Toxicity differences of polystyrene nanoplastics (PS-NPs) and polylactic acid nanoplastics (PLA-NPs) on the Paramecium bursaria - Chlorella symbiotic system.

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 (LC50) 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.