Geochemical, biological and clumped isotopologue evidence for substantial microbial methane production under carbon limitation in serpentinites of the Samail Ophiolite, Oman

International audienceIn hyperalkaline (pH > 10) fluids that have participated in low-temperature (< 150°C) serpentinization reactions, the dominant form of C is often methane (CH4), but the origin of this CH4 is uncertain. To assess CH4 origin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic pro29 files of fluid-hosted microbial communities, and measured isotopic compositions of hy30 drocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue (13CH3D and 12CH2D2) relative abundances less than equilibrium, consistent with substantial microbial CH4 pro33 duction. Further, we observed an inverse relationship between dissolved inorganic C concentrations and δ13 CCH4 across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is mod36 ulated by C availability. An additional source of CH4 is evidenced by the presence of CH4-bearing fluid inclusions in the Samail Ophiolite and our measurement of high δ13 C values of ethane and propane, which are similar to those reported in studies of CH4-rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methan41 otrophs, indicating that microbial consumption of CH4 in the ophiolite may further enrich CH4 in 13C. We conclude that substantial microbial CH4 is produced under vary43 ing degrees of C limitation and mixes with abiotic CH4 released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by wa45 ter/rock reactions