Synergistic Interaction between Ammonia-Oxidizing and Nitrite-Oxidizing Bacteria Enhances Stability of Partial Nitrification Systems Under Nitrite Shock Loading

The inhibition of nitrite-oxidizing bacteria (NOB) has long been regarded as a major challenge for achieving stable partial nitrification (PN) process. However, the persistence of NOB, even under inhibitory conditions, suggests its potential functional importance in PN systems. This study comparatively analyzed the response of PN systems from reactor performance to gene expression, under ammonium and nitrite shock loadings to elucidate the hidden role of NOB. Results demonstrated that PN systems exhibited greater resistance to nitrite shock, maintaining a 58.2% ammonium removal efficiency even at a nitrite concentration of 300 mg L–1. But this resistance impaired when NOB activity was suppressed. Unlike elevated ammonium, high nitrite concentrations stimulated the expression of amo, hao, nirSK, norBC, and nosZ genes, enhanced ammonia monooxygenase and nitrite reductase activities, and improved the overall activity of ammonia-oxidizing bacteria (AOB). Isotopic analysis using 15N-labeled nitrite revealed the production of 30N and 29N, indicating that nitrite reduction mitigated nitrite toxicity to AOB. Notably, NO was identified as a potential signaling molecular mediating synergistic interactions between AOB and NOB, contributing to support system stability. Overall, this study provides unique insights into the functional role of NOB in improving the resilience and stability of PN systems under stress conditions.