Concentration-specific effects of micropollutants on microbial communities and antibiotic resistance genes in activated sludge systems.

Micropollutants are widespread in wastewater systems and can impact microbial communities and the transfer of antibiotic resistance genes (ARGs). Nevertheless, the specific concentration thresholds for these effects under environmental conditions remain largely unknown. This study evaluated six micropollutants at five environmentally relevant concentrations (0.1-500 μg/L) to clarify the concentration-dependent effects on microbial ecology and ARG dynamics. The results showed micropollutant exposure generally reduced microbial diversity. Most micropollutants, except triclosan (TCS), influenced microbial community structure and ARG profiles at even 0.1-1 μg/L, indicating a close association between taxonomic shifts and resistome changes. In contrast, low concentration TCS (0.1-1 μg/L) affected ARG dynamics without altering community composition. Moreover, ARG abundance generally increased with micropollutant concentration. Nevertheless, the acetaminophen and naproxen posed lower horizontal transfer risks despite ARG elevation. In contrast, diclofenac and TCS markedly enhanced ARG transfer at ≥ 10 and 1 μg/L, respectively. Network analysis identified high-risk ARG hosts (e.g., Burkholderiales, Rhodocyclaceae) and highlighted mobile genetic elements involved in replication/recombination/repair. These findings demonstrate that micropollutant-induced resistance dissemination is both concentration-specific and compound-specific, guiding ARG management in wastewater.