Phylogenetic characterization of endosymbionts in three hydrothermal vent mussels: influence on host distributions

The bacterial endosymbionts of 3 hydrothermal vent mussels from Japanese wa ters were characterized by transmission electron microscopic (TEM) observat ion and phylogenetic analyses of 16S ribosomal RNA gene sequences. Endosymb ionts of Bathymodiolus septemdierum were related to sulfur-oxidizing bacter ia (thioautotrophs), while endosymbionts of B. platifrons and B. japonicus were related to methane-oxidizing bacteria (methanotrophs). This is the fir st report of deep-sea mussels containing only methanotrophs (lacking thioau totrophs) from hydrothermal vents. Comparison of methane and hydrogen sulfi de concentrations in end-member fluids from deep-sea hydrothermal vents ind icated that methane concentrations were much higher in habitats containing Bathymodiolus spp, which harbored only methanotrophs than in other habitats of hydrothermal vent mussels. The known distribution of other mussels cont aining only methanotrophs has thus far been limited to cold-seep environmen ts with high methane concentrations from the interstitial water. These resu lts suggest that the distribution of methanotrophic symbioses between deep- sea mussels and methanotrophs is strongly influenced by the methane or hydr ocarbon concentrations provided from hydrothermal vent and cold-seep activi ties (or that methane concentration is a possible limiting factor that rest ricts the distribution of methanotrophy-dependent symbioses in the deep sea ).