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

Nitrogen (N) is an essential element for life. N compounds such as ammonium ([Formula: see text]) may act as electron donors, while nitrate ([Formula: see text]) and nitrite ([Formula: see text]) 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 (H(2)) 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 Mg(2+)- [Formula: see text] type fluids comprised mostly of [Formula: see text] , and Ca(2+)-OH(−) fluids comprised primarily of ammonia (NH(3)). We infer that fixed N is introduced to the serpentinite aquifer as [Formula: see text]. High concentrations of [Formula: see text] (>100 μM) with a relict meteoric oxygen isotopic composition (δ(18)O ~ 22‰, Δ(17)O ~ 6‰) were observed in shallow aquifer fluids, indicative of [Formula: see text] sourced from atmospheric deposition (rainwater [Formula: see text]: δ(18)O of 53.7‰, Δ(17)O of 16.8‰) mixed with [Formula: see text] produced in situ through nitrification (estimated endmember δ(18)O and Δ(17)O of ~0‰). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH(3) (>100 μM) with little [Formula: see text] detectable. We interpret that NH(3) in hyperalkaline fluids is a product of [Formula: see text] reduction. The proportionality of the O and N isotope fractionation ((18)ε / (15)ε) measured in Samail Ophiolite [Formula: see text] was close to unity ((18)ε / (15)ε ~ 1), which is consistent with dissimilatory [Formula: see text] 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 [Formula: see text]. Production of [Formula: see text] as the end-product of [Formula: see text] 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.