Stable isotope evidence for a putative endosymbiont-based lithotrophic Bathymodiolus sp. mussel community atop a serpentine seamont

A putative endosymbiont-based benthic animal community composed of at least two kinds of bivalves has been found atop the South Chamorro serpentine seamount in the Mariana fore-arc, western Pacific. Multiple stable isotopic analyses (hydrogen, carbon, nitrogen, and sulfur) were carried out on the soft tissues of an undescribed mussel species belonging to the genus Bathymodiolus. Markedly higher stable carbon (δ¹³C: - 21.4 to - 18.9‰ vs. PDB) and lower sulfur (δ³⁴S: + 10.2 to + 10.6‰ vs. CDT) isotopic compositions of the mussel tissues suggested that they utilized both methane and sulfide as nutrient sources of carbon and sulfur: Analyses of in situ shimmering water and clayey sediment under the mussel bed supported these findings. These results strongly suggest that the mussels harbored both methanotrophic and thioautotrophic bacterial symbionts in their gills. This is the first stable isotopic evidence of a dual symbiotic Bathymodiolus sp. in the western Pacific. It is proposed that the major substrates for energy acquisition-methane and hydrogen sulfide-may be responsible for results from serpentinization in the lower crust. This abiotic process would be followed by abiotic or archaeal methane production and bacterial sulfate-reduction as evidenced by high δ¹³C value (- 14.6‰) of methane in the shimmering water and low δ³⁴S value (- 32.3‰) of sulfides in the immediate sediments, respectively. Due to the very low organic matter content (0.5 mg C/g dry sediment) of the immediate sediment, it is further proposed that sulfate-reduction occurred using hydrogen (or possibly methane) as an electron donor. Thus, the mussel community living independently of magmatism and sedimentary organic matter is a lithotrophic animal community.