Summary: We report here a molecular survey based on 16S rRNA genes of the bacterial diversity found in two deep‐sea vent niches at the Mid‐Atlantic Ridge: hydrothermal sediment (Rainbow site), and microcolonizers made of three different substrates (organic‐rich, iron‐rich and pumice) that were exposed for 15 days to a vent emission. Bacterial diversity in sediment samples was scattered through many bacterial divisions. The most abundant and diverse environmental sequences (phylotypes) in our libraries corresponded to the Gammaproteobacteria, followed by the Acidobacteria. We detected members of all the subdivisions within the Proteobacteria. Myxobacterial lineages were the most represented within the delta subdivision. Phylotypes ascribing to the Cytophaga‐Flavobacterium‐Bacteroides , Planctomycetales, high and low G + C Gram‐positives, Nitrospirae, and the candidate division TM7 were also identified. Compared to this broad taxonomic coverage, microcolonizers were almost exclusively colonized by epsilonproteobacteria, although these exhibited considerable morphological and phylogenetic in‐group diversity. No specificity for any of the substrates tested was seen. This observation further supports the idea of the ecological dominance of epsilonproteobacteria in the fluid–seawater interface environment. Because oxidation of reduced S species and/or sulphur‐reduction is thought to be essential for their energetic metabolism in these areas, we mapped different oxidation states of S in individual bacterial filaments from the iron‐rich microcolonizer. For this, we used high‐resolution, non‐destructive synchrotron micro‐X‐ray Absorption Near‐Edge Spectroscopy (micro‐XANES), which revealed the co‐existence of different S oxidation states, from sulphide to sulphate, at the level of individual cells. This suggests that these cells were metabolizing sulphur in situ .

Deep-sea mussels of the genus Bathymodiolus (Bivalvia: Mytilidae) harbor symbiotic bacteria in their gills and are among the dominant invertebrate species at cold seeps and hydrothermal vents. An undescribed Bathymodiolus species was collected at a depth of 3,150 m in a newly discovered cold seep area on the southeast Atlantic margin, close to the Zaire channel. Transmission electron microscopy, comparative 16S rRNA analysis, and fluorescence in situ hybridization indicated that this Bathymodiolus sp. lives in a dual symbiosis with sulfide- and methane-oxidizing bacteria. A distinct distribution pattern of the symbiotic bacteria in the gill epithelium was observed, with the thiotrophic symbiont dominating the apical region and the methanotrophic symbiont more abundant in the basal region of the bacteriocytes. No variations in this distribution pattern or in the relative abundances of the two symbionts were observed in mussels collected from three different mussel beds with methane concentrations ranging from 0.7 to 33.7 μM. The 16S rRNA sequence of the methanotrophic symbiont is most closely related to those of known methanotrophic symbionts from other bathymodiolid mussels. Surprisingly, the thiotrophic Bathymodiolus sp. 16S rRNA sequence does not fall into the monophyletic group of sequences from thiotrophic symbionts of all other Bathymodiolus hosts. While these mussel species all come from vents, this study describes the first thiotrophic sequence from a seep mussel and shows that it is most closely related (99% sequence identity) to an environmental clone sequence obtained from a hydrothermal plume near Japan.

Mussels of the subfamily Bathymodiolinae thrive around chimneys emitting hot fluids at deep sea hydrothermal vents, as well as at cold seeps and on sunken organic debris (sunken wood, whale falls). Despite the absence of light-driven primary production in these deep-sea ecosystems, mussels succeed reaching high biomasses in these harsh conditions thanks to chemosynthetic, carbon-fixing bacterial symbionts located in their gill tissue. Since the discovery of mussel symbioses about three decades ago our knowledge has increased, yet new findings are published regularly regarding their diversity, role and evolution. This article attempts to summarize current knowledge about symbiosis in Bathymodiolinae, focusing on mussel species for which information is available regarding both hosts and symbionts. Moreover, new data obtained from small mussels inhabiting sunken woods around the Philippines are provided. Indeed, mussel species from organic falls remain poorly studied compared to their vent and seep relatives despite their importance for the understanding of the evolution of symbiosis in the subfamily Bathymodiolinae.