The functional analysis of NirBD in microorganisms and the efficacies of NirBD on the soil nitrogen storage and N2O emission

The siroheme-containing nitrite reductase (NirBD), which is encoded by the <i>nirBD</i> gene, is a functional enzyme in the nitrogen cycle. The NirBD enzyme can regulate N₂O release through the two distinct pathways of assimilatory nitrate/nitrite reduction to biomass and dissimilatory nitrate/nitrite reduction to ammonium. Therefore, a thorough comprehension of the function of NirBD in microorganisms can enable us to better understanding the contributions for nitrogen retention and N₂O emission. However, the knowledge of the functions and expression mechanisms of <i>nirBD</i> gene across different microorganisms remains limited. This review synthesized the current research on the phylogenetic distribution and catalytic versatility of NirBD in fungi, bacteria, and actinomycetes. The contributions of NirBD for nitrogen retention and N₂O emission were extensively discussed under anaerobic and aerobic conditions. The expression mechanism of NirBD was demonstrated. The factors that affect the expression amount of the NirBD enzyme in microorganisms were clarified systematically. This review not only elucidated the unique role of NirBD in the nitrogen metabolism but also provided a critical theoretical foundation for developing future strategies to enhance soil nitrogen fertility and mitigate N₂O emissions.