A critical review of the oxygen paradox in mainstream anaerobic ammonium oxidation: dissolved oxygen as a cross-scale driver from microbial adaptation to process stability.

Anaerobic ammonium oxidation (anammox) is a promising low-carbon nitrogen removal pathway, but its mainstream application remains constrained by the oxygen paradox. In this review, the oxygen paradox is defined as the requirement for oxygen-dependent nitrite production by aerobic ammonia oxidizers and the simultaneous risk of oxygen-induced inhibition of anaerobic ammonium-oxidizing bacteria (AnAOB); mechanistically, it also reflects the dual role of dissolved oxygen (DO) as both a toxic stressor and a regulatory signal. Unlike recent reviews that mainly discuss DO inhibition, microbial competition, or biofilm protection separately, this review integrates these processes into a cross-scale framework linking molecular adaptation, ecological niche reconstruction, aggregate protection, contaminant-related interaction superposition, and engineering resilience. At the molecular scale, DO affects enzymatic activity, oxidative stress, anammoxosome membrane integrity, iron-oxygen homeostasis, and dormancy-resuscitation responses. At the community scale, DO gradients determine niche partitioning among ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), Comammox Nitrospira, and AnAOB, thereby shifting microbial interactions from competition to conditional cooperation. At the reactor scale, DO interacts with biomass structure, extracellular polymeric substances (EPS), antibiotics, and microplastics, influencing oxygen shielding, localized toxicity, and resilience under real wastewater fluctuations. The review further translates this framework into engineering guidance, emphasizing adjustable DO windows, intermittent aeration, structured biomass retention, NOB/Comammox control, and feedback based on NH4+-N, NO2--N, NO3--N, pH, oxidation-reduction potential (ORP), oxygen uptake rate, and post-disturbance recovery. This perspective supports a transition from empirical low-DO operation toward dynamic, structure-based, and physiology-informed mainstream anammox control.