The combined toxic effects of long-term exposure to environmentally relevant concentrations of imidacloprid and chromium on Xenopus laevis tadpoles: Growth, oxidative stress, and molecular mechanisms

Imidacloprid (IMI) and chromium (Cr), as widely co-occurring pesticide and heavy metal pollutants in freshwater ecosystems, pose potential threats to non-target aquatic organisms. Despite their frequent detection in the environment, systematic investigations into their combined toxicity mechanisms remain limited, particularly in ecologically significant amphibians. Therefore, in this study, the mechanisms of single and combined toxicity of IMI and Cr in Xenopus laevis tadpoles were investigated at environmentally relevant concentrations (41.4 μg/L IMI; 54.6 μg/L Cr). The results showed that the exposure to Cr alone and in combination with IMI significantly inhibited growth and induced oxidative stress in tadpoles. Furthermore, transcriptomic and metabolomic analyses demonstrated that the Cr-treated group exhibited a greater number of differentially expressed genes (DEGs) and differentially expressed metabolites than the IMI-treated group. Meanwhile, the group treated with both IMI and Cr demonstrated the highest number of DEGs and differentially expressed metabolites when compared with the control group. Joint analysis showed that these DEGs and differentially expressed metabolites mainly disrupted amino acid metabolism, carbohydrate metabolism, purine metabolism pathways and FoxO signalling pathways in tadpoles. In summary, exposure to IMI and Cr, either individually or in combination, adversely affected the growth and development, gene expression and metabolite profiles of Xenopus laevis tadpoles. These findings provide crucial toxicological evidence for assessing the ecological risks of contamination by multiple neonicotinoid pesticide and heavy metal contamination in amphibian populations.