Geochemical, Biological, and Clumped Isotopologue Evidence for Substantial Microbial Methane Production Under Carbon Limitation in Serpentinites of the Samail Ophiolite, Oman
Abstract
In hyperalkaline ((Formula presented.)) fluids that have participated in low-temperature (<150 (Formula presented.)) serpentinization reactions, the dominant form of C is often methane ((Formula presented.)), but the origin of this (Formula presented.) is uncertain. To assess (Formula presented.) origin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic profiles of fluid-hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue ((Formula presented.) D and (Formula presented.)) relative abundances less than equilibrium, consistent with substantial microbial (Formula presented.) production. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations and (Formula presented.) across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source of (Formula presented.) is evidenced by the presence of (Formula presented.) -bearing fluid inclusions in the Samail Ophiolite and our measurement of high (Formula presented.) values of ethane and propane, which are similar to those reported in studies of (Formula presented.) -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 methanotrophs, indicating that microbial consumption of (Formula presented.) in the ophiolite may further enrich (Formula presented.) in 13C. We conclude that substantial microbial (Formula presented.) is produced under varying degrees of C limitation and mixes with abiotic (Formula presented.) released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.