[B-CG03] Earth and Planetary Science Frontiers for Life and Global Environment

[BCG03-04] Ongoing archaeal methanogenesis in an organic- and iodine-rich deep aquifer: molecular evidence from coenzyme factor 430 and methane-specific radiocarbon measurements

*Atsushi Urai^1,2, Yoshinori Takano¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Shinshu University)

Keywords:deep aquifer, radiocarbon measurement, Iodine, methane, small sub-unit rRNA gene sequencing, methanogenic archaea

Deep methanogenesis in a subsurface environment is an important process for understanding global biogeochemical cycles. Biogenic methane is produced by methanogenic archaea living under anaerobic conditions, including deeply marine sediments down to 2.5 km below the seafloor (Inagaki et al., 2015). However, the ecology of methanogenic archaea, such as in situ biomass and activity, is still unclear. To elucidate the deep prokaryotic community including methanogenic archaea in a forearc basin at Southern Kanto gas field (Ishii et al., 2019), we conducted three parallel approaches; i) the lipid-based analysis, ii) small-subunit (SSU) rRNA gene sequencing, and iii) coenzyme factor 430 analysis (Kaneko et al., 2014), which is a key molecular of all methanogenic archaea (Thauer et al., 2019). To assess the origin of deep carbon, we performed radiocarbon measurements of methane and dissolved inorganic carbon (DIC). The Southern Kanto gas field is located on the Boso Peninsula and known as the largest natural gas dissolved in the water field in Japan. The deep aquifer, which originated from ancient seawater, has many unique features; high-purity of methane (>99%), high concentrations of iodine (∼2000 times of seawater) and dissolved organic matter (DOM).

The results of lipid-based analysis suggested that the biomass of domain archaea was less than 10% of the total prokaryote in deep aquifer, which was supported by the result of the SSU rRNA gene sequencing. The high concentration of native F430 (1.67×10⁷ femto mol/L; Urai et al., 2021) with the absence of the F430 epimers was confirmed in the brine-rich water, suggesting activated ongoing methanogenesis in the deep aquifer. Radiocarbon measurements of methane and dissolved inorganic carbon (DIC) revealed ¹⁴C-depleted (Δ¹⁴C_methane and Δ¹⁴C_DIC, <−997.4‰; Urai et al., 2021), indicating the independent deep habitat and active state of archaeal methanogenesis.

This study was supported in part by the Japan Society for the Promotion of Science (JSPS) with the joint research between the Kanto Natural Gas Development Co., Ltd. and JAMSTEC.



References:

Inagaki et al. (2015) Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor, Science, 349, 420-424.

Thauer, R. K. (2019). Methyl (alkyl)-coenzyme M reductases: nickel F-430-containing enzymes involved in anaerobic methane formation and in anaerobic oxidation of methane or of short chain alkanes. Biochemistry, 58, 5198-5220.

Kaneko et al. (2014) Quantitative analysis of coenzyme F430 in environmental samples: a new diagnostic tool for methanogenesis and anaerobic methane oxidation. Analytical Chemistry, 86, 3633-3638.

Ishii et al. (2019) Bioelectrochemical stimulation of electromethanogenesis at a seawater-based subsurface aquifer in a natural gas field. Frontiers in Energy Research 6, 144.

Urai et al. (2021) Origin of deep methane associated with a unique community of microorganisms in an organic- and iodine-rich aquifer, ACS Earth Space Chem., 5(1), 1-11.