Geochemical, biological and clumped isotopologue evidence for
substantial microbial methane production under carbon limitation in
serpentinites of the Samail Ophiolite, Oman
Abstract
In high-pH ($\text{pH}>10$) fluids that
have participated in low-temperature
($<150\,^{\circ}\text{C}$)
serpentinization, the dominant form of C is often methane
(CH$_{4}$), but the origin of this CH$_{4}$ is uncertain. To
assess CH$_{4}$ origin during low-temperature serpentinization, we
pumped fluids from 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 ($^{13}$CH$_{3}$D and
$^{12}$CH$_{2}$D$_{2}$) relative abundances less than
equilibrium, consistent with substantial microbial CH$_{4}$
production. Further, we observed an inverse relationship between
dissolved inorganic C concentrations and
$\delta^{13}\text{C}_{\text{CH}_{4}}$
across fluids bearing microbiological evidence of methanogenic activity,
suggesting that the apparent C isotope effect of microbial
methanogenesis is modulated by C availability. A second source of
CH$_{4}$ is evidenced by the presence of CH$_{4}$-bearing
fluid inclusions in the Samail Ophiolite and our measurement of high
$\delta^{13}\text{C}$ values of
ethane and propane, which are similar to those reported in studies of
CH$_{4}$-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 CH$_{4}$ in the ophiolite
may further enrich CH$_{4}$ in $^{13}$C. We conclude that
substantial microbial CH$_{4}$ is produced under varying degrees of
C limitation and mixes with abiotic CH$_{4}$ released from fluid
inclusions. This study lends insight into the functioning of microbial
ecosystems supported by water/rock reactions.