Response of methanogenic metabolism to polystyrene microplastics at varying concentrations: The trade-off between inhibitory and protective effects in anaerobic digestion

Polystyrene (PS) is an important raw material of food packages, and its induced microplastics (MPs) inevitably enter into the food waste (FW) with mechanical or hydrothermal pretreatment, which may affect the FW anaerobic digestion (AD). However, the different microbial responses to PS MPs in AD system have rarely been reported at varying PS MPs levels. In this study, the trade-off between positive and negative effects of PS MPs on syntrophic methanogenesis in AD for FW treatment was investigated when the dosage varied from 10 to 200 mg/L. The results showed that methane (CH) yield was upgraded by 4.72% at a lower PS MPs concentration (25 mg/L) compared with the control reactor without PS MPs dosing. In contrast, it would be inhibited by 10.13% when PS MPs concentration increased to 100 mg/L. More secretion of extracellular polymeric substances was the main reason for methanogenic metabolism enhancement at lower dosages of PS MPs due to its characteristics of cell-protection and electron transfer facilitation. Furthermore, the higher abundance of genes regarding superoxide dismutase and catalase in the reactor with PS MPs of 25 mg/L could impede reactive oxygen species formation and alleviate its toxicity. The findings of the study provide a comprehensive understanding of the promotion and inhibition mechamisms of syntrophic metabolism in the AD process by PS MPs and their released substances at varying concentrations.