Unraveling Biological Ammonium Oxidation in Toxic Petrochemical Wastewater Treatment: A Metagenomic Exploration with Practical Implications

Ammonia oxidation plays a pivotal role in biological nitrogen removal from toxic petrochemical wastewater, but its microbial stability under prolonged toxic stress remains poorly understood. This study employed a membrane bioreactor with a gradient dilution approach to treat real petrochemical wastewater, demonstrating that gradual acclimation to toxicity enabled sustained ammonia removal at 0.26 ± 0.02 kg of N·m^–3·d^–1. Progressive dilution selectively enriched the Nitrosomonas and amo genes. However, exposure to low-diluted wastewater triggered a 68.9% reduction in ex-situ ammonia oxidation activity. Notably, Comammox Nitrospira exhibited ecological resilience under high-stress conditions, with its amoA gene abundance increasing 7.6-fold (to 1.3 × 10⁸ copies gVSS^–1) and network centrality surpassing most Nitrosomonas species. Concurrently, Nitrospira maintained a stable nxrB gene abundance and harbored genes for toxic compound degradation, enhancing their ecological versatility. As a genus member, Comammox Nitrospira might leverage these adaptive traits to gain a competitive edge in high-stress environments. These findings reveal toxicity-dependent niche partitioning between Nitrosomonas and Comammox and emphasize the need to integrate microbial community dynamics into early prediction of performance shifts for optimizing industrial wastewater treatment under fluctuating toxic loads.