Isolation, Identification, and Condition Optimization of an Ammonia-Oxidizing Bacterium and Its Potential Application in Wastewater Treatment

Ammonia-oxidizing bacteria (AOB) are the rate-limiting microbial group in biological nitrogen removal. However, difficulties in isolation and cultivation, along with unclear metabolic regulation mechanisms, have long constrained their engineering applications. Existing research has mostly focused on the responses of model strains to environmental factors, while studies on the growth regulation mechanisms driven by key medium components remain scarce. Moreover, there is a lack of efficient strains suitable for complex wastewater with high ammonia and high salinity. To isolate an efficient strain and optimize its culture conditions for high-ammonia wastewater treatment, we collected water samples from a polluted river in Zhongshan City. After enrichment, a strain was isolated via gradient dilution and silica gel plating, identified by scanning electron microscopy and 16S rDNA sequencing as Nitrosomonas europaea W4 (99.93% similarity to the type strain). Single-factor medium optimization examined CaCO3 and Fe2+/Fe3+, while temperature and initial ammonia nitrogen effects were tested, and landfill leachate was used for verification. CaCO3 shortened the lag phase without affecting maximum specific growth rate; replacing Fe3+ with Fe2+ further reduced lag and enhanced the ammonia oxidation rate. Optimal growth occurred at 25–30 °C and an initial ammonia nitrogen concentration of ~2000 mg/L. In landfill leachate, the strain increased the ammonia degradation rate 6.3-fold, from 4.64 ± 0.11 mg L−1 d−1 in the uninoculated control group to 29.32 ± 0.07 mg L−1 d−1, and importantly, nitrite can be rapidly degraded by indigenous denitrifiers, posing no secondary pollution risk. Overall, N. europaea W4 exhibits high ammonia oxidation efficiency, and the optimized medium and conditions improve its proliferation and metabolic stability, providing a basis for cultivation and application in treating high-strength ammonia nitrogen wastewater.