Novel insights into self-defense function of anammox sludge under magnesium ions (Mg2+) stress based on Mg2+ transport system

[Display omitted] •0.8 mmol/L Mg2+ significantly enhances the performance of anammox systems.•Mg2+ strengthens genetic repair to improve the anti-shock capacity of AnAOB.•Mg2+ transport theory was introduced to reveal the response of AnAOB to Mg2+.•A self-defense function of AnAOB based on PhoQ/PhoP-MgtAB system was proposed. Magnesium ion (Mg2+) plays an important role in the accumulation and stability of anaerobic ammonium-oxidizing bacteria (AnAOB). In this study, the response of anammox sludge to Mg2+ was comprehensively investigated by performance evaluation and metagenomics analysis. Appropriate Mg2+ (0.8 mmol/L) could improve the nitrogen removal performance, AnAOB activity, and the synthesis potential of some hydrophobic substances, while high Mg2+ (>1.6 mmol/L) has a negative effect. Meanwhile, Mg2+ transmembrane transport theory was introduced to reveal the response principle of AnAOB to Mg2+ from a novel insight. AnAOB may have a self-defense function based on the PhoQ/PhoP-MgtAB system. Low extracellular Mg2+ will activate this function to enhance Mg2+ influx, thereby improving the intracellular metabolism of AnAOB. Excessive Mg2+, however, dormant this function and induces Mg2+ efflux, which may decrease the intracellular Mg2+ and thus affect AnAOB metabolism. These findings provide valuable references for the Mg2+ regulation of anammox-based process.