Biofilms on Plastics Slow Photo-Oxidation while Promoting Surface Degradation

Plastics in natural environments undergo physicochemical aging, yet the role of biofilms─microbial communities that colonize plastic surfaces─remains poorly understood. To evaluate this, polyethylene terephthalate (PET) drinking bottles were exposed to natural freshwater to facilitate biofilm formation, and subsequently subjected to ultraviolet (UV) irradiation to assess the influence of biofilms on weathering. Results showed that biofilms exert dual roles, as both protectors and aggressors. When combined with UV exposure, biofilms intensified surface morphological alterations, root-mean-square roughness increased from ∼22 nm for pristine PET to ∼874 nm, and altered polymer crystallinity, as indicated by a shift in the Raman spectrum. They increased brittleness and reduced impact strength, facilitating UV to penetrate deeper layers and form cavities, thereby enhancing specific surface area from 2 g m^–2 (pristine PET) to 22 g m^–2. In contrast, UV exposure alone induced extensive photo-oxidation, reflected in higher surface hydrophilicity and increased polar surface energy components. Biofilms may attenuate UV penetration but concurrently promote plastic fragmentation, which might indirectly favor additive leaching through mechanical weakening and structural degradation. These findings underscore the complexity of plastic weathering under natural conditions, which surpasses dynamics observed in laboratory studies on pristine materials.