A heterotrophic nitrification-aerobic denitrification bacterium Acinetobacter sp. WZ-1: the superior stress resistance and the unconventional nitrogen metabolic pathways

Heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are outstanding in nitrogen removal for wastewater treatment. In this study, Acinetobacter sp. WZ-1 and Rhodococcus sp. JY-5 were isolated and identified as HN-AD strains. The 15N isotope experiments demonstrated that WZ-1 and JY-5 both converted inorganic nitrogen to N2. Compared with JY-5, WZ-1 showed the superior ability on NH4+-N removal under environmental stress. WZ-1 removed 85.15% and 97.06% of NH4+-N at the initial pH values of 5 and 9, respectively. At the high temperature of 40 °C, NH4+-N removal efficiency by WZ-1 still reached to 80.66%. Moreover, high C/N ratios showed the significant inhibition on growth of WZ-1 and JY-5 under alkaline environment. WZ-1 still removed NH4+-N of 47.15–87.68% with the low C/N ratios of 4–8 at pH 9. It indicated the potential of WZ-1 to treat high-ammonia wastewater featured by the high pH values and low C/N ratios. The genome and intermediate products inferred the unconventional nitrification-denitrification pathway of WZ-1 strain (NH4+→NH2OH→NO→NO3−→NO2−→NO→N2→N2). Particularly, nitrate production is attributed to NO oxidation according the hmp gene. Traditional genes for denitrification (narGH, nirKS, norBD and nosZ) were not found in genome, while flavorubredoxins and NosD protein might involve in nitrite/NO reduction and N2O reduction, respectively. This study proved that certain unreported enzymes and proteins play important roles in nitrogen removal of the unconventional HN-AD bacteria.