Enzymatic biotransformation of polystyrene: Interfacial oxidative modification by a serine hydrolase from Pseudomonas...

Polystyrene (PS) is highly resistant to biodegradation, and the enzymatic processes capable of initiating its environmental transformation remain poorly defined. This study investigates the previously underexplored association between a purified serine hydrolase (SH) from Pseudomonas aeruginosa, a bacterium isolated from the gut microbiome of PS-fed Zophobas atratus, and the early-stage oxidative modification of PS at the polymer-water interface. Under aqueous conditions, SH-treated high-molecular-weight PS films exhibited pronounced nanoscale surface alterations and progressive oxygen functionalization, as demonstrated by field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). These surface chemical changes were accompanied by a modest reduction in number-average molecular weight (M_n) from 36.7 ± 0.5 kDa to 33.8 ± 0.2 kDa, while the weight-average molecular weight (M_w) remained largely unchanged, indicating limited molecular weight reduction without extensive polymer depolymerization. AEBSF inhibition further supported the involvement of SH activity in the observed PS surface modification. Gas chromatography-mass spectrometry (GC-MS) detected signals tentatively assigned to low-molecular-weight aromatic compounds, including oxygenated species, consistent with chemical transformation associated with SH treatment. Structural modeling and molecular docking suggested that SH adopts a conserved α/β-hydrolase fold with a surface-accessible aromatic binding cavity, potentially facilitating interactions with styrenic motifs at the polymer-water interface. Collectively, these findings reveal a novel association between SH activity and interfacial PS modification, highlighting enzyme-associated surface oxygenation as a potential early-stage process in plastic aging with implications for the environmental fate and potential chemical risks of persistent plastic pollutants.