[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-10] Anaerobic methane-oxidizing activity in a deep underground borehole demonstrated by in-situ pressure groundwater incubation

*Hiroki Nishimura¹, Mariko Kouduka¹, Akari Fukuda¹, Toyoho Ishimura², Yuki Amano^3,4, Hikari Beppu⁴, Kazuya Miyakawa³, Yohey Suzuki¹ (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Graduate School of Human and Environmental Studies, Kyoto University, 3.Horonobe Underground Research Center, Japan Atomic Energy Agency, 4.Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency)

Keywords:Anaerobic Oxidation of Methane, Ca. Methanoperedenaceae Archaea, Terrestrial Subsurface Biosphere

The family Candidatus Methanoperedenaceae archaea mediate anaerobic oxidation of methane (AOM) in various terrestrial environments. In near-surface environments, the metabolic details of AOM conducted by this archaeal family have been characterized through ambient pressure incubations. Although the dominance of Ca. Methanoperedenaceae archaea has been also reported from deep-subsurface groundwater samples, the in-situ metabolic activity of AOM is poorly constrained. In this study, we newly developed a high-pressure laboratory incubation system that maintains the hydraulic pressure ranging from ambient to 5 MPa. Using this system, we investigated geochemically similar groundwater samples from 214- and 249-m deep boreholes at Horonobe Underground Research Laboratory, Japan, where the high and low abundances of Ca. Methanoperedenaceae archaea have been revealed by genome-resolved metagenomic analysis, respectively. We incubated the groundwater samples with ¹³C-labelled methane as an AOM substrate and with or without amorphous Fe(III) as an electron acceptor at an in-situ pressure of 1.6 MPa. After three to seven-day incubation, AOM activities were not detected from the 249-m deep groundwater but from the 214-m deep groundwater. The AOM rates were 93.7 ± 40.6 and 27.7 ± 37.5 nM/day with or without Fe(III) amendment, proposing Fe(III)-dependent AOM. To clarify the differences in AOM activity between the 214- and 249-m deep groundwater samples, we characterized Fe(III)-bearing compounds in suspended particulates collected by filtration. The particulates were scarce in the 249-m deep groundwater on the filter, while the abundance of Fe(III)-bearing phyllosilicates was confirmed in the 214-m deep groundwater. These results support the in-situ activity of AOM in the deep subsurface borehole using Fe(III)-bearing insoluble electron acceptors.

The Ministry of Economy, Trade and Industry of Japan has funded a part of the work as “The project for validating near-field system assessment methodology in geological disposal system” (2020 FY, Grant Number: JPJ007597).