Simultaneous exposure to nanoplastics and cadmium mitigates microalgae cellular toxicity: Insights from molecular simulation and metabolomics

In the severe pollution area of nanoplastics (NPs) and cadmium ions (Cd²⁺), the joint effects of their high environmental concentrations on primary producers may differ from those of low environmental doses. Thus, we investigated the physiological changes, cell morphology, molecular dynamic simulation, phenotypic interactions, and metabolomics responses of C. pyrenoidosa to high environmental concentrations of NPs and Cd²⁺ after 12-d acclimation. After 12-d cultivation, mono-NPs and mono-Cd²⁺ reduced cell density and triggered antioxidant enzymes, extracellular polymeric substances (EPS) production, and cell aggregation to defend their unfavorable effects. Based on the molecular dynamic simulation, the chlorine atoms of the NPs and Cd²⁺ had charge attraction with the nitrogen and phosphorus atoms in the choline and phosphate groups in the cell membrane, thereby NPs and Cd²⁺ could adsorb on the cells to destroy them. In the joint exposure, NPs dominated the variations of ultrastructure and metabolomics and alleviated the toxicity of NPs and Cd²⁺. Due to its high environmental concentration, more NPs could compete with the microalgae for Cd²⁺ and thicken cell walls, diminishing the Cd²⁺ content and antioxidant enzymes of microalgae. NPs addition also decreased the EPS content, while the bound EPS with -CN bond was kept to detoxicate Cd²⁺. Metabolomics results showed that the NPs downregulated nucleotide, arachidonic acid, and tryptophan metabolisms, while the Cd²⁺ showed an opposite trend. Compared with their respective exposures, metabolomics results found the changes in metabolic molecules, suggesting the NPs_Cd²⁺ toxicity was mitigated by balancing nucleotide, arachidonic acid, tryptophan, and arginine and proline metabolisms. Consequently, this study provided new insights that simultaneous exposure to high environmental concentrations of NPs and Cd²⁺ mitigated microalgae cellular toxicity, which may change their fates and biogeochemical cycles in aquatic systems.