Exploring influence mechanism of small-molecule carbon source on heterotrophic nitrification-aerobic denitrification process from carbon metabolism, nitrogen metabolism and electron transport process

[Display omitted] •Acinetobacter johnsonii ZHL01 removes ammonia via the pathway of NH4+→NH2OH → NO.•Small-molecular carbon sources lead to different intracellular NADH concentrations.•Compared with other metabolic pathways, the TCA cycle is crucial to the HNAD process.•dnfABC genes play an important role for the nitrogen removal of Acinetobacter johnsonii ZHL01. The heterotrophic nitrification-aerobic denitrification (HNAD) process can remove nitrogen and organic carbon under aerobic conditions. To get the in-depth mechanism of the HAND process, a strain named Acinetobacter johnsonii ZHL01 was isolated, and enzyme activity, electron transport, energy production, and gene expression of the strain were studied with small-molecule carbon sources, including sodium citrate, sodium acetate, sodium fumarate, and sodium succinate. The HNAD pathway of ZHL01 was NH4+→NH2OH → NO, and nitrogen balance analysis shows that ZHL01 could assimilate and denitrify 58.29 ± 1.05 % and 16.58 ± 1.07 % of nitrogen, respectively. The assimilation, the nitrification/denitrification, and the respiration processes were regulated by the concentration of reduced nicotinamide adenine dinucleotide (NADH) produced from the different metabolic pathways of small-molecule carbon sources. The HNAD process occurs to reduce intracellular redox levels related to NADH concentrations. This discovery provides a theoretical basis for the practical application of HAND bacteria.