Oxidative surface biodeterioration of polystyrene by plastisphere-derived Paenibacillus strains from weathered EPS buoy debris

Expanded polystyrene (EPS) buoy debris is a persistent source of polystyrene (PS) particles in coastal environments, yet microbial processes occurring on weathered PS surfaces remain poorly resolved. This study investigated whether plastisphere-derived bacteria from weathered EPS buoy debris can induce measurable PS surface biodeterioration under carbon-limited conditions. Three Paenibacillus strains, BS8–1, BS8–2, and BS11, were isolated from weathered EPS buoy debris collected from the Korean coast. PS films were incubated with each strain for 30 days and evaluated using gravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, water contact angle measurement, thermogravimetric/differential scanning calorimetry, ATR-FTIR, 1 H NMR, GC–MS, whole-genome annotation, and transcriptomic profiling. All three strains colonized PS films and induced strain-dependent changes in polymer mass, surface morphology, wettability, thermal behavior, and chemical signatures. Strain-exposed PS films showed 5.13–9.62% apparent mass loss and decreased water contact angles from 83.26° in the uninoculated control to 60.62–71.23°. SEM revealed post-washing surface pitting and erosive features, while EDS, ATR-FTIR, and 1 H NMR indicated oxidation-associated surface modification and increased structural heterogeneity. GC–MS detected treatment-associated extractable organics relative to uninoculated controls. Genome annotation and transcriptomic profiling of BS8–2 revealed redox- and aromatic-compound-processing candidate functions, together with transport- and cell-surface-associated responses consistent with adaptation to PS-interface conditions. The combined evidence supports oxidative surface biodeterioration and partial chemical modification of PS rather than complete mineralization, definitive backbone cleavage, or confirmed assimilation of PS-derived carbon. These findings identify weathered EPS buoy debris as a reservoir of PS-interacting Paenibacillus strains and highlight the need for future carbon-fate, molecular-weight, and ecotoxicity assessments to determine the environmental fate and potential risk of PS-associated transformation products.