Effects of Typical Antibiotics on Denitrification Processes and Their Degradation Products in Aquatic Environments

The continuous input of antibiotics into groundwater poses a potential threat to nitrogen cycling, but the mechanisms affecting denitrification remain unclear. In this study, ciprofloxacin (CIP) and sulfamethoxazole (SMX) were selected to investigate the effects of single and combined exposure on denitrification performance, enzyme activities, microbial community, functional genes, and degradation product ecotoxicity. Results showed that CIP exhibited the strongest inhibitory effect, followed by CIP+SMX, while SMX showed weak and delayed inhibition. Antibiotics inhibited cell viability more significantly than cell abundance, consistent with changes in nitrogen transformation and enzyme activities. Pseudomonadaceae was decreasing under antibiotic stress, particularly in the CIP+SMX group. Functional gene analysis revealed significant reductions in nirK, nirS, and nosZ abundances in CIP-treated groups, while SMX inhibition emerged during recovery. Degradation pathway analysis identified multiple transformation routes, and ecotoxicity predictions indicated that some intermediates were more toxic than parent compounds. This study provides important insights into the ecotoxicological effects of antibiotics on denitrification in groundwater systems.