Polyethylene terephthalate microplastic aging mediated by Shewanella oneidensis via extracellular electron transfer under anoxic conditions

Polyethylene terephthalate microplastics (PET-MPs) accumulate widely in natural environments, where microorganisms can substantially affect their aging. While previous studies on the microbial aging of PET-MPs have predominantly focused on oxidative and hydrolytic pathways, this study investigated the potential reductive aging of PET-MPs under anaerobic conditions using the model extracellular respiratory bacterium Shewanella oneidensis MR-1. Our results showed that S. oneidensis MR-1 mediated the reductive aging of PET-MPs, as evidenced by substantial surface erosion, an 11.7% decrease in Young’s modulus, a 23.8% reduction in the O/C atomic ratio, and a 14.9%–26.5% loss of key oxygen-containing functional groups (C=O and C–O bonds). This aging process required extracellular electron transfer capability via the CymA-MtrABC-OmcA protein complex and flavin shuttles, as it was abolished by knockout of cymA and mtrABC-omcA and accelerated by the addition of exogenous flavin. Based on density functional theory calculations, including analysis of constrained Fukui functions and thermodynamic assessments, together with the detection of intermediates, a putative reductive aging pathway was proposed, in which the carbonyl group in the ester bond of PET-MPs was identified as the primary site susceptible to reduction. In the presence of PET-MPs, biomass retention and cell viability were both substantially higher than in the MP-free control, indicating that the reduction of PET-MPs sustained cellular survival under electron acceptor-limited conditions. These findings broadened the mechanistic understanding of the anaerobic PET aging process, which was underappreciated yet potentially important in oxygen-depleted environments.