Functional instability and community-level compensatory mechanisms in the anammox system under long-term acetamiprid...

Acetamiprid is a frequently detected neonicotinoid insecticide that is widely present in water bodies and may disrupt the stability of the anaerobic ammonium oxidation (anammox) process. This study investigated the response threshold and mechanistic transition of the anammox system under long-term acetamiprid stress. The system remained stable at 0-2.5 mg/L acetamiprid, and nitrogen removal efficiency (NRE) did not change significantly. At 5-15 mg/L acetamiprid, NRE was maintained at approximately 80%, whereas the NO₃^--N/NH₄⁺-N ratio increased to 0.40, and specific anammox activity (SAA) declined. This apparent maintenance of reactor performance was likely sustained by community-level functional compensation. At 50 mg/L acetamiprid, reactive oxygen species (ROS) levels increased by 74%, the protective effect of extracellular polymeric substances (EPS) weakened, and NRE decreased by 9.11%, indicating that the compensatory capacity of the microbial community had been exceeded and that the reactor had entered an unstable state. Overall, the reactor exhibited a stage-dependent transition from apparent stability to latent functional impairment and ultimately to overt instability. Community and metagenomic analyses further suggested that acetamiprid exposure reduced the ecological dominance and functional contributions of Candidatus Kuenenia and Candidatus Jettenia, while increasing the relative importance of Candidatus Brocadia and associated populations such as Ignavibacterium, and enhancing their stress response and xenobiotic-related functions. This transition indicates that the system shifted from a mode dominated by core anammox bacteria to a more distributed, multispecies compensatory state. These findings provide new insights into the stability boundaries and failure transitions of the anammox system under pesticide stress.