Mechanistic insights into microbial and functional responses to norfloxacin in a thiosulfate-mediated denitrification granular sludge system.

Thiosulfate-driven denitrification granular sludge systems offer a promising strategy for treating wastewater with low carbon-to-nitrogen ratios. In this study, the system achieved 97.0 % nitrate removal and 50.2 % norfloxacin removal within a 1.5-hour hydraulic retention time, without nitrite accumulation. Norfloxacin was degraded via acetylation, defluorination, and quinoline ring cleavage, reducing the ecotoxicity of transformation products. Functional analysis revealed a 65.2 % increase in cytochrome P450 abundance, along with elevated expression of acetyltransferases, dehalogenases, and dioxygenases. Microbial communities shifted from Thiomonas and Sulfurimonas to Sulfuricella and Thiobacillus, accompanied by upregulation of sulfur oxidation (soxABXYZ) and denitrification genes (napA, nirK, norB, nosZ). Norfloxacin exposure increased total antibiotic resistance abundance from 1784 to 2154 copies per gigabase, with plasmid-associated resistance rising from 26.9 % to 39.9 %, and mobile genetic elements increasing from 1653 to 4972 copies per gigabase. These findings demonstrate the system's potential for cost-effective, robust treatment of wastewater containing both nutrients and antibiotics.