Marie La Rivière

Transient shifts in bacterial communities associated with the temperate gorgonian Paramuricea clavata in the Northwestern Mediterranean Sea.

Background Bacterial communities that are associated with tropical reef-forming corals are being increasingly recognized for their role in host physiology and health. However, little is known about the microbial diversity of the communities associated with temperate gorgonian corals, even though these communities are key structural components of the ecosystem. In the Northwestern Mediterranean Sea, gorgonians undergo recurrent mass mortalities, but the potential relationship between these events and the structure of the associated bacterial communities remains unexplored. Because microbial assemblages may contribute to the overall health and disease resistance of their host, a detailed baseline of the associated bacterial diversity is required to better understand the functioning of the gorgonian holobiont. Methodology/Principal Findings The bacterial diversity associated with the gorgonian Paramuricea clavata was determined using denaturing gradient gel electrophoresis, terminal-restriction fragment length polymorphism and the construction of clone libraries of the bacterial 16S ribosomal DNA. Three study sites were monitored for 4 years to assess the variability of communities associated with healthy colonies. Bacterial assemblages were highly dominated by one Hahellaceae-related ribotype and exhibited low diversity. While this pattern was mostly conserved through space and time, in summer 2007, a deep shift in microbiota structure toward increased bacterial diversity and the transient disappearance of Hahellaceae was observed. Conclusion/Significance This is the first spatiotemporal study to investigate the bacterial diversity associated with a temperate shallow gorgonian. Our data revealed an established relationship between P. clavata and a specific bacterial group within the Oceanospirillales. These results suggest a potential symbiotic role of Hahellaceae in the host-microbe association, as recently suggested for tropical corals. However, a transient imbalance in bacterial associations can be tolerated by the holobiont without apparent symptoms of disease. The subsequent restoration of the Hahellaceae-dominated community is indicative of the specificity and resilience of the bacteria associated with the gorgonian host.

Evidence for host specificity among dominant bacterial symbionts in temperate gorgonian corals

Gorgonian corals serve as key engineering species within Mediterranean rocky-shore communities that have recently suffered from repeated mortality events during warm temperature anomalies. Among the factors that may link thermal conditions with disease outbreaks, a number of bacterial pathogens have been implicated; they may take advantage of decreases in the defenses and/or overall health of the gorgonian hosts. Considering the beneficial role of the resident bacteria in tropical coral holobionts, a detailed characterization of the gorgonian-associated microbial populations is required to better understand the relationships among native microbiota, host fitness, and pathogen susceptibility. In this study, the bacterial communities associated with three sympatric gorgonian species, Eunicella singularis, Eunicella cavolini, and Corallium rubrum, were investigated to provide insight into the stability and the specificity of host–microbe interactions. Natural variations in bacterial communities were detected using terminal restriction fragment length polymorphism (T-RFLP) of the 16S ribosomal DNA. No major differences were identified between individual colonies sampled in winter or in summer within each gorgonian species. Although hierarchical cluster analysis of the T-RFLP profiles revealed that the three species harbor distinct communities, comparison of the T-RFLP peaks indicated the presence of common bacterial ribotypes. From phylogenetic analysis of 16S rDNA clone libraries, we identified a bacterial lineage related to the Hahellaceae family within the Oceanospirillales that is shared among E. singularis, E. cavolini, and C. rubrum and that dominates the communities of both species of Eunicella. However, distinct clades of Hahellaceae are harbored by various gorgonian species from Mediterranean and tropical waters, suggesting that these bacteria have formed host-specific symbiotic relationships with gorgonian octocorals. In addition, the relatedness of symbionts from host species belonging to the same taxon but occurring in geographically remote areas is consistent with codivergence between gorgonians and their associated bacteria.

Regional and local environmental conditions do not shape the response to warming of a marine habitat-forming species

The differential response of marine populations to climate change remains poorly understood. Here, we combine common garden thermotolerance experiments in aquaria and population genetics to disentangle the factors driving the population response to thermal stress in a temperate habitat-forming species: the octocoral Paramuricea clavata. Using eight populations separated from tens of meters to hundreds of kilometers, which were differentially impacted by recent mortality events, we identify 25 °C as a critical thermal threshold. After one week of exposure at this temperature, seven of the eight populations were affected by tissue necrosis and after 30 days of exposure at this temperature, the mean % of affected colonies increased gradually from 3 to 97%. We then demonstrate the weak relation between the observed differential phenotypic responses and the local temperature regimes experienced by each population. A significant correlation was observed between these responses and the extent of genetic drift impacting each population. Local adaptation may thus be hindered by genetic drift, which seems to be the main driver of the differential response. Accordingly, conservation measures should promote connectivity and control density erosion in order to limit the impact of genetic drift on marine populations facing climate change.