Biomarkers of Lead Exposure in the Freshwater Mussel Elliptio complanata For Assessing Transportation Related Impacts

Abstract

The first phase of this study involved examination of lead (Pb) concentrations in adult Eastern elliptio (Elliptio complanata) mussel tissue and sediment at 40 sites throughout North Carolina, ranging in average daily traffic count, land-use patterns and watershed characteristics. Data from these sites revealed an increased Pb concentration of about 2 μg/g in sediment downstream of bridge crossings compared to upstream for high traffic areas of >20,000 vehicle crossings/day (vc/d), as well as a significantly greater average Pb concentration in mussels from sites with traffic count >500 vc/d compared to sites with <500 vc/d. Two 28-d toxicity tests with Pb and adult Eastern elliptio mussels were conducted in the second phase of the study to compare the field-derived Pb bioaccumulation data and to examine sublethal biomarkers of Pb exposure and toxicity. Endpoints for the tests included mussel survival, metal accumulation, shell length and weight. For the first test, δ-aminolevulinic acid dehydratase (ALAD) inhibition was assessed as a biomarker of exposure and effect in mussel hemolymph. In the second test, Na+,K+-ATPase activity in mussel gill tissue was examined as a biomarker of exposure and effect, along with measurements of hemolymph ion concentrations. Pb was measured in mussel hemolymph in both tests at near-exposure levels, and at the greatest exposure concentration in the first test (396 μg/L), the hemolymph Pb concentration was 4.5 times the amount present in test water. In contrast, the greatest exposure concentration in the second test (458 μg/L), had hemolymph concentrations only 0.12 times the amount present in test water. This finding suggested a threshold effect concentration for Pb exposure and hemolymph concentrations. The average Pb concentration in mussel tissue was strongly correlated with Pb exposure concentration (R²= 0.99) at day 28 for both tests. The enzyme ALAD was not present in measurable concentrations in freshwater mussel hemolymph, gill or mantle tissue. The Na+,K+-ATPase activity was found to be a strong biomarker of Pb exposure in freshwater mussel gill tissue (~50% of total ATPase) using a K-free salt solution for detection, and was found to be negatively correlated with average tissue Pb concentration (R2 = 0.85) on day 28. Pb accumulated rapidly in mussel tissue during the first two weeks of the second exposure, with lesser accumulation during the final two weeks, with only the 121 µg/L treatment group changing significantly. Because Pb was being removed from the test water throughout the duration of exposure, I conclude that the mussels were approaching equilibrium with the aqueous environment and were eliminating the Pb in lysosomes and granulocytes through pseudo-feces. Although the major contribution of Pb to the environment from now-banned leaded gasoline is still evident in North Carolina, the concentrations measured in mussel tissue and sediment were not at levels which would cause immediate or short-term reductions in Eastern elliptio populations. Moreover, the sub-lethal effects of Pb on Na+,K+-ATPase activity, although a strong biomarker of exposure, appear not to be significantly effected at environmentally relevant concentrations of Pb. However, other species of freshwater mussels may be more sensitive to Pb contamination and further assessment is needed to verify the role Pb may be having in the overall declines in mussel species diversity being observed worldwide.