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

Abstract

Abstract. Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the practicality 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 been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater column 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 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 reported values 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 (9.4/8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values determined herein are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach used herein to other habitats with methanotrophic activity will help reveal whether the small εH value observed herein is a ubiquitous feature across all marine systems.

First asteroid gas sample delivered by the Hayabusa2 mission: A treasure box from Ryugu

“…At JAMSTEC: Methane and ethane in the gas pipette (NT1P1B) and blank pipette (NT5P1D) were purified by purge-and-trap gas chromatography before the introduction into an isotope ratio mass spectrometer (MAT253, Thermo Fisher Scientific, Bremen, Germany) ( 42 , 43 ). Neither methane nor ethane was detected (cf.…”