Origin of Deep Methane Associated with a Unique Community of Microorganisms in an Organic- And Iodine-Rich Aquifer

Abstract

We performed coenzyme factor 430 (F430) analysis and radiocarbon measurements to identify deep microorganisms, including methanogenic archaea, and assess the origin of methane in a deep organic- and iodine-rich aquifer in a forearc basin at the Southern Kanto gas field, Boso Peninsula, Japan. We detected high concentrations of native F430 (approximately 1.67 × 104 femto mol L-1) in the absence of the F430 epimer. Both lipid and small-subunit rRNA gene analyses indicated that the biomass of domain archaea was less than 10% of the total prokaryotes, while methanogenic archaea (e.g., Methanomicrobia) was detected in the rRNA sequences. These results strongly suggest high active methanogenesis potential mediated by the subsurface microbes. A stable carbon isotope ratio of methane (δ13Cmethane, -67.9 to -71.4‰ vs Vienna Pee Dee Belemnite) and a huge reservoir of high-purity methane (>99%) indicated a typical biogenic origin. Radiocarbon measurements of methane and dissolved inorganic carbon (DIC) revealed 14C-depleted (both Δ14Cmethane and Δ14CDIC,