Independent phylogenetic origins of methanotrophic and chemoautotrophic bacterial endosymbioses in marine bivalves

The discovery of bacterium-bivalve symbioses capable of utilizing methane as a carbon and energy source indicates that the endosymbionts of hydrothermal vent and cold seep bivalves are not restricted to sulfur-oxidizing chemoautotrophic bacteria but also include methanotrophic bacteria. The phylogenetic origin of methanotrophic endosymbionts and their relationship to known symbiotic and free-living bacteria, however, have remained unexplored. In situ localization and phylogenetic analysis of a symbiont 16S rRNA gene cloned from the gills of a recently described deep-sea mussel species demonstrate that this symbiont represents a new taxon which is closely related to free-living, cultivable Type I methanotrophic bacteria. This symbiont is distinct from known chemoautotrophic symbionts. Thus, despite compelling similarities between the symbio-ses, chemoautotrophic and methanotrophic symbionts of marine bivalves have independent phylogenetic origins. The recent discovery of symbioses between methanotrophic bacteria and marine bivalves has demonstrated that bacterium-invertebrate associations of the types that are known to occur in the unique fauna of deep-sea hydrothermal vents and cold seeps are more diverse and more evolutionarily complex tha