Subsurface biogeochemical cycling of nitrogen in the actively serpentinizing Samail Ophiolite, Oman.

Abstract

Nitrogen (N) is an essential element for life. N compounds such as ammonium ( ) may act as electron donors, while nitrate ( ) and nitrite ( ) may serve as electron acceptors to support energy metabolism. However, little is known regarding the availability and forms of N in subsurface ecosystems, particularly in serpentinite-hosted settings where hydrogen (H2) generated through water-rock reactions promotes habitable conditions for microbial life. Here, we analyzed N and oxygen (O) isotope composition to investigate the source, abundance, and cycling of N species within the Samail Ophiolite of Oman. The dominant dissolved N species was dependent on the fluid type, with Mg2+- type fluids comprised mostly of , and Ca2+-OH- fluids comprised primarily of ammonia (NH3). We infer that fixed N is introduced to the serpentinite aquifer as . High concentrations of (>100 μM) with a relict meteoric oxygen isotopic composition (δ18O ~ 22‰, Δ17O ~ 6‰) were observed in shallow aquifer fluids, indicative of sourced from atmospheric deposition (rainwater : δ18O of 53.7‰, Δ17O of 16.8‰) mixed with produced in situ through nitrification (estimated endmember δ18O and Δ17O of ~0‰). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH3 (>100 μM) with little detectable. We interpret that NH3 in hyperalkaline fluids is a product of reduction. The proportionality of the O and N isotope fractionation (18ε / 15ε) measured in Samail Ophiolite was close to unity (18ε / 15ε ~ 1), which is consistent with dissimilatory reduction with a membrane-bound reductase (NarG); however, abiotic reduction processes may also be occurring. The presence of genes commonly involved in N reduction processes (narG, napA, nrfA) in the metagenomes of biomass sourced from aquifer fluids supports potential biological involvement in the consumption of . Production of as the end-product of reduction via dissimilatory nitrate reduction to ammonium (DNRA) could retain N in the subsurface and fuel nitrification in the oxygenated near surface. Elevated bioavailable N in all sampled fluids indicates that N is not likely limiting as a nutrient in serpentinites of the Samail Ophiolite.