Key enzymes and N2O emission in aerobic ammonia oxidation process

Abstract:

The increasing emission of nitrous oxide (N₂O) poses a serious threat to global ecological balance, to which microbes contribute a lot. Aerobic ammonia oxidation is the oxidation of ammonia to nitrite under aerobic conditions, which directly or indirectly affects the global production and release of N₂O. Ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), complete ammonia oxidizers (Comammox), and heterotrophic ammonia-oxidizing bacteria (HAOB) are major contributors to ammonia oxidation. It is important for the mitigation of N₂O emission to clarify how N₂O is produced by the four types of microorganisms. This paper reviewed the N₂O production pathways in AOA, AOB, Comammox and HAOB driven by the aerobic ammonia oxidation process. The roles of some key enzymes in the N₂O production pathways were elucidated through enzymatic analysis. Ammonia monooxygenase (AMO) is the first key enzyme in the autotrophic aerobic ammonia oxidation process, and the direct product of hydroxylamine oxidation is NO instead of NO₂^-, which is then converted to NO₂^- by an unknown enzyme. The aerobic ammonia oxidation process and related enzymes of HAOB are not completely clear, and the issue of N₂O emission needs to be further studied. This paper aims to lay a theoretical basis for regulating biological N₂O emission.