Effects of aging treatments on the biofilm formation on low density polyethylene microplastics in constructed wetland soils

Wetland soils are critical interfaces for contaminant retention and biogeochemical processing but are increasingly burdened by microplastics (MPs). Little is known about the effects of MP aging on characteristics and temporal dynamics of biofilm formation on their surface and associated bacterial communities in soils. To fill these knowledge gaps, we performed an in situ exposure experiment in constructed wetland soils with virgin and aged low density polyethylene (LDPE) MPs (hereafter referred to as “MPs”) as biofilm substrates. Results showed that aging enhanced MP oxidation with more metal oxides formed in biofilms after 330 days exposure. The biomass of biofilms on MPs increased with exposure time but more dead bacteria was observed in aged than virgin MPs. Bacterial communities in biofilms were significantly different from those in surrounding soils with lower diversity, which decreased first and then increased with exposure time. Meanwhile, aged MPs specifically enriched some methanotrophic bacteria. Network analysis suggested a less complex but more stable co-occurrence pattern of bacterial communities in aged than in virgin MPs biofilms. Community assembly null model analysis showed that aging shifted biofilm bacterial community assembly toward more deterministic processes. Moreover, genes involved in lignin degradation and methane oxidation, were more abundant in MP-attached biofilms than in soil. These findings reveal that MP aging modulates the wetland soil plastisphere by enhancing surface oxidation and metal oxide accumulation in biofilms to alter microhabitats, which in turn increases biofilm biomass while raising the proportion of dead bacteria, forms a less complex but more stable bacterial community network, promotes deterministic bacterial community assembly, and enriches functional genes related to methane metabolism. This work advances targeted understanding of how MP aging shapes soil plastisphere structure and function.