Mercury demethylation in waterbird livers: Dose–response thresholds and differences among species

Authors

  • Collin A. Eagles-Smith,

    Corresponding author
    1. Davis Field Station, Western Ecological Research Center, U.S. Geological Survey, One Shields Avenue, University of California, Davis, California 95616
    • Davis Field Station, Western Ecological Research Center, U.S. Geological Survey, One Shields Avenue, University of California, Davis, California 95616
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  • Joshua T. Ackerman,

    1. Davis Field Station, Western Ecological Research Center, U.S. Geological Survey, One Shields Avenue, University of California, Davis, California 95616
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  • Julie Yee,

    1. Western Ecological Research Center, U.S. Geological Survey, 3020 State University Drive East, Sacramento, California 95819
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  • Terrence L. Adelsbach

    1. Environmental Contaminants Division, U.S. Fish and Wildlife Service, 2800 Cottage Way, Suite W-2605, Sacramento, California 95825
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  • The use of trade, product, or firm names in the publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Abstract

We assessed methylmercury (MeHg) demethylation in the livers of adults and chicks of four waterbird species that commonly breed in San Francisco Bay: American avocets, black-necked stilts, Caspian terns, and Forster's terns. In adults (all species combined), we found strong evidence for a threshold model where MeHg demethylation occurred above a hepatic total mercury concentration threshold of 8.51 ± 0.93 μg/g dry weight, and there was a strong decline in %MeHg values as total mercury (THg) concentrations increased above 8.51 μg/g dry weight. Conversely, there was no evidence for a demethylation threshold in chicks, and we found that %MeHg values declined linearly with increasing THg concentrations. For adults, we also found taxonomic differences in the demethylation responses, with avocets and stilts showing a higher demethylation rate than that of terns when concentrations exceeded the threshold, whereas terns had a lower demethylation threshold (7.48 ± 1.48 μg/g dry wt) than that of avocets and stilts (9.91 ± 1.29 μg/g dry wt). Finally, we assessed the role of selenium (Se) in the demethylation process. Selenium concentrations were positively correlated with inorganic Hg in livers of birds above the demethylation threshold but not below. This suggests that Se may act as a binding site for demethylated Hg and may reduce the potential for secondary toxicity. Our findings indicate that waterbirds demethylate mercury in their livers if exposure exceeds a threshold value and suggest that taxonomic differences in demethylation ability may be an important factor in evaluating species-specific risk to MeHg exposure. Further, we provide strong evidence for a threshold of approximately 8.5 μg/g dry weight of THg in the liver where demethylation is initiated.

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