PhD student: Corentin Hochard, Thesis director : Pierre Galand
Thesis defended the 30th of may 2023
Tropical coral reefs depend on complex microbial communities that drive biogeochemical cycles, maintain host health, and support ecosystem homeostasis. Understanding the complex ecology of coral reef microorganisms is essential for the preservation of these precious ecosystems. However, the precise functional role of the reef microbial communities remains poorly known. In particular, the association between corals and bacteria of the Endozoicomonadaceae family, believed to be a crucial coral bacterial symbiont, is still not well defined. Microorganisms such as Endozoicomonadaceae appear essential for the survival of the adult coral host, but larval settlement is another important element for the corals’ fitness. The success of larval recruitment has recently been shown to depend on the Crustose Coraline Algae (CCA) on which they settle. More precisely, microbial communities associated with CCAs may play a crucial role, yet we know very little about these communities. The overall objectives of this thesis were to study the species diversity and the functional potential of the microbial communities associated with tropical corals and crustose coralline algae (CCA). Chapter 2 focused on Endozoicomonadaceae associated to three coral species across the Pacific Ocean. It revealed that different coral species exhibit distinct strategies of host-symbiont relationships. We identified three new symbiont species, each with distinct functional adaptations that may drive the host-symbiont relationship. The environment had generally only a small effect on Endozoicomonadaceae community composition, while the genetic lineage of the host was important in some corals. We suggest that the relation between Endozoicomonadaceae and the coral can range from stable co-dependent relationships to opportunistic associations. In Chapter 3, we described the microbial communities associated to different CCA species across spatial scales and defined the factors controlling their composition. We also tested if their were some links between the CCA and coral larvae microbial communities. Our results suggest that the CCA microbiome does not act as a microbial reservoir for the developing coral larvae. However, we observed that the microbial communities of coral recruits differed depending on their association with different types of algae. We conclude that CCAs and their associated bacteria influence the composition of the coral recruits’ microbiome. Additionally, we showed that different CCA species exhibit distinct microbial communities, with potential signal of phylosymbiosis, suggesting adaptability of the microbiome through evolutionary time. In Chapter 4, we studied the functional potential of CCA microbial communities. We demonstrate that CCA harbor distinct functional communities despite sharing a strong core functional metabolisms. The microbial community of the two CCA species that we targeted did not show clear differences in their ability to produce coral larvae inducers. However, inducing functional capabilities were not homogenous across microbial genera between CCA species. We suggest that microbial communities do not directly determine the behaviour of larvae settlement, but rather enhance or mitigate the response induced by the CCA and the environment.
Ecology of bacterial symbionts associated with corals and crustose coralline algae in the Pacific Ocean: from genomes to communities
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