Discovery of a novel sulfur-oxidizing endosymbiont (Ca. Vesicomyosocius atacamensis) associated with a newly described Archivesica species from the Atacama Trench.

Here we report the microbiome composition and lipid (molecular and isotopic) profile of gills from Archivesica sp. Atacama., a new species of deep-sea bivalve family Vesicomyidae collected at 2839 m depth on the eastern slope of the Atacama Trench. Metabarcoding unveiled that 99.44% of the microbial ASVs (Amplicon Sequence Variant) obtained from this bivalve's gills belonged to Ca. Vesicomyosocius sp. atacamensis, a bacterium closely related to symbionts of other vesicomycoids based on the 16 S rRNA phylogeny (a putative chemoautotrophic sulfide-oxidizing bacterium Form I RubisCO). Additional ASVs included microbes from taxa known for their ability to oxidize sulfur. Consistent with the microbiome composition, the analysis of lipid biomarkers in the gills revealed a high abundance of C16:1ω7 and C18:1ω7 fatty acids, well-known markers of sulfide-oxidizing (thiotrophic) bacterial metabolisms. The δ¹³C values of the bivalve's bulk gills (-35.5‰) and of individual fatty acids (-40.0 to -46.5‰) were typical of bivalves hosting thiotrophic endosymbionts utilizing form I RubisCO for carbon fixation. In addition, nearby sediments showed a significant presence of terminal branched (iso/anteiso C13-C17), mid branched (10Me-C16 and 10Me-C18) and cyclopropyl (Cy17 and Cy19) fatty acids, coherent with sulfate-reducing bacterial (SRB) communities found by metabarcoding. These findings confirm that thiotrophic symbiosis provides energy for the new deep-sea Archivesica bivalve reported here.