Toxicity of brominated flame retardants in fish, with emphasis on endocrine effects and reproduction

Abstract

Abstract The abundant use of brominated flame retardants (BFRs) in modern polymers has over the passed decades resulted in contamination of the environment, and BFRs are increasingly found in fish. Laboratory studies have shown that a number of BFRs and BFR-metabolites can interfere with thyroid and (sex)steroid hormone function (endocrine disruption). The studies presented in this thesis investigate whether exposure to environmentally relevant BFRs is likely to cause endocrine disruption and failure of reproduction in fish. To address this question, we carried out long term exposure studies with the most frequently found BFRs: tetrabromobisphenol A (TBBPA), hexabromocyclododecane (HBCD), and a brominated diphenylether mixture (PentaBDE). Juvenile flounder (Platichthys flesus, a common flatfish in estuaries along the European coastal zone) and adult reproducing zebrafish (Danio rerio, a freshwater cyprinid) were used as test species. Flounders were exposed to TBBPA for 105 days via the water, and to HBCD and PBDE (78 and 101 days, respectively) via food and/or sediment. Zebrafish were exposed to the test substances via the water. Exposure of adults was for four weeks, followed by exposure of eggs and juveniles until six weeks after hatching, which enabled monitoring of reproduction and juvenile development (partial life-cycle approach). Exposed fish were examined macroscopically and histologically with emphasis on reproductive and endocrine organs. Plasma thyroid hormone (T3 and T4) concentrations were determined in all flounder studies and in zebrafish exposed to PentaBDE. Cytochrome P4501A (CYP1A) activity (EROD) was determined in livers from flounder as indication for dioxin-like effects. Activities of the steroidogenic enzyme CYP19 (aromatase) were determined in flounder gonads, and production of the yolk precursor protein vitellogenin (VTG) was determined in plasma from flounder exposed to TBBPA. Concentrations of BFRs in tissue from exposed fish were evaluated to provide a dose background for risk assessment. All exposures resulted in a linear increase of internal BFR concentrations with exposure concentrations. Whereas the concentrations in zebrafish were generally higher, the range in flounder included environmentally relevant concentrations. Exposure of flounder to TBBPA resulted in an increase in the concentration of thyroid hormone T4 in plasma and a mild increase of aromatase activity in testes. In zebrafish, egg production and juvenile survival were reduced. Significant effects occurred at internal concentrations that were at least 10 times the highest levels observed in fish in the environment. Exposure to HBCD did not result in adverse effects in flounder or zebrafish (reported elsewhere). Exposure to a mixture of PBDEs resulted in juvenile mortality in zebrafish at the highest PBDE concentrations, and a mild decrease of aromatase activity in ovaries, and plasma T4 concentrations in flounder. Dose-response modeling indicates that T4 concentrations and CYP19 activities could be affected at internal concentrations that have in exceptional cases been observed in fresh water fish in the wild. However, concentrations in estuarine and marine fish are usually around two orders of magnitude lower indicating a limited risk for estuarine and marine fish. In general, the studies presented in this thesis show minimal indications for endocrine effects of exposure to BFRs in fish at concentrations observed in the environment.