Thallium's Threat to Aquatic Life: Stage-Specific Toxicity in Zebrafish Embryos and Larvae

The escalating frequency of thallium (Tl) contamination incidents amplifies its environmental risk. However, the potential risk of Tl to aquatic organisms, especially across varying developmental stages, remains poorly understood. In this study, we employed zebrafish as a representative model organism and exposed zebrafish embryos and larvae at distinct developmental periods (specifically, 6 h postfertilization (hpf) and 72 hpf) to low concentrations of Tl(I) (0.25 and 0.50 mg/L). The exposure was performed for a short duration of 24 h, followed by a 96 h depuration period. Our results revealed that Tl(I) exerted disparate biological effects on zebrafish at different developmental stages. Embryos exhibited negligible uptake of Tl(I), whereas larvae showed a significant accumulation of Tl(I) and struggled with its rapid elimination. Notably, Tl(I) was able to permeate the blood-brain barrier, thereby posing a risk to the nervous system. Transcriptomic analysis indicated that Tl(I) triggered distinct toxicological pathways in embryos and larvae. It mainly interfered with metabolic processes in embryos, while in larvae, it mainly disrupted intracellular ion homeostasis, both consequently provoking neurotoxicity. This emphasizes that the multifaceted nature of Tl(I) toxicity depends on the developmental stages of the organism. This study clearly shows that the bioeffects of Tl are intricately related to the developmental stage of zebrafish, offering a valuable perspective for the pollutant toxicity assessment.