Evidence for Methyl-Compound-Activated Life in Coal Bed System 2 km Below Sea Floor

Abstract:

IODP Expedition 337 set the record for deepest marine scientific drilling down to 2.4 kmbsf. This cruise also had the unique opportunity to retrieve deep cores from the Shimokita coal bed system in Japan with the aseptic and anaerobic conditions necessary to look for deep life. Onboard scientists prepared nearly 1,700 microbiology samples shared among five different countries to study life in the deep biosphere. Samples spanned over 1 km in sampling depths and include representatives of shale, sandstone, and coal lithologies. Findings from previous IODP and deep mine expeditions suggest the genetic potential for methylotrophy in the deep subsurface, but it has yet to be observed in incubations. A subset of Expedition 337 anoxic incubations were prepared with a range of ¹³C-methyl substrates (methane, methylamine, and methanol) and maintained near in situ temperatures. To observe ¹³C methyl compound metabolism over time, we monitored the δ¹³C of the dissolved inorganic carbon (by-product of methyl compound metabolism) over a period of 1.5 years. Elemental analysis (EA), ion chromatograph (IC), ¹³C volatile fatty acid (VFA), and mineral-associated microscopy data were also collected to constrain initial and endpoint conditions in these incubations. Our geochemical evidence suggests that the coal horizon incubated with ¹³C-methane showed the highest activity of all methyl incubations. This provides the first known observation of methane-activated metabolism in the deep biosphere, and suggests there are not only active cells in the deeply buried terrigenous coal bed at Shimokita, but the presence of a microbial community activated by methylotrophic compounds.