Symbioses bactériennes de bivalves mytilidés associés aux sources de fluides en domaine océanique profond : diversité, rôle nutritionnel et influence de l'environnement

Symbiotic associations between chemosynthetic bacteria and invertebrates are one of the major sources of primary production in deep-sea chemosynthesis-based ecosystems. Mussels of the genus Bathymodiolus (Bivalvia: Mytilidae) occur worldwide and can dominate the animal community at methane-rich cold seeps and sulfide-rich hydrothermal vents. Some species are associated with both sulfide-oxidizing and methanotrophic bacteria. These mussels are thus a good model for studying the diversity, role and ecology of multiple symbiotic associations, which are still poorly understood. For the present study, several species of mytilids were collected from seep and vent sites situated at depths from 850 to 3300 meters using submersibles. Symbiotic bacteria associated with these mytilids were characterized using comparative 16S rRNA and functional gene sequence analysis. In two species, the diversity of symbionts was much greater than expected, with four bacterial phylotypes in B. heckerae from the Gulf of Mexico and six phylotypes in Idas sp., a small mytilid from the eastern Mediterranean. The existence of an environmental influence on the abundance of the different bacterial phylotypes was assessed using fluorescence in situ hybridization and quantitative RNA hybridization. For Bathymodiolus sp. from the Gulf of Guinea, results were compared to environmental data, and no variation was seen between specimens collected on mussel beds with different methane concentrations. Relative and overall abundances of bacteria varied between species and sites in B. brooksii (Gulf of Mexico), B. azoricus and B. puteoserpentis (Mid-Atlantic Ridge). For the latter two species, the variation could be linked to characteristics of hydrothermal fluids, in particular to methane concentrations. The nutritional role of the symbiotic bacteria was investigated using stable isotope analyses, performed on whole tissues and specific biomarkers such as lipids and RNA. In B. heckerae, B. brooksii and B. childressii from the Gulf of Mexico, in which methanotrophs are the most abundant symbionts, methanotrophy was confirmed as the main source of carbon nutrition. This study emphasizes the relevance of combining molecular and stable isotope approaches with deep-sea environment investigations to understand the symbioses that fuel the metazoan communities occurring in cold seep and hydrothermal ecosystems.