The single and combined effects of deltamethrin and polyethylene microplastics on the development and biochemical responses of Xenopus laevis in early life stages.

The co-occurrence of microplastics and pesticides in freshwater ecosystems raises concerns for aquatic organisms. However, their combined effects remain poorly understood. In this study, the single and combined effects of deltamethrin (DEL) and polyethylene microplastics (PE-MPs) were investigated in the early life stages of Xenopus laevis. Embryonic developmental toxicity was evaluated under standardized Frog Embryo Teratogenesis Assay–Xenopus (FETAX) conditions in embryos, and sublethal biochemical responses were also assessed in embryos and tadpoles. In the first phase, embryos were exposed for 96 h to a concentration range of DEL (3.125–6,400 µg/L) and PE-MPs (50–1,000 mg/L) to estimate the median lethal concentration (LC50) and the median effective concentration for malformation (EC50), as well as the teratogenic index (TI). In the second phase, assays were performed at DEL 2.72 and 13.6 µg/L (alone or combined with 100 mg/L PE-MPs) and PE-MPs 50 and 250 mg/L alone, assessing malformation and lethality in embryos and enzymatic biomarkers in both embryos and tadpoles. DEL caused significant embryotoxicity, with 96 h LC50 and EC50 values of 68.0 µg/L and 7.58 µg/L, respectively, and a TI of 8.97. PE-MPs alone did not induce lethality or malformations but caused biochemical alterations. Combined exposures produced additive (glutathione S-transferase, GST; carboxylesterase, CaE) and antagonistic (catalase, CAT; acetylcholinesterase, AChE) patterns; notably, CAT showed a synergistic pattern in embryos based on interaction index analysis. In tadpoles, 250 mg/L PE-MP concentration inhibited glutathione reductase (GR) and AChE activity, suggesting life stage–dependent sensitivity. These findings underscore the complexity of mixture toxicity and emphasize the need for combined exposure assessments in amphibian ecotoxicology.