Exposure to Synthetic Antioxidants Disrupt Early Development in the Frogs Silurana Tropicalis and Lithobates Pipiens

Abstract

Many chemicals in commonly used household and industrial products are being released into the environment yet their toxic acute and chronic effects on humans and wildlife are not well understood. Some of these chemicals behave as endocrine disruptors (EDCs), altering development in many wildlife species including amphibians, which are sensitive to these compounds. Producing toxicological information is the first step to evaluate the effects these chemicals may have on the environment and wildlife. Two synthetic phenolic antioxidants, 4,4’-thiobis(6-t-butyl-m-cresol) (CAS 96-69-5; TBBC) and 2,4-di-tert-butylphenol (CAS 96-76-4; DTBP) were chosen to evaluate their potential toxicity and developmental disruption on two amphibian species. Both, TBBC and DTBP, are present in many common products such as rubber and plastic products, they are listed on the National Chemicals Management Plan as potential EDCs and they are potentially toxic to aquatic life. Furthermore, given that their chemical structure resembles the thyroid hormones (TH), the present thesis hypothesis is that they interfere with TH-dependent developmental processes on the frogs Silurana tropicalis and Lithobates pipiens and affect their early stage development. To test this hypothesis, S. tropicalis embryos and L. pipiens tadpoles were exposed at the Nieuwkoop-Faber (NF) stage 9-10 and Gosner (G) stage 24-25, respectively, to these synthetic antioxidants’ concentrations at several concentrations (0, 0.025, 0.05, 0.075, 0.1, 0.2 and 0.4 mg/l). The FETAX protocol was followed to determine the 96h lethal concentrations and sublethal effects, evaluating survival, growth and development. A chronic exposure on S. tropicalis was also conducted exposing tadpoles from stage NF47-48 to three sublethal TBBC concentrations (0, 0.002, 0.1 and 5 µg/l) for 7 weeks to evaluate effects on growth and metamorphosis. The TBBC 96h lethal and effective concentrations (LC50 and EC50) were 0.076 and 0.078 mg/l, respectively, for S. tropicalis. For L. pipiens exposed to TBBC the LC50 was 0.17 mg/l and for those exposed to DTBP the lethal concentration was 0.52 mg/l. Acute exposure to all TBBC concentrations affected S. tropicalis growth and was lethal at 0.2 and 0.4 mg/l. The TBBC compound also affected L. pipiens body size and was lethal at 0.4 mg/l. All L. pipiens tadpoles died at 0.8 mg/l of acute exposure to DTBP. Chronic exposure to sublethal concentrations of TBBC reduced body size by 8% at 5 µg/l and body mass by 17% at 0.002 µg/ when metamorphosis was completed. This study confirms these two synthetic antioxidants are toxic in vivo to these amphibian species. Also, TBBC induces malformations and inhibits tadpole growth at 0.025 mg/l after acute exposure and affects body mass of metamorphs at 0.002 µg/l after chronic exposure. These findings call for further investigations on how these synthetic antioxidants affect growth in S. tropicalis and L. pipiens.