Intracellular O ceanospirillales inhabit the gills of the hydrothermal vent snail A lviniconcha with chemosynthetic, γ‐ P roteobacterial symbionts

Summary Associations between bacteria from the γ‐Proteobacterial order O ceanospirillales and marine invertebrates are quite common. Members of the O ceanospirillales exhibit a diversity of interactions with their various hosts, ranging from the catabolism of complex compounds that benefit host growth to attacking and bursting host nuclei. Here, we describe the association between a novel O ceanospirillales phylotype and the hydrothermal vent snail A lviniconcha . A lviniconcha typically harbour chemoautotrophic γ‐ or ε‐Proteobacterial symbionts inside their gill cells. Via fluorescence in situ hybridization and transmission electron microscopy, we observed an O ceanospirillales phylotype (named AOP for ‘ A lviniconcha   O ceanospirillales phylotype’) in membrane‐bound vacuoles that were separate from the known γ‐ or ε‐Proteobacterial symbionts. Using quantitative polymerase chain reaction, we surveyed 181 A lviniconcha hosting γ‐Proteobacterial symbionts and 102 hosting ε‐Proteobacterial symbionts, and found that the population size of AOP was always minor relative to the canonical symbionts (median 0.53% of the total quantified 16S rRNA genes). Additionally, we detected AOP more frequently in A lviniconcha hosting γ‐Proteobacterial symbionts than in those hosting ε‐Proteobacterial symbionts (96% and 5% of individuals respectively). The high incidence of AOP in γ‐Proteobacteria hosting A lviniconcha implies that it could play a significant ecological role either as a host parasite or as an additional symbiont with unknown physiological capacities.