Comparison of the effects of continuous and accumulative exposure to nanoplastics on microalga Chlorella pyrenoidosa during chronic toxicity

Most previous studies have focused on the continuous exposure of aquatic organisms to nanoplastics. However, persistent pollutants in natural aquatic surroundings are a threat, and their concentrations are continuously increasing. The discussion and research into the effects of accumulative exposure to these materials are limited. Therefore, this study aimed to compare the effects of continuous and accumulative exposure to polystyrene (PS) nanoplastics (80 nm) on Chlorella pyrenoidosa during chronic toxicity. The results indicated that under conditions of continuous exposure, this alga exhibited self-recovery to defend against the negative effects of PS nanoplastics during 15-21 days of exposure (the 21-d inhibitory rate was 1.41%). However, one unanticipated finding was that during the same period of accumulative exposure, nanoplastics retained a substantial and stable inhibitory effect on the algal growth (the 21-d inhibitory rate was 6.79% in accumulative exposure for twice), indicating the invalid self-recovery of algae. The results of scanning electron microscopy demonstrated that on day 21, the degree of damage to the algal cells under accumulative exposure was more severe than that under continuous exposure. Hence, nanoplastics exerted an irreversibly negative effect on aquatic organisms depending on the pattern, frequency, concentration, and duration of exposure. This project evaluated the practical significance of nanoplastics in aquatic ecosystems.