Serpentinization can generate highly reduced fluids replete with hydrogen (H2) and methane (CH4), potent reductants capable of driving both microbial methanogenesis and methanotrophy. However, serpentinized fluids can also be hyperalkaline in pH and have low reduction potentials, characteristics that limit the availability of dissolved inorganic carbon and oxidants for methanogenesis and/or methanotrophy. We examine the potential for microbial CH4 cycling in serpentinization-impacted subsurface fracture fluids intersecting multiple rock and water types within the Samail Ophiolite in the Sultanate of Oman using molecular approaches. Both 16S rRNA gene and metatranscriptomic analyses reveal the methanogenic genus, Methanobacterium, dominate the most hyperalkaline, H2- and CH4-rich fluid communities. Additionally, 16S rRNA genes associated with an aerobic methanotrophic genus, Methylococcus, were detected in wells across varied fluid geochemistry, and sequences affiliated with anaerobic methane oxidizing taxa were detected in low abundance. Multiple pathways of microbial CH4 production and consumption were detected in fracture fluid metagenomes and metatranscriptomes of key genes in methanogenesis were identified, particularly in communities residing in the most hyperalkaline fluids. These results indicate methanogenic activity is possible at hyperalkaline pH, support a role for microorganisms in CH4 production and consumption in aquifers undergoing low-temperature serpentinization, and demonstrate the need to reconceptualize the relationship between microbial activity and δ13CCH4 in systems under inorganic carbon limited conditions. These findings indicate microorganisms influence methane cycling in subsurface serpentinites, are likely to influence the isotopic composition of methane in these systems and further expands upon what is known of microbially-driven geochemical reactions in geologically relevant serpentinizing systems.

Molecular evidence for an active microbial methane cycle in subsurface serpentinite-hosted fluids in the Samail Ophiolite, Oman

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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