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

Molecular hydrogen (H₂) 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 H₂ in 20°-375°C hydrothermal fluids to evaluate usefulness of the isotope ratio as a tracer to explore the H₂- 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 H₂ within an analytical error of 10‰ in the δD_H2 value. The δD_H2 values in the high-temperature fluids were between -405‰ and -330‰, indicating the achievement of the hydrogen isotopic equilibrium between H₂ and H₂O at around the hydrothermal end-member temperature. In contrast, several low-temperature fluids showed apparently smaller δD_H2 values than those in the high-temperature fluids in spite of a negligible δD_H2 change due to fluid-seawater mixing, suggesting the possibility of δD_H2 change in the low-temperature fluids and the surrounding environments. Since the δD_H2 change in low-temperature environments is not well explained by the very sluggish abiotic thermal isotopic equilibrium between H₂ and H₂O, it could be associated with (micro)biological H₂-consuming and/or H ₂-generating metabolisms that would strongly promote the isotopic equilibrium at low temperatures. Our first detection of the δD _H2 variation in deep-sea hydrothermal systems presents the availability of the δD_H2 value as a new tracer for microbes whose enzymes catalyze D/H exchange in H₂.