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

Shinsuke Kawagucci,Yuji Onishi,Takuro Nunoura,Akiko Makabe,Yohei Matsui,Tatsuhiro Fukuba,Taichi Yokokawa

doi:10.5194/bg-18-5351-2021

Abstract

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.

Highlights

Stable isotope ratios have been widely used for tracing biogeochemical cycles and microbial activities (e.g., Ohkouchi et al, 2010; Sharp, 2017)
Where the subscripts designate initial reactant (r0) and reactant remaining in a sample, and f represents the fraction of the reactant consumed after the reaction
Vertical profiles of biogeochemical parameters drawn by altitude from the seafloor are presented in Fig. 2, and with all data provided as Table S1

Summary

Introduction

Stable isotope ratios have been widely used for tracing biogeochemical cycles and microbial activities (e.g., Ohkouchi et al, 2010; Sharp, 2017). To ensure the reliable use of isotope tracers in biogeoscience, it is important to understand the isotope fractionation factors associated with expected processes in the environment of interest. The isotope fractionation factor associated with microbial metabolism is known to be variable due to physiological responses to environmental conditions such as temperature and substrate availability (e.g., Valentine et al, 2004). S. Kawagucci et al.: Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation the fractionation factor in the natural environment is most appropriately done via observation

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