Characterization of tissue-associated bacterial community of two Bathymodiolus species from the adjacent cold seep and hydrothermal vent environments.

Deep-sea mussels are widely distributed in marine chemosynthetic ecosystems. Bathymodiolus platifrons and B. japonicus, occurring at both cold seeps and hydrothermal vents, have been reported to house exclusively methanotrophic symbionts in the gill. However, the comparison of microbiota associated with different tissues between these two species from two contrasting habitats is still limited. In this study, using B. platifrons and B. japonicus collected from the adjacent cold seep and hydrothermal vent environments, we sampled different tissues (gill, adductor muscle, mantle, foot, and visceral mass including the gut) to decipher the microbial community structure at the tissue scale by employing 16S rRNA gene sequencing strategy. In the gill of both seep mussels and vent mussels, the symbiont gammaproteobacterial Methylomonaceae was the predominant lineage, and methane oxidation was identified as one of the most abundant putative function. In comparison, abundant families in other tissues were Pseudomonadaceae and Enterobacteriaceae in seep mussels and vent mussels, respectively, which may get involved in element cycling. The results revealed high similarity of community structure between two mussel species from the same habitat. The gill showed distinctive bacterial community structure compared with other tissues within the same environment, while the gill communities from two environments were more similar. Remarkably structural variations of adductor muscle, mantle, foot, and visceral mass were observed between two environments. This study can extend the understanding on the characteristics of tissue-associated microbiota of deep-sea mussels from the adjacent cold seep and hydrothermal vent environments.