Research Progresses on Mechanism of N2O Production and its coupling with carbon cycling by Marine Ammonia-Oxidizing Archaea

In recent years, the contribution of ammonia-oxidizing archaea (AOA) to N₂O in marine nitrification processes and its coupling with carbon cycling has become a hot topic under global climate change conditions. To mitigate the greenhouse effect of N₂O, significant efforts have been made to better understand the mechanism of archaeal N₂O production. Reviewing the latest progress of archaeal N₂O production leads to the following implications: (1) Changes in the abundance of AOA under different conditions do not necessarily parallel the accompanying production of archaeal N₂O. (2) With the development of selective inhibitors, isotope techniques, and molecular technologies, the important role of AOA in global nitrogen cycling and N₂O emissions is gradually being confirmed. The expansion of global ocean acidification further alters the marine ecological environment, restricting ammonia oxidation. The expansion of oceanic oxygen minimum zones will change the vertical and horizontal distribution of dissolved oxygen (DO) in the ocean, thereby altering the N₂O metabolic processes of AOA. Given the current state of research on marine AOA production of N₂O, the study of N₂O production mechanisms and ammonia-oxidizing microorganisms is still in its infancy. It is necessary to conduct further research in the following areas: (1) Previous studies on N₂O generation mechanisms in the ocean have mainly focused on nitrifying bacteria and denitrifying bacteria, with insufficient understanding of marine AOA and a lack of sufficient AOA and N₂O distribution data to quantify AOA's contribution to N₂O generation. (2) There are very few pure strains of marine AOA that can be cultured, which makes it impossible to reproduce the nitrogen metabolic processes of marine AOA in the laboratory, representing a current challenge in N₂O generation mechanisms and AOA research. Future research needs to improve the culture medium based on the nutritional deficiencies of AOA and guide the configuration of culture media for different AOA groups by combining metagenomics. (3) The genomics of key enzymes in the aerobic/anaerobic ammonia oxidation metabolism of archaea and the electron transfer process in the ammonia oxidation process are still uncertain, requiring further exploration using isotope techniques and metagenomics. (4) There is little understanding of the in situ ecological function of marine AOA, necessitating in situ incubation experiments to discuss the contribution of AOA to the N₂O generation mechanism from the perspective of in situ ecological function. (5) Existing studies have used double 15N-18O labeling technology to determine the significant contribution of NH2OH oxidation to archaeal N₂O production and have clearly described the pathways and kinetic processes of N₂O production in AOA. This research has improved our understanding of marine N₂O production, and the multiple sources of N and O atoms in N₂O determined here should provide information for biogeochemical models aimed at solving marine nitrogen and carbon cycling. Dual isotope labeling technology can be combined with manipulation experiments of temperature, pH, DO, etc., to explore the response rate and pathways of archaeal N₂O production to ocean warming, acidification, and deoxygenation. In summary, AOA, as a major driver of marine nitrogen and carbon cycling, has become a new field in the effect of N₂O on global climate change.