Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the reliable use of isotope tracers in biogeosciences at large. Methane is a key component of the subsurface biosphere and a notable greenhouse gas, making the accurate evaluation of methane cycles, including microbial methanotrophy, imperative. Although the isotope fractionation factors associated with methanotrophy have been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater remain unclear. Here, we investigated hydrogen and carbon isotope ratios of methane as well as the relevant biogeochemical parameters and microbial community compositions in hydrothermal plumes in the Okinawa Trough. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, and we succeeded in simultaneously determining hydrogen and carbon isotope fractionation factors associated with the aerobic oxidation of methane (epsilon(H) = 49:4 +/- 5:0 %, epsilon(C) = 5 2 +/- 0 :4 %) - the former being the first of its kind ever reported. This epsilon(H) value is comparable with values reported from terrestrial ecosystems but clearly lower than those from aerobic and anaerobic methanotroph enrichment cultures, as well as incubations of methanotrophic isolates. The covariation factor between delta(CH4)-C-13C and delta(CH4)-C-D, Lambda (9.4 or 8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach to other habitats with methanotrophic activity will help reveal whether the small epsilon(H) value observed is a ubiquitous feature across all marine systems.

Automated summary

The study investigated hydrogen and carbon isotope ratios of methane as well as the associated fractionation factors in hydrothermal plumes in the Okinawa Trough, shedding light on the dynamics of methane cycling in the marine realm. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, providing important insights into microbial communities in this region.