A specific and widespread association between deep-sea Bathymodiolus mussels and a novel family of Epsilonproteobacteria

Bathymodiolus mussels dominate animal communities at many hydrothermal vents and cold seeps. Essential to the mussels' ecological and evolutionary success is their association with symbiotic methaneand sulfur-oxidizing gammaproteobacteria, which provide them with nutrition. In addition to these wellknown gammaproteobacterial endosymbionts, we found epsilonproteobacterial sequences in metatranscriptomes, metagenomes and 16S rRNA clone libraries as well as by polymerase chain reaction screening of Bathymodiolus species sampled from vents and seeps around the world. These epsilonproteobacterial sequences were closely related, indicating that the association is highly specific. The Bathymodiolus-associated epsilonproteobacterial 16S rRNA sequences were at most 87.6% identical to the closest cultured relative, and 91.2% identical to the closest sequences in public databases. This clade therefore represents a novel family within the Epsilonproteobacteria. Fluorescence in situ hybridization and transmission electron microscopy showed that the bacteria are filamentous epibionts associated with the gill epithelia in two Bathymodiolus species. In animals that host highly specific symbioses with one or a few types of endosymbionts, other lessabundant members of the microbiota can be easily overlooked. Our work highlights how widespread and specific associations with less-abundant microbes can be. Possibly, these microbes play an important role in the survival and health of their animal hosts