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Toxicity Differences of Polystyrene Nanoplastics ( PS-NPs ) and Polylactic Acid Nanoplastics ( PLA-NPs) on the Paramecium Bursaria - Chlorella Symbiotic System
Summary
"Biodegradable" plastics are often marketed as the safer, eco-friendly choice, but this study found that nanoparticles from biodegradable plastic (PLA) were actually more toxic to a single-celled organism than nanoparticles from regular plastic (polystyrene), causing more cell damage and stress at lower doses. This matters because as companies swap conventional plastics for biodegradable alternatives, tiny nanoplastic fragments from these "greener" products may not automatically be gentler on living cells, including potentially our own, so their safety shouldn't be assumed without direct testing.
Biodegradable plastics are often presumed to pose lower biological risks, yet their nanoparticulate forms may not follow this assumption. This study compared the 24-h acute toxicity and associated biological responses of conventional polystyrene nanoplastics (PS-NPs) and biodegradable polylactic acid nanoplastics (PLA-NPs) using the Paramecium bursaria-Chlorella symbiotic system. The 24-h median lethal concentrations (LC) were 27.41 mg/L for PS-NPs and 5.55 mg/L for PLA-NPs, indicating greater acute lethal toxicity of PLA-NPs under the tested conditions. After exposure and washing, pronounced NP-associated fluorescence was detected for both NP types. Both materials produced concentration-related increases in ROS-associated fluorescence and MDA content, together with concentration-specific changes in SOD and CAT activities. TEM observations revealed ultrastructural alterations in mitochondria, the pellicle, nucleoli, digestive vacuoles, and symbiotic Chlorella. NP exposure also altered chlorophyll a and carotenoid contents, indicating disruption of photosynthetic pigment homeostasis. RT-qPCR further showed concentration-specific expression changes in selected genes associated with photosynthesis, antioxidant defense, proteostasis, mitochondrial respiration, and vesicular transport. Overall, PLA-NPs exhibited greater acute lethal toxicity than PS-NPs during short-term exposure. These findings demonstrate that biodegradability does not necessarily correspond to lower biological toxicity when polymers occur in nanoparticulate form and highlight the need to consider nano-specific effects in the environmental safety assessment of biodegradable plastics.