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

[BBG01-P06] Extensive and Active Methane-Metabolizing Microbiomes Unveiled in Deep Sedimentary Rocks

*Hiroki Nishimura^1,2, Mariko Kouduka¹, Akari Fukuda¹, Toyoho Ishimura³, Yuki Amano^5,4, Hikari Beppu⁵, Kazuya Miyakawa⁴, Yohey Suzuki¹ (1.Graduate School of Science, Department of Earth and Planetary Science, The University of Tokyo, 2.Biogeochemistry Research Center (BGC), Research Institute for Marine Resources Utilization (MRU), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3.Kyoto University, Graduate School of Human and Environmental Studies, 4.Horonobe Underground Research Center, Japan Atomic Energy Agency, 5.Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency)

Keywords:Methanotrophy, Subsurface Biosphere, Rock Core Incubation

The terrestrial subsurface microbiology is considered to harbor 10% of the total biomass and 90% of prokaryotic biomass on Earth where buried organic matter and hydrogen are major energy sources. However, recent studies have indicated that methane is another major energy source in the terrestrial subsurface, mainly based on omics data from groundwater samples. To directly demonstrate methane oxidation activities indicated by the omics data, we previously conducted high-pressure incubation experiments of groundwater samples retrieved from the 214-m deep subsurface of Horonobe Underground Research Laboratory (URL), Hokkaido, Japan (Nishimura et al., 2023). Although results unambiguously showed methane oxidation activities in groundwater, it remains unknown whether methane oxidation activities are mediated by microbes that inhabit rocks. This information is critical for understanding of the extent and intensity of microbial activities on Earth, given that the subsurface is mostly made up of rocks rather than groundwater.

We obtained 210 to 320-m deep rock core samples drilled near Horonobe URL. As previously conducted for other terrestrial deep drilling programs, a contamination evaluation method using a fluorescent dye was adopted for our drilling. We measured the concentrations of fluorescent dye in the drilling fluid and the exterior and interior of rock core samples. Interior subsamples were not associated with detectable levels of the fluorescent dye, whereas contamination was evident at the core exterior. The interior subsamples were incubated at ambient pressure for one week with ¹³C-labelled methane as an energy source. After the incubation experiments, changes in carbon isotope ratio of dissolved inorganic carbon (DIC) were measured by an isotope ratio mass spectrometer (IRMS) equipped with a customized continuous-flow gas preparation system. During incubation, 700 – 1000‰ enrichment in ¹³C was obtained at depths of 210, 230, 260, 290 and 320 m, demonstrating extensive and active methane oxidation activities at this drilling site. Methane oxidation rates in the core samples were several orders higher than those in a 214-m deep groundwater sample. Taken together, our results support the concept that methane is a major energy source in the deep terrestrial subsurface.

This study was carried out as a part of R&D supporting program titled “Research and development on Groundwater Flow Evaluation Technology in Bedrock” and “The project for validating near-field system assessment methodology in geological disposal” under the contract with Ministry of Economy, Trade and Industry (METI) (2020, 2021 FY, Grant Number: JPJ007597).

Reference: Nishimura et al., 2023, Environmental Microbiology Reports, EMI413146.