Dietary uptake and toxicity of coal ash and selenium to larval Hyla versicolor

Abstract

In 2009, approximately 24 % of the estimated 92 million tons of coal ash (CA) produced in the U.S. was disposed of in aquatic settling basins. Amphibians are especially at risk of exposure to CA disposed in this manner, as they often breed in ash basins or habitat contaminated by basin effluent. Coal ash is a complex and variable mixture. Although trace elements make up a small percentage of the total mineral content, these CA constituents are of primary concern in the environment. Elevated selenium (Se) inputs to aquatic habitats from CA were associated with increased larval amphibian mortality and malformations. Selenium is an essential micronutrient with a narrow therapeutic concentration range and a propensity to biotransform and bioaccumulate in aquatic food chains. This research contrasts the toxicological effects of dietary exposures of CA, an organic Se-containing compound, selenomethionine (SeMet) and an inorganic Se-containing compound, selenium dioxide (SeO₂) to a larval amphibian (Hyla versicolor). Exposure to 50μg Se g⁻¹ wet mass (ww) nominal concentration of SeMet reduced larval metabolic rates, larval growth rates and was lethal to larvae by the conclusion of the 78 d study. The SeMet Low (5μg Se g⁻¹ ww nominal concentration) dose reduced the number of individuals to initiate metamorphosis, reduced size at metamorphosis, reduced survival to initiate and complete metamorphosis, and increased the frequency of malformations during metamorphosis. In contrast, there was little evidence of CA toxicity relative to control treatments. The SeO₂ High dose (50μg Se g⁻¹ ww nominal concentration) had comparable effects on survival, metamorphosis, and masses of recent metamorphs as the SeMet Low dose. Neither SeO₂ dose (50μg Se g⁻¹ ww or 5μg Se g⁻¹ ww nominal concentration) induced malformations or caused mortality during metamorphic climax. This research provides evidence of Se species-specific toxicity to a larval anuran. Results of this study indicate SeMet is more toxic and bioavailable to H. versicolor than SeO₂ and directly link 5μg Se g⁻¹ ww nominal SeMet exposures to rear limb malformations developed during metamorphosis.