Molecular study of bacterial diversity within the trophosome of the vestimentiferan tubeworm Ridgeia piscesae

A large proportion of the faunal biomass in hydrothermal vent ecosystems relies on symbiotic relationships, with bacteria as a source of nutrition. Whereas multiple symbioses have been observed in diverse vent hosts, siboglinid tubeworms have been thought to harbour a single endosymbiont phylotype affiliated to the Gammaproteobacteria. In the case of the Northeast Pacific vestimentiferan Ridgeia piscesae, two previous studies suggested the presence of more than one symbiont. The possibility of multiple, and possibly habitat-specific, symbionts in R. piscesae provided a potential explanation for the tube-worm's broad ecological niche, compared with other hydrothermal vent siboglinids. This study further explored the diversity of trophosome bacteria in R. piscesae using two methodological approaches not yet applied to this symbiosis. We carried out 454-pyrosequencing on trophosome samples from 46 individual worms and used catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) to verify the presence of the major groups detected in the pyrotag data. Both methods yielded inconsistent and sometimes contradictory results between sampling sites, and neither provided irrefutable evidence for the presence of symbionts other than the expected Gammaproteobacteria. We therefore conclude that the other adaptive mechanisms must be considered to explain the broad physico-chemical niche occupied by the different growth forms of R. piscesae.