Effects of polyethylene microplastics on the fate of antibiotic resistance genes and microbial communities in anaerobic digestion of dairy wastes

Microplastics (MPs) have been confirmed to affect the digestion performance of the anaerobic digestion (AD) and to influence the occurrence of antibiotic resistance genes (ARGs) in the environment. In this study, the effects of polyethylene (PE) MPs on digestion performance and the fate of ARGs and microbial communities in thermophilic AD (55 °C) and hyperthermophilic AD (65 °C) of dairy wastes were investigated. The results showed that the effects of PE MPs were similar at different temperatures. At 55 °C and 65 °C, the presence of PE MPs improved methane production by 8.4% and 41.2% and enhanced COD removal rates to 52.8% and 52.4%, respectively, mainly by increasing the abundances of microorganisms involved in hydrolysis, acidogenesis and acid utilization (e.g. Clostridium , Dechloromonas and Caldicoprobacter ). PE MPs significantly increased the fold changes in the abundances of tet C, tet G and tet W from 0.71 to 12.63 to 1.15–28.62. According to redundancy analysis and correlation analysis, free ammonia contributed the most (51% of the total variation) to the changes in host bacteria, which probably changed the fate of ARGs in different AD system. This study indicated the discharge of PE MPs presents a novel challenge in reducing the risks of ARG transmission. • The effects of polyethylene microplastics on anaerobic digestion were investigated. • Polyethylene efficiently increased the methane production and COD removal rate. • Polyethylene significantly increased the abundances of tet C, tet G and tet W. • Polyethylene shifted the microbial communities involving gas production.