Metagenomics Reveals the Particle Size Effects of Polyethylene Terephthalate Microplastics on Anaerobic Digestion

Polyethylene terephthalate microplastics (PET-MPs) are among the most prevalent microplastics in activated sludge; however, the mechanism underlying its impact on anaerobic digestion (AD) remain unclear. Through batch experiments and simulated substrate experiments, this study found that PET-MPs exhibited particle size-dependent effects across the various phases of AD. Results indicated that 50 μm PET-MPs particles enhanced methane production, whereas 200 μm particles inhibited the process. Specifically, 50 μm PET-MPs primarily accelerated the acidogenesis and methanogenesis phases, while 200 μm PET-MPs predominantly affected the dissolution and hydrolysis phases. Scanning electron microscopy (SEM) coupled with metagenomic analysis revealed that the surface of PET-MPs became roughened during AD, leading to an increased specific surface area. This morphological change selectively enriched hydrogenotrophic methanogens such as Candidatus_Methanoculleus_thermohydrogenotrophicum and activated their corresponding metabolic pathways, including mthB and mcrA . Mechanistic analysis indicated that PET-MPs enhance methane production by (i) providing a biointerface for hydrogenotrophic methanogen colonization, (ii) promoting protein-derived substrate fluxes that elevate H 2 and acetate availability, and (iii) enabling coactivation of acetoclastic and hydrogenotrophic pathways. This study provides new insights into microplastic removal and the enhancement of sludge resource utilization.