Substrate Competition of Diazotrophic Nitrous Oxide Assimilation Over Dinitrogen Fixation

Abstract Nitrous oxide (N 2 O) is a potent greenhouse gas and is depleting the stratospheric ozone layer. Diazotrophic N 2 O assimilation to biomass represents a novel biological N 2 O consumption pathway in addition to canonical denitrification. Thermodynamically, N 2 O assimilation is more favorable than dinitrogen (N 2 ) fixation in natural environments, especially under higher N 2 O concentration and cooler conditions. Via isotopic tracing experiments, N 2 O 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 [N 2 O]/[N 2 ] conditions (0.0005–0.01) within 24‐hr incubation. The rates of N 2 O assimilation during the light and dark periods were statistically insignificant compared with N 2 fixation activity. In a eutrophic estuary, N 2 O assimilation was not detected in the absence of diazotrophic activity. A competitive substrate kinetic model with experimentally calibrated parameters successfully quantified rate ratios of N 2 O assimilation and N 2 fixation in varying substrate concentrations. The low [N 2 O]/[N 2 ] ratio in natural conditions leads to N 2 O assimilation rate being <0.1% of N 2 fixation rate, rendering negligible impact of N 2 O assimilation. The model was also used to predict the time required for experimental detection of N 2 O assimilation in isotopic tracing experiments under varying [N 2 O]/[N 2 ] ratios. This study enhances the mechanistic understanding of N 2 O assimilation by diazotrophs, broadening the microbial nitrogen cycle by a potential N 2 O sink and nitrogen source for production.