Abstract
The potential for molecular hydrogen (H2) 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 400m‐deep, peridotite‐hosted wells in the Samail Ophiolite, Oman. Isolation and pumping of discrete intervals as deep as 108m to 132m 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 Ca2+−OH− groundwaters bearing up to 4.05μmol⋅L−1H2, 3.81μmol⋅L−1 methane (CH4) and 946μmol⋅L−1 sulfate (SO42−) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H2 oxidation coupled to SO42− reduction. In shallower, oxidized Mg2+−HCO3− groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High δ13C and δD of CH4 (up to 23.9‰VPDB and 45‰VSMOW, respectively) indicated microbial CH4 oxidation, particularly in Ca2+−OH− waters with evidence of mixing with Mg2+−HCO3− 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.
Plain Language Summary
Peridotite rocks can react with water to form hydrogen gas. Microbes can combine hydrogen with oxidants to power their cells. Rocks similar to peridotite have been abundant throughout the history of Earth and the Solar System. Therefore, peridotite‐water interaction is important for understanding the history and distribution of life. Prior studies investigating these processes have sampled waters from the surface of peridotite exposures or from open wells. These sampling methods risk contaminating deep, peridotite‐hosted waters with shallower waters influenced by the atmosphere. In this study, we used packers (tools that can be used to pump waters from separate regions of the subsurface in isolation) to better understand the distribution of microbes and nutrients in subsurface peridotites. We sampled waters from separate subsurface zones as deep as 108–132 m in two wells in peridotite. Waters from different depths had distinct chemical compositions and microbial communities. Sulfate reducing bacteria were dominant in waters that had most extensively reacted with peridotite in isolation, while microbes that consume nitrate or oxygen were also prevalent in waters with more evidence of atmospheric influence. The advanced sampling techniques we used help to distinguish where and how microbes live in the subsurface.
Key Points
Packers were used to sample groundwaters from discrete peridotite aquifers
The discrete aquifers contained waters with distinct chemical compositions and microbial communities
Chemolithoheterotrophic sulfate reduction was a dominant metabolic strategy inferred from 16S rRNA gene homology in highly reacted fluids