Are alternatives to polyethylene less toxic than conventional polyethylene? A case study of MP toxicity on Scrobicularia plana using an Integrative Biomarker Response index (IBR-T)

Microplastics (MP) are now widely contaminating multiple ecosystems. To mitigate this pollution, the development of plastic alternatives is being promoted. However, scientific data on the absence of toxicity of these alternatives is still lacking. The present study assesses the impact of petroleum-based fragmentable MP, oxo-polyethylene (oxoPE), and biobased biodegradable MP, polylactic acid (PLA), on the bivalve Scrobicularia plana. Clams were exposed for 21 days to environmental concentrations (0.008, 10 and 100 μg L^-1) of both PLA and oxoPE MP and impacts were evaluated from individual to molecular levels. The effects of alternative MP were then compared with polyethylene MP ones, resulted from a previous study. An Integrative Biomarker Response - Threshold index (IBR-T) was calculated to assess the differences in toxicity between alternative and conventional plastics. An impact of PLA and oxoPE MP was shown on energy reserves, with a reduction in glycogen concentration in organisms exposed to the highest concentration (100 μg L^-1). A decrease in burrowing was also reported in S. plana exposed to 0.008 and 100 µg L^-1 of PLA MP. At cellular and molecular levels, catalase (CAT) and glutathione-S-transferases (GST) activities decreased after exposure to PLA MP (10 and 100 μg L^-1 respectively) suggesting a disruption of antioxidant and detoxication systems, while no impact was noted on DNA damage, whatever the biodegradable MP. Analysis using IBR-T revealed greater effects of PLA (100 μg L^-1) on S. plana physiology compared to the different types of PE MP tested, while oxoPE MP showed limited toxicity. These findings support the importance of assessing the toxic effects of biodegradable and fragmentable MP to evaluate whether they can really be considered as a reliable alternative to conventional plastics.