Insights into the Differential Impacts of Polystyrene Microplastics and Nanoplastics on Sequencing Batch Reactor Performance: From Physiological Stress to Antibiotic Resistance Gene Dissemination

The widespread occurrence of microplastics (MPs) and nanoplastics (NPs) in wastewater treatment plants (WWTPs) has raised notable ecological concerns. This study investigated the multi-level impacts of polystyrene (PS) MPs and NPs on activated sludge in sequencing batch reactors (SBRs). Although long-term exposure to PS particles (up to 200 mg/L MPs and 50 mg/L NPs) did not compromise the macro-performance of the SBRs (with COD, NH4+-N, and TP removal efficiencies remaining >90%), the physical and biological integrity of the sludge was significantly altered. Sludge volume index (SVI) analysis revealed a decline in settleability, particularly under NP stress, which was corroborated by laser scanning confocal microscopy (LSCM) and transmission electron microscopy (TEM) observations showing the internalization of NPs into microbial cells. Physiologically, PS NPs induced higher levels of reactive oxygen species (ROS) and lactate dehydrogenase (LDH) release, indicating substantial oxidative stress and membrane damage. Furthermore, high-throughput sequencing and qPCR analysis demonstrated that PS particles reshaped the microbial community, enriching the antibiotic-resistant genus Acinetobacter and various antibiotic resistance genes (ARGs) such as sulII and tetG. Notably, PS NPs exhibited a more pronounced effect on ARG dissemination than MPs, even at lower mass concentrations, by facilitating horizontal gene transfer (HGT) through increased cell membrane permeability. These findings elucidate the distinct toxicological mechanisms of NPs in biological treatment systems and highlight their role as catalysts for the environmental spread of antibiotic resistance.