Perchlorate toxicity in fish: trophic transfer, developmental windows, and histological biomarkers

Abstract

The perchlorate anion is an oxygenated chlorine compound often used by the military as an oxidizer in solid rocket propellant and by industry in numerous other applications. It is a known endocrine disruptor and competitively inhibits the uptake of iodide into thyroid tissue in a concentration dependent manner, effectively reducing production of thyroid hormones. Perchlorate is highly water soluble, kinetically inert and has low adsorption tendency, making it a persistent and mobile aquatic contaminant. Perchlorate has been detected in drinking water sources throughout the United States and is present in many commercially available food and drink products. The objective of this dissertation was to better understand the bioaccumulation, toxicodynamics and morphological changes caused by perchlorate exposure utilizing northern pike (Esox lucius) and the threespine stickleback species (Gasterosteus aculeatus) model. The first research chapter (chapter 2) examines the potential for bioaccumulation and trophic transfer of perchlorate in northern pike exposed to 10 and 100 mg/L perchlorate via ambient water and food. As expected, perchlorate does not biomagnify, but does concentrate in the gastrointestinal tract tissue of pike. At the lower exposure concentration (10 mg/L) for combined water and food exposure, greater than additive tissue concentrations were detected indicating the need for regulatory testing to consider not only contaminated water but the associated contaminated food in the contaminated ecosystem (e.g., some studies provide food that is not contaminated while fish are exposed only to contaminated water). The following two chapters (chapters 3 and 4) examine the morphologic effects of exposure timing and duration on developing stickleback utilizing a unique upshift/downshift exposure regime to determine if critical developmental windows of perchlorate sensitivity exist for two exposure concentrations (30 and 100 mg/L). In chapter three, gross morphology (body size and skeletal armor traits) were quantified in sexually mature fish. The results demonstrate that growth can be suppressed with continuous exposure beginning within the first 14 days post fertilization (dpf). Skeletal armor traits responded variably to perchlorate exposure, with some increasing, some decreasing and others developing normally. The traits measured in this study (excluding standard length) were not sensitive to the timing of exposure (i.e., no definitive critical windows), but responses were concentration-dependent. Chapter four examines thyroid tissue histomorphological endpoints, sex ratio, and gonadal maturity in stickleback exposed to perchlorate (30 and 100 mg/L) for varying times and durations. Thyroid tissue responded with increased follicle hyperplasia, decreased area of colloid, increased angiogenesis, and follicle cell hypertrophy. Within the first 42 dpf, a critical window emerged for follicle hyperplasia and area of colloid. Stickleback rescued (removed from perchlorate contaminated water) anytime up to 305 dpf recovered from follicle hyperplasia and reduced colloid area. Angiogenesis increased in fish exposed to perchlorate and a critical window was detected for fish exposed to 30 mg/L anytime between 7 and 154 dpf. Recovery from angiogenesis did not occur. The ratio of males to females and gonadal development were altered in stickleback exposed to perchlorate continuously beginning within the first 14 dpf. Sex ratio was skewed toward males in a concentration-dependent manner, which could be due either to a masculinizing effect of perchlorate on sexual development or to differential survival of the sexes. Additionally, gonadal maturation was delayed for both sexes as the proportion of late stage testes and oocytes decreased in perchlorate exposed fish. Overall, these results demonstrate that the effects of perchlorate on aquatic vertebrates are complex. Movement within and between organisms is complicated due to the iodide concentrating mechanism of some tissues. Abnormalities of growth and skeletal armor traits are caused by perchlorate exposure and both are important to the survival and reproductive success of stickleback. In addition, as expected, the histomorphology of thyroid tissue is a responsive biomarker of perchlorate exposure in stickleback. Critical windows of sensitivity to perchlorate exist during early development and future research should scrutinize the biochemical mechanisms driving changes in thyroid condition and abnormal development, particularly for reproductive endpoints.