Additivity, biochemical impacts, and bioavailability of contaminant mixtures

Research into the biological effects of chemical mixtures has been sparse to date. This dissertation explores several aspects of mixture toxicology: mixtures of different chemical classes, additivity among contaminants sharing a similar mode of action, and how different forms of the same contaminant may affect bioavailability.; Juvenile chinook salmon (Oncorhynchus tshawytscha) were used to study the effect of estrogens and estrogen mimics on organophosphate pesticide toxicity. Organophosphates and many estrogen mimics are measurable in surface waters, and likely co-occur in the environment. Therefore, fish were aqueously exposed to the organophosphate pesticide chlorpyrifos and estradiol or the estrogen mimic nonylphenol. Nonylphenol significantly alleviated brain acetylcholinesterase inhibition due to pesticide toxicity, however this effect was not mediated through changes in hepatic CYP1A metabolism of the pesticide. Additionally, fish were exposed to chlorpyrifos and the estrogen mimics β-sitosterol, nonylphenol and ethylnylestradiol either singly or in mixture. Results indicate that the non-steroidal estrogens mitigate acetylcholinesterase inhibition, possibly acting additively. The mechanism for this effect is unclear: it is unrelated to traditional hormonal activity. Substrate competition for available metabolizing enzymes is currently the most plausible hypothesis.; Wild juvenile salmon and water samples from the Sacramento River, CA, USA were collected around the city of Sacramento to determine whether environmentally realistic contaminant concentrations impact fish. This area receives high loads of acetylcholinesterase-inhibiting pesticides, and outflow containing estrogens from an urban municipal wastewater facility. Caged fish were also placed above and below the municipal outfall to examine its impact on potential pesticide toxicity. Contrary to previous years, anticholinesterase pesticide concentrations in the river were quite low and did not produce any measurable acetylcholinesterase inhibition in wild or caged fish.; A novel wastewater treatment technology designed to remove selenium from agricultural tile drainage by bacterial reduction was also examined. Invertebrates in the treatment system were monitored, microcosm exposures conducted, and water selenium speciation determined. Selenium bioavailability is determined in part by the mixture of selenium species present, and it was substantially increased after treatment. Results suggest that a large pool of highly toxic and bioavailable organic selenium is created from the incoming selenate, probably due to algal assimilation.