N2O Assimilation, a New N2O Sink and Organic Nitrogen Source in Aquatic Ecosystems

Nitrous oxide (N₂O) is a strong greenhouse gas with ozone layer destruction ability, and its atmospheric concentration has been increasing rapidly due to anthropogenic activities. N₂O reduction to dinitrogen (N₂), the last step of denitrification, was recognized as the only biological N₂O sink. Recently, diazotrophic N₂O assimilation to organic nitrogen in biomass by nitrogenase has been discovered in the eastern South Pacific Ocean and cultured diazotroph Crocosphaera and Trichodesmium. N₂O assimilation to organic nitrogen is thermodynamically more favored than N₂ fixation in higher N₂O concentration and cooler environments, but the distribution and detailed mechanism of this new N₂O sink are still unclear. We applied isotopic tracing experiments to validate and measure N₂O assimilation and built an enzymatic kinetics model for a mechanistic explanation. Cultured diazotroph Crocosphaera (WH8501) and Trichodesmium (IMS101) both showed evident N₂O assimilation rates of 0.751 nM N h^-1 for Crocosphaera at [N₂O]/[N₂] = 0.0075, 0.690 nM N h^-1 for Trichodesmium at [N₂O]/[N₂] = 0.01, and 0.481 nM N h^-1 for Trichodesmium at [N₂O]/[N₂] = 0.0005. Although N₂O assimilation was assumed to be carried out by nitrogenase, it was asynchronous with the diel rhythmicity of N₂ fixation. Field samples from the Pearl River Estuary did not demonstrate the presence of N₂O assimilation. Since N₂ fixation was absent as well, the isotopic tracer ⁴⁶N₂O barely introduced influences on nitrogen isotopic composition compared to photosynthesis and remineralization, indicating that N₂O assimilation is an insignificant N₂O sink in eutrophic estuarine waters. Our enzymatic kinetic model revealed that N₂ rather than N₂O dominated the overall growth rates of cultured diazotrophs. The model indicated the [N₂O]/[N₂] required for the presence of N₂O assimilation in isotopic tracing experiments and explained the absence of this process under natural N₂ concentration environments. The insights from this study may suggest new engineering methods to control N₂O emissions.