Roles of bio-based microplastics in modulating the toxic effects of the herbicide metolachlor on the South American native species Palaemon argentinus: Single and co-exposure effects.

Microplastics (MPs), including those from bio-based plastics (BBPs), are increasingly detected in aquatic ecosystems, raising concerns about their role in modifying pollutant toxicity. This study investigated environmentally relevant concentrations of polylactic acid microplastics (PLA-MPs) on the South American native shrimp Palaemon argentinus, evaluating single and co-exposure scenarios with metolachlor (MET) over a 7-day bioassay. Characterization of PLA-MPs revealed a fragment shape with typical PLA spectroscopic pattern and an elemental composition of >80 % C and ∼15 % O. Both PLA-MPs and MET accumulated in the shrimp, causing oxidative stress, neurotoxicity, and tissue damage. Significant inhibition of acetylcholinesterase in shrimp exposed to PLA-MPs suggests impaired locomotion and behavior, which may affect population dynamics and ecosystem function. The MET adsorption on PLA-MPs was confirmed under tested conditions. The co-exposure increased MET prevalence in the cephalothorax, indicating the role of BBP particles as pesticide transporters in aquatic ecosystems. In addition, calculated bioaccumulation factors suggested a minimal trophic transfer of MET under the tested conditions. The integrated biomarker response revealed co-exposure stress levels intermediate between MET exposure and PLA-MPs treatment, suggesting partial antagonistic interactions. Reduced cytoplasmic MET bioavailability via PLA-MP adsorption was hypothesized as the underlying mechanism. These results indicate complex MP-chemical interactions, with possible antagonistic effects at the biochemical and tissue levels, and highlight the need for further research on the ecotoxicity of BBPs. Greater efforts are needed to elucidate the role of plastic additives in toxicity and the role of MPs in the bioaccumulation processes of pesticides. This would be useful to assess the environmental impact of BBPs and determine their viability as a sustainable alternative to conventional plastics.