Insights into norfloxacin biodegradation and the fate of antibiotics resistance genes associated with sulfur-driven autotrophic denitrification

Abstract

The co-occurrence of nitrogen contaminants and norfloxacin (NOR) in low C/N wastewater poses significant treatment challenges. This study establishes a sulfur-based autotrophic denitrification (SAD) system for the simultaneous removal of nitrate and NOR. Batch and reactor experiments demonstrated that the system achieved up to 97.8 % nitrate and 85.2 % NOR removal efficiencies, with specific NOR removal rates reaching 0.60 mg/g-volatile suspended solids/day. NOR removal primarily occurred through co-metabolic degradation with nitrate, and adsorption also contributed. Long-term exposure to 0.1 mg/L NOR enhanced extracellular polymeric substances production by 22.4 %, increasing microbial resistance and NOR adsorption capacity. Seven NOR transformation products were identified, and three main degradation pathways were proposed based on Fukui function analysis. Genes coding cytochrome P450 enzymes, the key to NOR biodegradation, were significantly upregulated (64.5 % increase), and Thiobacillus exhibited strong NOR tolerance. NOR exposure also altered the abundance of nitrogen and sulfur metabolic genes. Metagenomic analysis revealed a nearly 3-fold increase in antibiotic resistance gene (ARG) abundance, with efflux and target modification as dominant resistance mechanisms, potentially facilitated by mobile genetic elements. This study provides novel insights into antibiotic degradation and ARG dynamics in SAD systems, offering a promising strategy for treating low C/N wastewater with concurrent removal of nitrogen and emerging contaminants.