Systematic analysis of an integrated ultraviolet pretreatment and mixotrophic denitrification system: Insights into carbon, sulfur, and nitrogen metabolism, microbial community dynamics, and the fate of antibiotic resistance genes

Antibiotics and nitrate are frequently detected in mariculture wastewater, and their simultaneous removal presents significant challenges. We developed an integrated system that combines an ultraviolet (UV) pretreatment unit with a thiosulfate-driven mixotrophic denitrification unit (UV-TDMD). This study aimed to examine the impact of different UV irradiation durations on microbial response behaviors, synergy and competition in metabolic pathways, and the fate of antibiotic resistance genes (ARGs). Although reducing UV irradiation time from 30 min to 0 min did not impair the denitrification performance of the mixotrophic denitrification unit, it led to an accumulation of sulfamethoxazole (SMX), which in turn triggered the production of reactive oxygen species (ROS). To counteract the oxidative damage caused by SMX, the microbial antioxidant system was activated, and the secretion of extracellular polymeric substance (EPS) increased. Metagenomic analysis showed that higher SMX levels promoted the dominance of Desulfocapsa, associated with sulfur reduction, while suppressing heterotrophic denitrifying bacteria. Elevated SMX stress diminished the electrons and energy supply in the absence of UV pretreatment, thereby intensifying competition among microbial communities, particularly affecting carbon-sulfur-nitrogen metabolism. Notably, a strong synergy (69.54 % positive correlation) was observed between 25 potential ARG hosts and 20 ARGs, which helped alleviate SMX stress. This study provides comprehensive insights into the critical role of the UV pretreatment unit in controlling antibiotics in the mixotrophic denitrification system.