Single and combined chronic toxicity of polystyrene nanoplastics (PSNP) and clothianidin on collembolans and enchytraeids

This study assessed the chronic toxicity of polystyrene nanoplastics (PSNP) and clothianidin (CLO), individually and as a binary mixture, on enchytraeids Enchytraeus crypticus and collembolans Folsomia candida. The assays were performed according to ISO protocols, using a tropical soil (Entisol) as the test substrate. The mixture assays were performed using equitoxic concentration, standardized according to the preliminary EC₅₀ values for each contaminant (1 Toxic Unit = reproductive EC₅₀ average), and the toxicological interactions were assessed through the Abbott model. Individually, CLO was much more toxic than PSNP (LOEC-based) to both enchytraeids (CLO = 2.67 mg kg^-1, PSNP = 128 mg kg^-1) and collembolans (CLO = 0.02 mg kg^-1, PSNP = 56.25 mg kg^-1). Collembolans were more sensitive to both contaminants compared to enchytraeids, but the difference in sensitivity was higher for CLO (> 100 times) than for PSNP (2.3 times). The binary mixtures showed additive effects of PSNP + CLO for both species at all mixture concentrations. For collembolans, concentrations that caused no significant effects individually (18.75 mg PSNP kg^-1 and 0.01 mg CLO kg^-1) induced significant negative effects when in mixture, indicating that the co-occurrence of individually non-toxic concentrations might not be safe for collembolans.