Aerobic denitrification in different dissolved oxygen conditions: nitrogen metabolism and electron distribution

Aerobic denitrification broadens the practical range of biological nitrogen removal processes in wastewater treatment. However, comprehensive examination of the impact of dissolved oxygen (DO) levels on the metabolic pathways of aerobic denitrifiers remains lacking. Here, we investigated the effects of DO concentration on the nitrogen removal processes of the aerobic denitrifier Pseudomonas mendocina HITSZ-D1 (strain D1): DO markedly alters its nitrogen removal performance. Whole-genome sequencing identified numerous nitrogen metabolism-related genes in strain D1, including for periplasmic nitrate reductase and the other typical aerobic denitrification enzymes. Compared with anaerobic conditions, the total nitrogen removal rate of strain D1 decreased by 17.71 % and 23.56 % in moderate- (average 4.04 mg·L^-1) and high-DO (average 7.39 mg·L^-1) conditions, respectively. Transcriptomic, enzymatic, and nitrogen balance analyses showed that increasing the DO level from anaerobic to aerobic causes strain D1 to switch its main nitrogen metabolism pathway from denitrification to assimilation. Although the strain demonstrated greater cellular activity and electron transfer efficiency in aerobic conditions, variation in the activities of denitrification and ammonia assimilation enzymes caused most electrons to be used for cellular respiration and assimilation. Strain D1 was applied in an aerobic sequencing batch reactor used for wastewater treatment. The flexible switching of nitrogen metabolic pathways enabled the bacterium to significantly improve the nitrogen removal efficiency of the reactor. This study reveals the different patterns of nitrogen metabolism of a denitrifier in aerobic and anaerobic conditions from the perspectives of substance transformation and electron distribution, providing a new basis for the practical application of aerobic denitrification.