Serpentinization of ultramafic rock in ophiolite complexes along continental margins leads to the mobilization of volatiles and reduced carbon compounds that can be used as sources of energy by subsurface microbial communities. The extent to which sulfur compounds can serve as electron acceptors in anoxic serpentinizing systems and their role in biogenic carbon cycling remains to be elucidated. Large scale processes at CROMO were studied using geochemical analyses, bioenergetics calculations, microscopic cell counts, and 16S rRNA sequencing to identify the population of sulfate reducers and methane cyclers. Shotgun metagenomic and metatranscriptomic sequencing identified the production of key genes for sulfate reduction, sulfide oxidation, and thiosulfate disproportionation. Small scale processes at CROMO were identified through a depth profile of CSW1.1. With water pumped directly from the well, microcosms were created to measure the growth of microbial communities in the presence of 13CH4. Thiosulfate or Fe(OH)3 were injected as electron acceptors, with the addition of O2 gas in designated “oxic” bottles. The highest cell growth and biogenic 13DIC production occurred in “anoxic” 13CH4 + thiosulfate amended bottles, with Trueperaceae dominating both the profile of CSW1.1 and the microcosms. The biogeochemistry of CROMO yields insight into the potential for sulfur and methane cycling within this cryptic serpentinite environment found throughout the world.
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