cf. ovata en Méditerranée Nord Occidentale. Ecosystèmes. Sorbonne Université, 2019. Français. ⟨NNT : 2019SORUS292⟩. ⟨tel-03015932⟩

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