Nanoplastics and the Role of the Corona in the Biological Responses of Daphnia magna

In the aquatic environment, organisms predominantly interact with nanoplastics coated with environmental biomolecules rather than with bare plastics. This study investigates the biological response of Daphnia magna neonates to bare and biomolecule-coated nanoplastics over 24 and 48 h. The commercially available fetal bovine serum (FBS), biomolecules collected from D. magna secretions (DSB), and algae biomolecules (AB), were used to coat (hard corona) 20 nm fluorescent nanoplastics. The results show that neonate exposure to both bare and coated nanoplastics resulted in the production of biomolecules by the neonates, which interacted with the nanoplastic surface, uniquely altering their properties through binding and exchange of biomolecules, affecting their internalization and toxicity. Neonates exposed to bare nanoplastics exhibited the highest mortality rates, while nanoplastics coated with FBS, DSB, and AB resulted in lower mortality, suggesting that corona composition significantly influences nanoplastic toxicity. Confocal microscopy revealed the highest internalization for the bare, DSB, and AB coated nanoplastics compared to the FBS coated nanoplastics. Swimming behavior was mostly affected by the bare nanoplastics compared to the corona-coated nanoplastics. The antioxidant defense mechanisms, including glutathione S-transferase (GST), catalase (CAT), and superoxide dismutase (SOD) were analyzed in D. magnahomogenates exposed to both bare and protein-coated nanoplastics, in order to better understand the overall oxidative stress responses in neonates. Results demonstrated higher enzyme activities at 24 h of exposure to all nanoplastics, particularly for SOD and GST. At 48 h, CAT activity increased for all nanoplastics, indicating sustained oxidative stress and activation of mechanisms to neutralize ROS and mitigate cellular damage. This study underscores the critical role of the coronas in modulating nanoplastic toxicity, advocating for their consideration in toxicological risk assessments.