Metagenomic insights into the "window" effect of static magnetic field on nitrous oxide emission from biological nitrogen removal process at low temperature.

This study aimed to explore whether the "window" effect of static magnetic field (SMF) on nitrous oxide (NO) emission from the biological nitrogen removal process at low temperature existed and reveal its biological mechanism at the gene level. Four sequencing batch reactors (SBRs) with SMFs of 0, 10, 45, and 75 mT were operated continuously for 110 days at 10 °C and the lowest NO-Gas cumulative emission (5.50 mg N/day) and NO conversion rate (4.28 %) in 45 mT SMF-SBR verified the existence of the "window" effect. In 45 mT SMF-SBR, nearly all enzymatic activities related to NO reduction and corresponding functional gene abundances improved significantly. Metagenomic high-throughput sequencing analysis revealed that Alicycliphilus denitricans, Paracoccus denitrificans, Rhodopseudomonas palustris, Pseudomonas stutzeri, and Dechloromonas aromatica, as species related to NO reduction, could be separately enriched by applying suitable SMF intensity. Gene functions annotation based on KEGG and CAZy databases indicated that SMF not only accelerated the rate of free ammonia into ammonia-oxidizing bacteria and electrons delivered to the corresponding denitrification reductases, but also enhanced the degradation of complex organic matter into smaller molecules, and thus reducing the production of NO via nitrifier denitrification and incomplete denitrification pathways at 10 °C. These findings provided a guideline and presented a blueprint of ecophysiology for the future application of magnetic field to the reduction of NO emission in wastewater treatment plants in the cold region.