Environmentally relevant concentrations of thiamethoxam cause cytotoxic and genotoxic damage in bullfrog (Lithobates catesbeianus) tadpoles.

Neonicotinoids are emerging pollutants of major concern due to their widespread use and potential risks to biodiversity, yet their genetic and lethal toxicity, key parameters in environmental risk assessment, are still poorly explored in anurans. In this study, we evaluated the acute toxicity of thiamethoxam (96 h) in Lithobates catesbeianus tadpoles, determining lethal concentrations (LC10-96 h, 50-96 h, 90-96 h) and assessing subindividual effects, including genotoxicity (Comet assay), mutagenicity (Micronucleus test), and cytotoxicity, at environmentally relevant concentrations (lethal: 14.40-3516 µg a.i./L; subindividual: 9.4-59 µg a.i./L). The LC10-96 h, 50-96 h, 90-96 h were defined as 624.44, 1070.13, and 1833.03 µg a.i./L, confidence interval 95%, respectively. Subindividual exposure induced significant genotoxic effects at a concentration of 31 µg a.i./L, and mutagenic responses with an increased frequency of lobed nuclei, nuclear budding, and anucleated cells, especially at a concentration of 59 µg a.i./L. A positive correlation was observed between Comet assay parameters and nuclear abnormalities. Taken together, these findings demonstrate dose-dependent genomic damage and suggest links between distinct cellular alterations. Integrated with the lethal toxicity data, the results indicate that thiamethoxam disrupts essential cellular processes in amphibians, highlighting L. catesbeianus as a sensitive bioindicator, reinforcing the relevance of genotoxic and mutagenic biomarkers in assessing the impacts of neonicotinoids on aquatic ecosystems.