Adaptation of microbial gene polymorphisms of the partial nitritation-anammox process for maintaining denitrification performance and robustness under salinity stress

To explore the adaptive mechanisms of the partial nitritation-anammox (PNA) process under high salinity stress during kitchen wastewater treatment, focusing on their physiological and molecular responses through metagenomic analysis. An airlift inner-circulation partition bioreactor (AIPBR) was developed, featuring an inner cylinder and a flow guide tube to create distinct oxygen gradients, facilitating the study of microbial adaptation under varying salt conditions. The AIPBR was operated with synthetic wastewater containing ammonium concentrations of 1800 ± 100 mg/L and salinity gradients ranging from 1 to 10 g/L, followed by a fixed salinity period at 6 g/L, with ammonium concentrations approximately 850 mg/L. High-throughput metagenomic analysis revealed shifts in functional genes and metabolic pathways in response to salinity stress. Anammox bacteria adapted by enriching genes involved in the synthesis of osmoprotective compounds and activating energy-producing pathways like the tricarboxylic acid cycle (TCA). These adaptations, along with modifications in membrane composition, were essential for sustaining system stability under elevated salinity. Under prolonged high salinity stress, anaerobic ammonium oxidizing (AnAOB) exhibited improved salt tolerance, maintaining a total nitrogen removal efficiency above 85 % and stabilizing after an adaptation phase. The metagenomic data revealed a marked enrichment of genes associated with ion transport, stress response mechanisms, and DNA repair pathways. Changes in microbial community composition favored salt-tolerant species, supporting system stability. These findings highlight the applicability of the developed bioreactor for scaling up the PNA process to handle high-salinity wastewater, providing a promising avenue for sustainable nitrogen removal in challenging environments.