Effects of photoaged polystyrene microplastics and nanoplastics on the extracellular aggregation and intracellular accumulation of ZnO nanoparticles to algae.

Microplastics (MPs) and nanoplastics (NPs) can alter the behavior of co-existing pollutants in aquatic environments through their interactions. However, limited information was available on how the interactions between photoaged MPs/NPs and nanoparticles affect the extracellular aggregation and intracellular accumulation of nanoparticles and their associated ecological risk. Here, we investigated the effects of interactions between photoaged polystyrene MPs and NPs (aged PS MPs/NPs) and zinc oxide nanoparticles (nano-ZnO) on the green alga Chlorella vulgaris. Results proved that the adsorption of nano-ZnO on aged PS MPs/NPs, particularly aged PS NPs, via electrostatic force and hydrogen bonds was enhanced compared to that on pristine PS MPs/NPs. Molecular dynamic simulations confirmed stronger electrostatic force and van der Waals force between nano-ZnO and aged PS MPs/NPs. Furthermore, the binding affinity dissociation constant of nano-ZnO to the algal cells (32.4 and 45.0 µmol/L) in the presence of aged PS MPs/NPs was significantly higher than that of pristine PS MPs/NPs (24.0 and 7.0 µmol/L). The enhanced extracellular aggregation of nano-ZnO by aged PS MPs/NPs inhibited the Zn intracellular accumulation in algal cells. Nevertheless, this inhibitory effect was relatively weak for PS NPs due to their internalization carrying nano-ZnO into the algal cells. Despite the reduction of Zn intracellular accumulation, aged PS MPs/NPs still increased the ecological risk of nano-ZnO to aquatic organisms from medium to high risk through aggregation. These findings provide deeper insights into the environmental behavior and ecological risk of aged MPs/NPs.