Salinity propels inward HNO₂ transport to robustly suppress nitrite oxidizers with implications for nitrite recovery

A sustainable strategy for sewage nitrogen recovery via mainstream NH₄⁺ exchange combined with side-stream partial nitrification (PN) inherently creates a side-stream environment with co-existing high salinity and nitrite. However, the molecular/ionic mechanisms underlying their synergistic suppression of nitrite-oxidizing bacteria (NOB) remain poorly understood. This study demonstrates that 3.3 g/L salinity with >0.06 mg N/L free nitrous acid (FNA) synergistically and selectively inhibited NOB, initiating stable PN within 17 days. This synergy suppressed NOB activity completely and selected out NOB effectively for ~50 days by overcoming the adaptive revival of NOB. Mechanistically, salinity approaching the isotonic point facilitated transmembrane uptake of FNA and protons, enhancing intracellular FNA accumulation and H⁺-induced bacteriolysis. Differentially, under the synergistic stress, the activity of ammonia-oxidizing bacteria (AOB) increased by 292%, driven by the enrichment of Nitrosomonas. Based on nitrogen balance analysis, microbial evolution and Nir activity changes, Nitrosomonas was found to establish a protective metabolic loop of nitrite reduction to NO / NO reoxidation to nitrite with assistance of denitrifiers to resist the salinity-enhanced FNA inhibition. This study provides an in-depth insight into the role of salinity and FNA in shaping evolutionary trajectories of AOB and NOB, offering a novel framework for sustainable nitrogen recovery in sewage treatment. (Figure presented.)