[CRIOBE EPHE-PSL] Soutenance de thèse

[EN] On coral reefs, the cumulative impact of human-driven stressors has resulted in a replacement of the dominant benthic members where phase-shifts from coral to macroalgal dominance prevail. The persistence of macroalgae is favored by an intense competition against corals, limiting the recovery of coral communities. Upon contact, macroalgae can vector allelochemicals and microbes causing necrosis and coral mortality, yet their diffusion and impacts beyond the space that algae physically occupy remain unclear. This thesis aimed to address two major questions: how macroalgae modify chemical and microbial waterscapes, and how they impact coral holobiont health and recruitment. By manipulating the presence/absence of macroalgae, this thesis revealed a fine-scale spatial structuring of microbes and metabolites according to macroalgal abundance and boundary layers (i.e., benthic and momentum). Algal-associated waters were enriched with opportunistic bacteria, potentially pathogenic, and toxic molecules (i.e., diterpenes). This research demonstrated that exposure to macroalgae alter the microbiome of coral larvae (Pocillopora acuta) and negatively impact larval and recruit survival through interacting parental and environmental effects. The relative influence of contact vs. water-mediated (i.e., 2 cm) effects on coral microbiome and metabolome was specifically tested using the allelopathic alga Dictyota bartayresiana. Each type of interaction distinctly disturbed the coral microbiome and metabolome, suggesting an adjustment in lipid metabolism to meet the energetic cost of competition and the production of defense metabolites. By combining metabarcoding and non-targeted metabolomics, this thesis has described the identity and distribution of microbes and metabolites associated with coral-algae competition, providing a better understanding of the consequences of phase shifts on the resilience of coral reefs.

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Thèse dirigée par :
Maggy Nugues (EPHE) et co-encadrée par Isabelle Bonnard (UPVD) & Camille Clérissi (EPHE)

Jury composé de: