Comparative toxic effects of microplastics and nanoplastics on Chlamydomonas reinhardtii: Growth inhibition, oxidative stress, and cell morphology

Microplastics (MPs) have become an emerging global environmental pollution issue, while studies on the impact of nanoplastics (NPs) on aquatic environments, especially in freshwater environments, are still limited. In this study, we compared the toxic effects of micro- and nano-sized polystyrene microbeads (PS-MPs and PS-NPs) on a widespread freshwater algal species, Chlamydomonas reinhardtii (C. reinhardtii). The results revealed that both PS-MPs and PS-NPs had negative effects on the growth of C. reinhardtii in the range of 50 to 500 mg/L, reducing its biomass and photosynthetic pigment. The growth inhibitory rate (IR) of C. reinhardtii was 61.7% and 23.3% when exposed to 500 mg/L PS-NPs and PS-MPs for 96 h, respectively, demonstrating a higher toxic effect of the NPs than the MPs. In comparison to the control group, reactive oxygen species (ROS) and malondialdehyde (MDA) levels in algae cells increased, enhancing cell membrane permeability and then accelerating PS-NP internalization. MDA levels increased by 1.3 and 2.2 times under PS-MPs and PS-NPs exposure (500 mg/L, 96 h), respectively. Furthermore, the content of the extracellular polymeric substances and the antioxidant enzymatic (superoxide dismutase, peroxidase, and catalase) activities were all enhanced to resist the adverse effects caused by PS-MPs and PS-NPs. It is interesting that the internalized NPs were found to be clearly encapsulated in the vacuoles of the algal cells. These findings suggest that the ecological effect of NPs might need more attention, especially considering the continuing fragmentation of MPs in the environment.