Abstract. 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 (εH=49.4±5.0 ‰, εC=5.2±0.4 ‰) – the former being the first of its kind ever
reported. This εH 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 δ13CCH4 and δDCH4, Λ (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
εH value observed is a ubiquitous feature across all
marine systems.
Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water
