Salinity-dependent nitrifier adaptation shapes partial nitritation resilience under dynamic saline conditions: Mechanistic insights into Nitrosomonas salt adaptation.

Salinity stress is a promising strategy for establishing partial nitritation (PN). Nevertheless, the adaptation of nitrifiers and its implications for PN stability under dynamic saline conditions remain poorly understood. In this study, a continuous-flow self-circulating up-flow fluidized bed reactor (AOAN-Zier) was employed to systematically investigate the effects of salt loading rate (SLR) fluctuations on nitrifier functional responses and PN resilience under different salinity backgrounds. PN was rapidly established within 7 d under salt-free conditions, achieving a nitrite accumulation ratio (NAR) of 96% on day 9. Subsequent responses to SLR reduction differed depending on salinity history. At 10 g/L salinity, reduced SLR alleviated nitrite-oxidizing bacteria (NOB) inhibition and resulted in PN deterioration. In contrast, after long-term exposure to 30 g/L salinity, the system maintained an average NAR of 99% despite SLR reduction, owing to the sustained loss of NOB functional activity. Activity assays, microbial succession, and metagenomic analyses revealed that PN resilience was primarily determined by NOB absolute activity rather than the relative activity balance between AOB and NOB. Long-term saline selection promoted functional divergence among nitrifiers, with salt-adapted Nitrosomonas maintaining ammonia oxidation capacity through coordinated osmotic regulation, nitrogen metabolism, oxidative stress defense, and energy conservation. Moreover, the salt-adapted sludge retained high AOB activity after desalination under mainstream-like low-ammonium conditions. This study provides new insights into salinity-dependent nitrifier adaptation and advances the understanding of microbial mechanisms underlying PN resilience under dynamic saline conditions.