Microplastics enhanced the toxic effects of sulfamethoxazole on aerobic granular sludge and enriched antibiotic resistance genes

Aerobic granular sludge (AGS) is an effective antibiotic wastewater treatment technology. However, although sulfonamide antibiotics (SAs) and microplastics (MPs) are frequently and simultaneously detected, their combined effects on AGS remain unclear. Herein, the effects of a combination of polystyrene (PS) and sulfamethoxazole (SMX) on the granulation, performance, and antibiotic resistance genes (ARGs) of AGS in sequencing batch reactor (SBR) systems were evaluated. The response mechanism of AGS microorganisms to the combined toxicity was investigated using metagenomics and metabolomics. The results revealed that the combined stress decreased the removal rate of COD and NO3--N in the SBR system, delayed the process of AGS granulation, and increased the abundance of ARGs by 62.02% compared with SMX stress alone. The increased concentration of malondialdehyde and phospholipid metabolites suggested that PS enhanced the toxicity of SMX toward microorganisms by disrupting the cell membrane, thereby increasing the secretion of extracellular polymeric substances (EPS) by 28.16% and decreasing the proportion of viable cells by 23.16%. In contrast, the abundance of carbon-source utilization, denitrifying bacteria, and corresponding key functional genes decreased. Microbes respond to combined toxic shocks by increasing the synthesis of "cheap" amino acids and polysaccharides to secrete a greater amount of EPS. This study will promote the understanding of the impact of the coexistence of MPs and SAs on AGS.