title: Biogeochemical signals from deep microbial life in terrestrial crust.
abstract: In contrast to the deep subseafloor biosphere, a volumetrically vast and stable habitat for microbial life in the terrestrial crust remains poorly explored. For the long-term sustainability of a crustal biome, high-energy fluxes derived from hydrothermal circulation and water radiolysis in uranium-enriched rocks are seemingly essential. However, the crustal habitability depending on a low supply of energy is unknown. We present multi-isotopic evidence of microbially mediated sulfate reduction in a granitic aquifer, a representative of the terrestrial crust habitat. Deep meteoric groundwater was collected from underground boreholes drilled into Cretaceous Toki granite (central Japan). A large sulfur isotopic fractionation of 20-60‰ diagnostic to microbial sulfate reduction is associated with the investigated groundwater containing sulfate below 0.2 mM. In contrast, a small carbon isotopic fractionation (<30‰) is not indicative of methanogenesis. Except for 2011, the concentrations of H2 ranged mostly from 1 to 5 nM, which is also consistent with an aquifer where a terminal electron accepting process is dominantly controlled by ongoing sulfate reduction. High isotopic ratios of mantle-derived 3He relative to radiogenic 4He in groundwater and the flux of H2 along adjacent faults suggest that, in addition to low concentrations of organic matter (<70 µM), H2 from deeper sources might partly fuel metabolic activities. Our results demonstrate that the deep biosphere in the terrestrial crust is metabolically active and playing a crucial role in the formation of reducing groundwater even under low-energy fluxes.
authors.affiliation: Graduate School of Science, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033, Japan; Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
authors.forename: Yohey
authors.initials: Y
authors.lastname: Suzuki
authors.affiliation: Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
authors.forename: Uta
authors.initials: U
authors.lastname: Konno
authors.affiliation: Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan; Japan Atomic Energy Agency, 1-64 Yamanouchi, Akiyo-cho, Mizunami, Gifu 509-6132, Japan.
authors.forename: Akari
authors.initials: A
authors.lastname: Fukuda
authors.affiliation: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
authors.forename: Daisuke D
authors.initials: DD
authors.lastname: Komatsu
authors.affiliation: Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
authors.forename: Akinari
authors.initials: A
authors.lastname: Hirota
authors.affiliation: Graduate School of Science, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Katsuaki
authors.initials: K
authors.lastname: Watanabe
authors.affiliation: Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Togo
authors.affiliation: Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
authors.forename: Noritoshi
authors.initials: N
authors.lastname: Morikawa
authors.affiliation: Japan Atomic Energy Agency, 1-64 Yamanouchi, Akiyo-cho, Mizunami, Gifu 509-6132, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Hagiwara
authors.affiliation: Japan Atomic Energy Agency, 1-64 Yamanouchi, Akiyo-cho, Mizunami, Gifu 509-6132, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Aosai
authors.affiliation: Japan Atomic Energy Agency, 1-64 Yamanouchi, Akiyo-cho, Mizunami, Gifu 509-6132, Japan.
authors.forename: Teruki
authors.initials: T
authors.lastname: Iwatsuki
authors.affiliation: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
authors.forename: Urumu
authors.initials: U
authors.lastname: Tsunogai
authors.affiliation: Low Level Radioactivity Laboratory, Kanazawa University, 24, Wake, Nomi, Ishikawa 923-1224, Japan.
authors.forename: Seiya
authors.initials: S
authors.lastname: Nagao
authors.affiliation: Geological Survey of Japan, Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
authors.forename: Kazumasa
authors.initials: K
authors.lastname: Ito
authors.affiliation: Japan Atomic Energy Agency, 1-64 Yamanouchi, Akiyo-cho, Mizunami, Gifu 509-6132, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Mizuno
mesh_terms: D017753:Ecosystem; D004734:Energy Metabolism; D019015:Geologic Sediments / Q000737:chemistry / Q000382:microbiology; D060587:Groundwater / Q000737:chemistry / Q000382:microbiology; D006863:Hydrogen-Ion Concentration; D007564:Japan; D008697:Methane / Q000737:chemistry / Q000378:metabolism; D008829:Microbiology; D009792:Oceans and Seas; D012822:Silicon Dioxide / Q000737:chemistry / Q000378:metabolism; D013431:Sulfates / Q000737:chemistry / Q000378:metabolism; D013696:Temperature
chemical_list: D013431:Sulfates; C007886:granite; D012822:Silicon Dioxide; D008697:Methane