Polystyrene Nanoplastics Disrupt the Conversion of Dimethylsulfoniopropionate in Marine Roseovarius nubinhibens

Nanoplastic pollution is increasingly pervasive in marine environments, yet how their interfacial behavior modulates microbially biogeochemical function under saline conditions remains poorly understood. Here, we investigated the effects of aminated (PS-NH2) and sulfonated (PS-SO3H) polystyrene nanoplastics on the dimethylsulfoniopropionate (DMSP)-catabolizing marine bacterium, Roseovarius nubinhibens. Both particles underwent pronounced aggregation and charge screening in the marine basal medium. At 5 mg/L, only PS-NH2 inhibited bacterial growth and increased oxidative stress. At their respective observed-effect concentrations (20 and 400 mg·L−1), both treatments induced comparable growth inhibition but contrasting membrane-permeability responses. Tightly bound extracellular polymeric substances (EPSs) exhibited more pronounced changes in composition and structure than loosely bound EPSs, with a greater aggregation-promoting effect on PS-NH2 than on PS-SO3H. DMSP conversion remained largely unchanged at 5 mg·L−1 but reduced modestly under PS-NH2 exposure and more markedly under PS-SO3H exposure at the respective observed-effect concentrations, accompanied by significant changes in the consumption of extracellular 3-methylmercaptopropionate (MMPA) and acrylate. Transcriptomic analysis identified 1167 and 1590 differentially expressed genes under PS-NH2 and PS-SO3H exposure, respectively, involving genes associated with sulfur oxidation, assimilatory sulfur reduction, cysteine metabolism, and redox processes. Together, these findings reveal distinct EPS responses alongside alterations in DMSP-related sulfur metabolism, highlighting the importance of considering both extracellular responses and microbial functional endpoints when assessing nanoplastic effects under saline conditions.