Aqueous Geochemical and Microbial Variation Across Discrete Depth Intervals in a Peridotite Aquifer Assessed Using a Packer System in the Samail Ophiolite, Oman

The potential for molecular hydrogen ((Formula presented.)) generated via serpentinization to fuel subsurface microbial ecosystems independent from photosynthesis has prompted biogeochemical investigations of serpentinization-influenced fluids. However, investigations typically sample via surface seeps or open-borehole pumping, which can mix chemically distinct waters from different depths. Depth-indiscriminate sampling methods could thus hinder understanding of the spatial controls on nutrient availability for microbial life. To resolve distinct groundwaters in a low-temperature serpentinizing environment, we deployed packers (tools that seal against borehole walls during pumping) in two (Formula presented.) -deep, peridotite-hosted wells in the Samail Ophiolite, Oman. Isolation and pumping of discrete intervals as deep as (Formula presented.) to (Formula presented.) below ground level revealed multiple aquifers that ranged in pH from 8 to 11. Chemical analyses and 16S rRNA gene sequencing of deep, highly reacted (Formula presented.) groundwaters bearing up to (Formula presented.), (Formula presented.) methane ((Formula presented.)) and (Formula presented.) sulfate ((Formula presented.)) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by (Formula presented.) oxidation coupled to (Formula presented.) reduction. In shallower, oxidized (Formula presented.) groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High (Formula presented.) and (Formula presented.) of (Formula presented.) (up to (Formula presented.) and (Formula presented.), respectively) indicated microbial (Formula presented.) oxidation, particularly in (Formula presented.) waters with evidence of mixing with (Formula presented.) waters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.