Accelerating enrichment of ARGs and MGEs with increasing ammonium removal during partial nitrification treating...

Free ammonia (FA) and free nitrous acid (FNA) are critical operational parameters to maintain partial nitrification (PN) in treating high-strength ammonia wastewater. However, whether FA and FNA, as non-antibiotic antimicrobial chemicals, could directly enhance antibiotic resistance genes (ARGs) enrichment during PN is still unclear. This study deciphered the prevalence of ARGs, potential mobility, and hosts under different ammonium removal efficiencies (ARE) (different FA and FNA) during stable PN. The results showed that when ARE increased from 52.2 ± 6.2 % to 91.8 ± 3.3 %, total abundances of ARGs and mobile gene elements (MGEs) were stepwise increased. In particular, four types of ARGs (aminoglycoside, bacitracin, peptide, and sulfonamide) and six subtypes of ARGs (bacA, ugd, sul2, baeR, aph(3")-I and aph(6)-I) increased with the increasing ARE. Under selection pressures of FA and FNA, plasmids tended to encode aminoglycoside and MLS, while chromosomes had a tendency to carry multidrug, bacitracin, peptide, and fosmidomycin. Furthermore, FA and FNA enhanced the spread of ARGs mediated by the dominated transposase and recombinase. Nitrosomonas dominated the host of ARGs co-occurring with MGEs during the operation of PN. This study highlights the effect of FA and FNA on ARGs and provides theoretical support for the control of ARGs in biological nitrogen removal processes via nitrite pathway.