Substrate Competition of Diazotrophic Nitrous Oxide Assimilation Over Dinitrogen Fixation.

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. Plain Language Summary: Nitrous oxide (N2O) contributes to climate warming and can consume the ozone layer. In aquatic ecosystems, N2O is generally consumed only by denitrification in the absence of oxygen. Recently, nitrogen‐fixing organisms (diazotrophs) which utilize nitrogen gas (N2) to make biomass are able to assimilate N2O into biomass, demonstrating a novel N2O consumption pathway in oxygenated environments. Experimental evidence demonstrated that actively growing diazotrophs can assimilate N2O and N2 at the same time. Both observation in cultured diazotrophs and numerical model proved that the relative availabilities of N2O and N2 regulate the competition of N2O assimilation and N2 fixation. N2O assimilation is more favorable under a higher concentration ratio of N2O to N2. Model simulation could determine the time required for successful determination of N2O assimilation during experiments. Contrastingly, at high nutrient estuarine waters where diazotrophs are inactive, N2O assimilation was not detected even at high N2O concentration. Overall, N2O assimilation may be negligible in natural conditions. Key Points: Concentration ratio of N2O to N2 positively correlates with rate ratio of N2O assimilation to N2 fixationActive diazotrophs are key organism performing N2O assimilationN2O assimilation has different circadian rhythms compared with N2 fixation [ABSTRACT FROM AUTHOR]