Bacterial endosymbiont of Oligobrachia sp. (Frenulata) from an active mud volcano in the Canadian Beaufort Sea

Siboglinid tubeworms of the genus Oligobrachia that thrive in obligatory association with endosymbionts have been predominantly observed in Arctic and high-latitude Atlantic cold seeps. Metabolic features of endosymbionts provide fundamental understanding for the survival strategy of tubeworms in cold seeps. However, no information on the bacterial endosymbionts of tubeworms from the Canadian Beaufort Sea has been available until now. In this study, we investigated the phylogeny and metabolic potential of a bacterial endosymbiont of siboglinid tubeworms from an active mud volcano in the Canadian Beaufort Sea using Illumina MiSeq sequencing of 16S rRNA gene amplicons. The siboglinid tubeworm shared 99.9% 18S rRNA gene sequence similarity with Oligobrachia haakonmosbiensis and 99.7-99.8% mitochondrial cytochrome C oxidase subunit I gene similarity with members of Oligobrachia sp. CPL-clade and was designated 'Oligobrachia sp. BS1.' The endosymbiont of Oligobrachia sp. BS1, which belongs to the Gammaproteobacteria, was most closely related to endosymbionts of Oligobrachia sp. CPL-clade, revealing the close relationships between the endosymbionts and their hosts. The bacterial endosymbiont of Oligobrachia sp. BS1 contained the key gene required for sulfur oxidation, aprA gene encoding the alpha-subunit of adenosine 1,5-phosphosulfate reductase, suggesting that this endosymbiont is capable of using sulfide as an energy source. The bacterial endosymbiont of an Oligobrachia species from an active mud volcano in the Canadian Beaufort Sea presented here expands our knowledge of the identities and thiotrophic metabolism of endosymbionts that are associated with hosts that dominate a wide range of methane seep habitats in the Arctic.