Physicochemical transformation of polystyrene mediated by Pontibacter strains isolated from discarded expanded...

Polystyrene (PS), including expanded polystyrene used in marine buoys, is a persistent source of coastal plastic pollution, yet evidence for its microbial transformation under marine-relevant conditions remains limited. In this study, three PS-associated bacterial strains (BS91, BS92, and BS100) were isolated from discarded expanded polystyrene buoy debris collected on the coast of Busan, Republic of Korea, and identified as members of the genus Pontibacter. Their PS-surface transforming potential was evaluated using gravimetric analysis, water contact angle measurement, scanning electron microscopy, thermogravimetric and differential scanning calorimetry, X-ray diffraction, attenuated total reflectance Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and gas chromatography-mass spectrometry. All three strains induced measurable physicochemical changes in PS films, with the strongest effects observed in carbon-free basal medium. BS100 showed the greatest apparent weight loss (4.47 ± 0.93%), the largest decrease in water contact angle, and the most pronounced surface disruption. Thermal and diffraction analyses indicated moderate changes in thermal behavior and short-range structural order without loss of the overall PS framework. Spectroscopic analyses further showed strain-dependent oxidative surface functionalization, including increased hydroxyl, carbonyl, and C-O-related signals and marked oxygen enrichment, particularly in BS100. Putative treatment-associated low-molecular-weight aromatic, aliphatic, cycloaliphatic, and oxygenated compounds were also detected after bacterial treatment. Together, these results support that Pontibacter strains associated with marine buoy debris can induce reproducible early-stage surface-level physicochemical transformation of PS and broaden the known diversity of marine bacteria implicated in PS surface modification under coastal plastic-debris conditions.