Polystyrene nanoplastics alter the ecotoxicological effects of diclofenac on freshwater microalgae Scenedesmus obliquus†

Due to the escalating risk of plastic pollution, nanoplastics have attracted considerable attention in the recent past. They can co-exist and interact with other contaminants like pharmaceuticals in the aquatic environment. Therefore, it is pertinent to understand how these pollutants interact with one another in the ecosystem. The current study examined the individual and combined effects of fluorescent polystyrene nanoplastics (FNPs) and diclofenac (DCF) on Scenedesmus obliquus using a full factorial design. The toxicity of S. obliquus significantly increased in a dose-dependent manner upon exposure to pristine forms of DCF and FNPs. The major cause of individual toxicity of DCF and FNPs in S. obliquus was oxidative stress. In the combined toxicity tests when FNPs (0.01, 0.1, and 1 mg L⁻¹) and DCF (1 mg L⁻¹) were mixed, a synergistic effect was noted compared to the respective pristine FNPs. However, when the DCF concentration in the mixture was decreased to 0.25 mg L⁻¹, the combined toxicity with FNPs (0.01, 0.1, and 1 mg L⁻¹) reduced indicating an antagonistic effect. The independent action model also showed an antagonistic effect for low-dose combinations of DCF and a synergistic effect for high-dose combinations. The estimation of oxidative stress parameters, antioxidant enzyme activity, and photosynthetic pigment content in the algae further validated the cytotoxicity data. The mean hydrodynamic diameter and surface charge analyses further indicated that the colloidal stability of the FNPs in the medium was affected when they were combined with DCF. The key reason for differences in the cytotoxicity of combinations could be observed variations in the aggregation of FNPs and differential adsorption patterns of DCF on the FNPs. These factors efficiently altered cell–particle interactions in the mixture demonstrating a hormesis effect. Thus, this current study highlighted the hazardous nature of the nanoplastics and their co-exposure risks with pharmaceuticals on microalgae in freshwater environments.