Serpentinization and Fuel for Biofilms: Using Geochemistry and Thermodynamics to Identify Metabolic Niches in the Subsurface

Hydrogen produced by serpentinization has the potential to fuel subsurface microbial metabolisms. In a zone of active serpentinization, the solids in this habitat comprise ultramafic parent rocks derived from the Earth's mantle, serpentine group minerals, veins of hydroxides, and accessory magnetite and/or other metal-rich grains (Schulte et al., 2006). The fluid that occurs with these solids is altered, trapped seawater and/or meteoric water. It is ultrabasic, with a pH of ~10.5 to 12.1, dominated by Ca²⁺ of 5 to 80 mg/L and OH^- of 6 to 70 mg/L (Barnes et al., 1967). The fluid is predicted to be reducing; hydrogen, a powerful reducing agent, is generated when Fe²⁺ in Fe(OH)₂ is oxidized to magnetite, coupled to the reduction of water to diatomic hydrogen. At serpentinizing seeps on land, H_2(aq) concentrations are predicted to be on the order of 300 µm, whereas concentrations that are orders of magnitude greater are possible where fluids are in equilibrium with serpentinite at depth (Sleep et al., 2004). These data describe a fluid out of equilibrium with the surface Earth, which may support microbial populations catalyzing available redox processes. These processes are chemical reactions, constrained by thermodynamics; the thermodynamic spontaneity of specific reactions can be inferred from empirical data by modeling the free energy change associated with serpentinization. Historic data for serpentinizing fluids in northern California and Oregon and new field observations in that region provide an opportunity to assess the subsurface habitability of the Klamath Mountains serpentine domain. Results indicate that metabolic niches do exist for methanogenesis, ferric iron, sulfate, and nitrate reduction reactions under the H_2(aq) activities considered.