Microbial sulfate reduction rates in low temperature serpentinizing mantle rocks

Earth's upper mantle is dominated by peridotite rock and these rock type was also found in the subsurface of Mars and probably comprise the rocky bodies of the icy moons Europa and Enceladus. When in contact with water, the reducing rock undergoes serpentinization reaction abiotically generating hydrogen. In such system hydrogen can abiotically reduce CO₂ and generate methane or low molecular weight organic acids which have been found in serpentinizing fluids on Earth. Hydrogen as well as organic acids represent valuable energy substrates for microbial metabolism such as methanogenesis and sulfate reduction rendering low temperature serpentinizing rocks potential habitats for microbial life on and beyond Earth. In particular microbial sulfate reduction is greatly understudied in the context of serpentinization. Using radiotracer incubations with ³⁵S-labeled sulfate, we have studied the presence and activity (process rates) of microbial sulfate reduction in fluids and rocks recovered from the Semail ophiolite in Oman and in fluids from the Coast Range ophiolite in Northern California. Both locations are characterized by ongoing low temperature serpentinization. Fluids representing a range of pH, sulfate and H₂ concentrations were collected from wells in California and Oman and intact rock samples were retrieved by diamond coring during the Oman Drilling Project. We find active microbial sulfate reduction in the fluid and rock samples at comparably low rates, between a few fmol to two pmol per cm³ fluid or rock, confirming the presence of active sulfate reduction in the system. The low activity points to strong limitations for microbial life in this environment where lowest activities correspond with hyperalkaline (>pH 11) fluids and sulfate concentrations below 150 μM. The majority of the active sulfate reduction seems to be present in rocks with fluids of moderate alkaline pH and highly negative redox potential. We suggest that the hyperalkaline pH represents a major limitation for life in such systems.