101 / 2024-09-08 14:48:09
Competition between diazotrophic nitrous oxide assimilation and dinitrogen fixation: the role of substrate availability
Nitrous Oxide,Nitrogen fixation,diazotroph
Session 3 - The nitrogen cycle towards a sustainable ocean: from microbes to global biogeochemistry
Abstract Accepted
Nitrous oxide (N2O) is a potent greenhouse gas, which has long been thought to be only consumed by the last step of denitrification in anoxic environments. Since the last decade, N2O assimilation into diazotrophic biomass in oxic environments has been reported in the marine and a freshwater system, representing a novel N2O sink. However, there’s a lack of understanding of the regulating factors for this new biological N2O consumption pathway, despite the fact that assimilation of N2O is thermodynamically more favorable over dinitrogen (N2) fixation. Here, we used isotopic tracing experiments to detect the N2O assimilation in cultured diazotrophs, Crocosphaera (WH8501) and Trichodesmium (IMS101), and water samples from a eutrophic estuary. Results showed that only under elevated [N2O]/[N2] conditions (0.0005 to 0.01) can N2O assimilation be detected on cultured diazotrophs, with specific rates from 1.27 × 10-4 to 2.00 × 10-4 h-1. The eutrophic estuarine water samples lacking active N2 fixation were not detected for N2O assimilation. A competitive substrate kinetic model calibrated by experimental data was used to quantify the rate ratios of N2O assimilation and N2 fixation in varying [N2O]/[N2] conditions. It also well characterizes the N2O assimilation and N2 fixation rate ratios reported in former studies. The model explains the absence of N2O assimilation in natural [N2O]/[N2] ratios and enhances insights among present and previous studies on the mechanism of this aerobic biological N2O sink, which will assist in improving the N2O flux characterization for aquatic ecosystems.