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

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 x 10(4) 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 (delta C-13(methane), -67.9 to -71.4 parts per thousand 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 C-14-depleted (both Delta C-14(methane) and Delta C-14(DIC) <-997.4 parts per thousand), suggesting that the entire deep habitat and methanogenesis represents an isolated subterranean microbial ecosystem.

Automated summary

The study identified deep microorganisms, particularly methanogenic archaea, in a deep aquifer in the Southern Kanto gas field in Japan using F430 analysis and radiocarbon measurements. The presence of high concentrations of native F430, along with the detection of Methanomicrobia in the rRNA sequences, indicated active methanogenesis potential mediated by subsurface microbes. Stable carbon isotope ratios and radiocarbon measurements of methane and DIC further supported the biogenic origin of methane and revealed an isolated subterranean microbial ecosystem.