In-sewer microplastics drive microbial metabolic shifts toward enhanced methanogenesis

Microplastics (MPs) in sewer systems can be transported extensively before entering wastewater treatment plants. Sewer systems harbor complex microbial communities under low-oxygen, sulfide-rich conditions that drive key biogeochemical cycles. These conditions drive microplastic aging, whereas these particles concurrently perturb sewer microbial ecology and metabolic functions. However, the underlying mechanisms of in-sewer microplastic aging and their subsequent impacts on sewer microbiomes remain unclear. Here we show that hydroxyl radicals preferentially attack ester bonds (C-O) in polyethylene terephthalate (PET) and polybutylene adipate terephthalate (PBAT) MPs, increasing surface roughness, reducing particle size, promoting surface oxidation, and ultimately inducing polymer chain scission. Exposure to PET and PBAT MPs at 30-500 particles L^-1 intensified oxidative stress, disrupted membrane integrity and permeability, impaired microbial activity, and suppressed sulfide production in a dose-dependent manner. These disruptions coincided with weakened microbial co-occurrence networks and a shift from stochastic toward deterministic community assembly. High doses of PET and PBAT MPs reduced hydrolytic/fermentative bacteria and sulfate-reducing bacteria by up to 63.4% and 49.7%, respectively, while enriching hydrogen-producing acetogenic bacteria and methanogenic archaea by 48.4-67.0%, consistent with reduced sulfidogenic potential and enhanced methanogenic potential. Changes in genes related to antioxidant defense, SOS response, quorum sensing (e.g., sodA, katG, lexA, and luxS), and redox signaling suggested potential mechanisms of microbial metabolic perturbations aggravated by PET and PBAT MPs. Our results indicate that sewer systems are not passive conduits but active reactors that promote MP aging, and that MPs reshape microbial functions. Microplastic control may therefore help reduce downstream particle pollution and limit perturbations to urban sewage biogeochemistry.