Energetically Informed Niche Models of Hydrogenotrophs Detected in Sediments of Serpentinized Fluids of the Samail Ophiolite of Oman

A geochemical gradient established by mixing between reduced, hyperalkaline (pH > 11), H₂-rich fluids generated through the process of serpentinization and surrounding surface water (pH ∼ 8) in the Samail Ophiolite of Oman provides an opportunity to characterize the geochemical and biological factors that influence the distribution of H₂ oxidizing chemotrophs, hydrogenotrophs. In this study, 16S rRNA gene amplicon sequencing was implemented to characterize hydrogenotrophs in sediments underlying surface expressed serpentinized fluids in Oman. Hydrogenotroph phylotype distribution was evaluated as functions of chemical energy supplies for their given metabolic redox reactions. Through this approach, it was discovered that hydrogenotrophic taxa are likely constrained to sediments with overlying fluids that have <∼60 μm O₂, including microorganisms of the genus, Hydrogenophaga. Sulfate reducers of the family, Thermodesulfovibrionaceae, likely require >∼10 μm SO₄⁻² for survival. In sediments with fluids having >∼10 μm SO₄⁻², sulfate reducers likely outcompete microorganisms of the methanogen genus, Methanobacterium, for H₂. Additionally, differences in distribution between Thermodesulfovibrionaceae and Methanobacterium may be driven by the availability of electron acceptors and the redox reaction that is most energy yielding in the fluid. Taken together, observations from the Oman geochemical gradient result in a hydrogenotroph niche model that can be used to evaluate global distribution patterns of hydrogenotrophs in continental serpentinized fluids. On a global scale, based on previous studies, Methanobacterium is constrained to fluids that have <∼10 μm SO₄⁻².