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 ( ${\text{NH}}_4^{+}$ ) may act as electron donors, while nitrate ( ${\text{NO}}_3^{-}$ ) and nitrite ( ${\text{NO}}_2^{-}$ ) 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⁺- ${\text{HCO}}_3^{-}$ type fluids comprised mostly of ${\text{NO}}_3^{-}$ , and Ca²⁺-OH^- fluids comprised primarily of ammonia (NH₃). We infer that fixed N is introduced to the serpentinite aquifer as ${\text{NO}}_3^{-}$ . High concentrations of ${\text{NO}}_3^{-}$ (>100 μM) with a relict meteoric oxygen isotopic composition (δ¹⁸O ~ 22‰, Δ¹⁷O ~ 6‰) were observed in shallow aquifer fluids, indicative of ${\text{NO}}_3^{-}$ sourced from atmospheric deposition (rainwater ${\text{NO}}_3^{-}$ : δ¹⁸O of 53.7‰, Δ¹⁷O of 16.8‰) mixed with ${\text{NO}}_3^{-}$ 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 ${\text{NO}}_3^{-}$ detectable. We interpret that NH₃ in hyperalkaline fluids is a product of ${\text{NO}}_3^{-}$ reduction. The proportionality of the O and N isotope fractionation (¹⁸ε / ¹⁵ε) measured in Samail Ophiolite ${\text{NO}}_3^{-}$ was close to unity (¹⁸ε / ¹⁵ε ~ 1), which is consistent with dissimilatory ${\text{NO}}_3^{-}$ 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 ${\text{NO}}_3^{-}$ . Production of ${\text{NH}}_4^{+}$ as the end-product of ${\text{NO}}_3^{-}$ 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.

Keywords: Samail Ophiolite; deep subsurface biosphere; nitrate; nitrogen; nitrogen isotopes; serpentinization; water–rock interaction.