A multi-isotope study of deep-sea mussels at three different hydrothermal vent sites in the northwestern Pacific

To investigate symbiotic bacterial ecosystems at different deep-sea hydrothermal systems in the northwestern Pacific, compound-specific carbon and hydrogen isotope analyses of lipid biomarkers have been performed in addition to bulk C, H, N, S and O isotope analyses on Bathymodiolus mussels from the Hatoma seamount (B. platifrons), the Daiyon-Yonaguni (Yonaguni) knoll (B. platifrons), and the Suiyo seamount (B. septemdierum). The two B. platifrons contain large amounts of diploptene, while no hopanoid is detected in B. septemdierum, suggesting that B. platifrons and B. septemdierum harbors methanotrophic and thiotrophic bacteria, respectively. In spite of the same symbiont, the large bulk δ¹³C difference between the Hatoma (- 44.8‰) and Yonaguni (- 24.5‰) mussels reflects isotopically distinct hydrothermal CH₄ (Hatoma: ~ - 48‰, Yonaguni: ~ -26‰) as a carbon source. Fatty acids of the Hatoma and Yonaguni mussels are more enriched in D (- 144 to - 101‰) than the Suiyo mussel (- 265 to - 162‰), suggesting that D-depleted magmatic water or D-enriched hydrogen derived from CH₄ could be a partial hydrogen source for methanotrophy or thiotrophy, respectively. Apparent positive correlations are observed between δ¹³C and δD of the bulk and biomarkers for each mussel due to similar biochemical processes during de novo synthesis. The compound-specific δ¹³C and δD variations have provided much information on not only distinct carbon and hydrogen sources but also the lipid synthesis with respect to different symbiotic bacterial ecosystems.