MICROBIAL OXIDATION OF METHANE IN KARST CAVES: ISOTOPIC EVIDENCE

Subsurface karstic caves are a critical but neglected terrestrial atmospheric-methane sink. Despite of the dominance of Upland Soil Cluster gamma clade (USCγ) in methanotrophic communities in this vast terrestrial habitat, the direct involvement of USCγ in cave methane oxidation remain elusive due to their recalcitration to culturing. Based on the ¹³CH₄ feeding microcosm experiments and the combination of NanoSIMS, d¹³C determinations of CO₂ and individual phospholipid fatty acid (PLFA), metagenomic analyses and metatranscriptomic analyses, we are able to provide compelling evidence that demonstrates the active involvement of USCγ methanotrophs in cave methane oxidation. When cave sediments were incubated with ¹³CH₄, substantial ¹³C enrichment was observed in single cells and PLFAs as well as in the CO₂ respired, strongly supporting the microbial utilization of ¹³CH₄ both as a carbon and energy source. Importantly, USCγ members oxidized methane at rates comparable to or even higher than those previously reported for forest and grassland soils worldwide. Given the global distribution of USCγ, subsurface caves are estimated here a methane sink as important as soils.