Thallium (Tl) is a trace element with high toxicity to mammals. It belongs to a category of technology-critical elements that are key components for the new or development technologies. The anthropic processes of Tl entering the aquatic environment include mining, coal combustion and cement production that release significant amount of Tl. Tl is a priority pollutant suggested by the U.S. EPA and its environmental fate and hazardous risk to aquatic life and human health is gaining serious attention recently. Tl exists in the environment as monovalent Tl(I) and trivalent Tl(III) states. Tl(I) is thought to be more thermodynamically stable than Tl(III); however, recent studies indicate that Tl(III) may exist in surface water, and seems much more toxic than Tl(I) to aquatic invertebrates. However, environmental transformation between Tl(I) and Tl(II) and their toxicity to higher tropic levels of aquatic organisms such as fish remain unclear. In this study, we aim to understand the stability of Tl(I) and Tl(III) in water under environmental relevant conditions. The toxic effect of different Tl species and associated modes of toxic action are investigated using medaka fish (<i>Oryzias latipes</i>) as a model organism. Our results showed that Tl(I) appeared to induce higher acute lethality to medaka larvae than Tl(III) at the tested concentration ranges and water conditions. To reveal toxic mechanism of Tl, we compared the transcriptome in medaka larvae from Tl(I)-treated and control groups using next generation sequencing (NGS). Differential gene expression (DGE) profiles showed that Tl(I) significantly induced alteration in cholesterol synthesis, steroid hormone metabolism and phase I metabolism related genes. These Tl-induced transcriptional alteration was confirmed by qPCR analysis and related enzymatic assays. The correlation between Ti speciation and toxicity in water is under further investigation. Based on our results, we imply that Tl has significant ecotoxicological impact to the aquatic ecosystem.

Investigation of environmental fate and toxic mechanisms of monovalent and trivalent thallium

254th National Meeting (2017)

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