The treated wastewater enhances the biodegradation of sulfonamide antibiotics in biofilm-sediment downstream of the receiving river outlet.

Although the treated wastewater meets the discharge standards, it can still become a potential transmitted stressor that affects aquatic organisms in receiving rivers. Biofilms and sediments as the main solid-phase substances in natural aquatic environments can biodegrade micropollutants. However, most of the current studies have selected a single solid-phase material, and there are relatively few studies that comprehensively consider the effect of treated wastewater on the dissipation of micropollutants in a composite biofilm-sediment system. Therefore, this study investigated the dissipation pathways of six sulfonamide antibiotics (SAs) in biofilm-sediment and the effect of treated wastewater on SAs dissipation. The results showed that biodegradation was the main pathway for SAs dissipation in biofilm-sediment. The input of treated wastewater increased the abundance of dominant degradation bacteria Burkholderiales and Pseudomonadale, thereby improving the biodegradation rate of SAs (approximately 1.5 times higher than upstream degradation rate). These genera could also be further integrated into downstream communities to continuously mediate the biodegradation of SAs. Through mass spectrometry and metagenomic sequencing analysis, it was found that the common degradation pathways of SAs in biofilm-sediment affected by treated wastewater are acetylation, formylation, hydroxylation, and bond cleavage. Acetyltransferase played an important role in the biodegradation of SAs. In addition, the enrichment of antibiotic resistant genes during biodegradation increased the risk of their spread in the aquatic environment. These findings provide new insights into the fate of antibiotics in aquatic environments and the impact of treated wastewater on downstream bacterial communities.