Substrate Competition of Diazotrophic Nitrous Oxide Assimilation Over Dinitrogen Fixation

Abstract

Nitrous oxide (N₂O) is a potent greenhouse gas and is depleting the stratospheric ozone layer. Diazotrophic N₂O assimilation to biomass represents a novel biological N₂O consumption pathway in addition to canonical denitrification. Thermodynamically, N₂O assimilation is more favorable than dinitrogen (N₂) fixation in natural environments, especially under higher N₂O concentration and cooler conditions. Via isotopic tracing experiments, N₂O assimilation was detected on cultured diazotrophs Crocosphaera and Trichodesmium with specific rates from 1.27 ± 0.16 × 10⁻⁴ to 2.00 ± 0.25 × 10⁻⁴ hr⁻¹ under elevated [N₂O]/[N₂] conditions (0.0005–0.01) within 24-hr incubation. The rates of N₂O assimilation during the light and dark periods were statistically insignificant compared with N₂ fixation activity. In a eutrophic estuary, N₂O assimilation was not detected in the absence of diazotrophic activity. A competitive substrate kinetic model with experimentally calibrated parameters successfully quantified rate ratios of N₂O assimilation and N₂ fixation in varying substrate concentrations. The low [N₂O]/[N₂] ratio in natural conditions leads to N₂O assimilation rate being <0.1% of N₂ fixation rate, rendering negligible impact of N₂O assimilation. The model was also used to predict the time required for experimental detection of N₂O assimilation in isotopic tracing experiments under varying [N₂O]/[N₂] ratios. This study enhances the mechanistic understanding of N₂O assimilation by diazotrophs, broadening the microbial nitrogen cycle by a potential N₂O sink and nitrogen source for production.