Shifts in RelativeTissue §15N Values In Snowy Egret Nestlings with Dletaiy Mercuiy Exposure: A Marker for Increased Protein Degradation
PATRICIA L. SHAW-ALLEN, CHRISTOPHER S. ROMANEK, A. L. BRYAN, JR., HEATHER BRANT, AND CHARLES H. JAGOE
Department of Geology and Ecotoxicology, Remediation, and Risk Assessment Group, Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802
Shifts in tissue nitrogen isotope composition may be a more sensitive
general indicator of stress than measurement of h!gh-turnover defensive
biomolecules such as metallothionein and glutathlone. As a physical resource
transmitted along the trophic web, perturbations in protein nitrogen metabolism
may also help resolve issues concerning the effects of contaminants on
organisms and their consequential hierarchical linkages in ecotoxicology.
Snowy egret nestlings (Egretta thula) fed mercury-contaminated
diets of constant nitrogen isotope composition exhibited increased relative
015N values in whole liver (p = 0.0011) and the acid
soluble fraction (ASF) of the liver (p = 0.0005) when compared
to nestlings fed a reference diet. When nitrogen isotope data were adjusted
for the source term of the diet, liver mercury concentrations corresponded
with both whole liver relative 15N enrichment (r2-0.79,
slope 0.009, p < 0.0001) and relative 15N enrichment
in the acid-soluble fraction of the liver (r2 = 0.85,
slope 0.026, p < 0.0001). Meanwhile, significant differences
were not observed in hepatic levels of the metal-binding peptides metallothionein
and glutathione despite a nearly 3-fold difference in liver mercury content.
Because increases in tissue 015N values result from increased
rates of protein breakdown relative to synthesis, we propose that the
increased relative liver 15N values reflect a shift in protein
metabolism. The relationship between ASF and mercury was signigicantly
stronger (p < 0.0001) than that for whole liver, suggesting
that the relationship is driven by an increase in bodily derived amino
acids in the acid-soluble, free amino acid pool.
SREL Reprint #2876
Shaw-Allen, P. L., C. S. Romanek, A. L. Bryan, Jr., H. A. Brant and C. H. Jagoe. 2005. Shifts in relative tissue §15N values in snowy egret nestlings with dietary mercury exposure: A marker for increased protein degradation. Environmental Science & Technology 39:4226-4233.
