Isotopic variation of molecular hydrogen in 20°–375°C hydrothermal fluids as detected by a new analytical method

Molecular hydrogen (H 2 ) is one of the most important energy sources for subseafloor chemolithoautotrophic microbial ecosystems in the deep‐sea hydrothermal environments. This study investigated stable isotope ratios of H 2 in 20°–375°C hydrothermal fluids to evaluate usefulness of the isotope ratio as a tracer to explore the H 2 ‐metabolisms. Prior to the observation, we developed an improved analytical method for the determination of concentration and stable isotope ratio of H 2 . This method achieved a relatively high sensitivity with a detection limit of 1 nmol H 2 within an analytical error of 10‰ in the δ D H2 value. The δ D H2 values in the high‐temperature fluids were between −405‰ and −330‰, indicating the achievement of the hydrogen isotopic equilibrium between H 2 and H 2 O at around the hydrothermal end‐member temperature. In contrast, several low‐temperature fluids showed apparently smaller δ D H2 values than those in the high‐temperature fluids in spite of a negligible δ D H2 change due to fluid‐seawater mixing, suggesting the possibility of δ D H2 change in the low‐temperature fluids and the surrounding environments. Since the δ D H2 change in low‐temperature environments is not well explained by the very sluggish abiotic thermal isotopic equilibrium between H 2 and H 2 O, it could be associated with (micro)biological H 2 ‐consuming and/or H 2 ‐generating metabolisms that would strongly promote the isotopic equilibrium at low temperatures. Our first detection of the δ D H2 variation in deep‐sea hydrothermal systems presents the availability of the δ D H2 value as a new tracer for microbes whose enzymes catalyze D/H exchange in H 2 .