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 (NH+ 4 ) may act as electron donors, while nitrate (NO− 3 ) and nitrite (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+-HCO− 3 type fluids comprised mostly of NO− 3 , and Ca2+-OH− fluids comprised primarily of ammonia (NH3). We infer that fixed N is introduced to the serpentinite aquifer as NO− 3 . High concentrations of NO− 3 (>100µM) with a relict meteoric oxygen isotopic composition (δ 18O ∼ 22‰, 117O ∼ 6‰) were observed in shallow aquifer fluids, indicative of NO− 3 sourced from atmospheric deposition (rainwater NO− 3 : δ 18O of 53.7‰, 117O of 16.8‰) mixed with NO− 3 produced in situ through nitrification (estimated endmember δ 18O and 117O of ∼0‰). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH3 (>100µM) with little NO− 3 detectable. We interpret that NH3 in hyperalkaline fluids is a product of NO− 3 reduction. The proportionality of the O and N isotope fractionation (18ε / 15ε) measured in Samail Ophiolite NO− 3 was close to unity (18ε / 15ε ∼ 1), which is consistent with dissimilatory 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 NO− 3 . Production of NH+ 4 as the end-product of 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.

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ORCID for Juerg M. Matter: orcid.org/0000-0002-1070-7371

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