Polylactic acid versus polypropylene microplastics: ecotoxicological effects on Gammarus aequicauda using a multi-level approach

This study compared the effects of microplastics (MPs) originating from a fossil-based polymer (polypropylene, PP) and a bioplastic (polylactic acid, PLA) on the amphipod Gammarus aequicauda. Microplastics (38-220 μm) derived from commercial disposable products were used to evaluate acute toxicity, yielding 96-h LC₅₀ values of >100 mg/L for PLA and 34.71 mg/L for PP. Both MP types were further tested under sub-lethal conditions (5 mg/L) over a 60-day exposure period to assess impacts on growth and reproduction. Chronic exposure resulted in a significant reduction in growth in both PLA- and PP-treated groups beginning at day 45, with PP-exposed individuals also showing the lowest survival rates. Additionally, PLA and PP exposure prolonged embryonic development compared with controls. Reproductive impairment was most pronounced in the PP treatment, which exhibited significant decreases in the number of ovigerous females and in total offspring, alongside an increase in aborted eggs. Seven stress-related genes were identified for the first time in G. aequicauda, and their expression was quantified by RT-qPCR following 15, 30, and 45 days of exposure. Gene expression profiles showed complex, time-dependent responses, with both MP types inducing largely similar transcriptional patterns by day 45, characterized by up-regulation of most target genes. Overall, the results demonstrate that commercially derived PP and PLA microplastics can elicit physiological and reproductive stress in G. aequicauda, underscoring their potential ecological risk.