Superfund Research Program
An Integrated Approach to Assess Sediment Toxicity
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Background: Sediments are the recipients of most organic and inorganic contaminants released into the environment. They are often contaminated with mixtures of polycyclic aromatic hydrocarbons (PAHs), as well as polychlorinated biphenyls (PCBs), pesticides, heavy metals, and other compounds. While there are extensive data to confirm that PAHs and other sediment contaminants are toxic to aquatic receptors, information about the bioavailability and genotoxicity of complex mixtures to aquatic organisms is limited.
The results of chemical analysis generally do not predict toxicity in a biological system, particularly for complex pollutant mixtures such as PAHs or PCBs. Dr. K.C. Donnelly, Texas A&M University SBRP, believes that a battery of biomarkers provides a valuable tool that can adequately characterize subtle, sublethal, or genotoxic effects of chemical mixtures. Much like different instruments or detectors can be used to detect different classes of chemicals, a battery of biological tests can be used to detect a range of toxic or genotoxic effects. A collaboration between six university-based programs (Baylor, Duke, Michigan State, UC-Davis, UC-San Diego and Texas A&M) resulted in the formation of a National Bioassay Network to develop a protocol for analysis of sediments. The goal of the National Bioassay Network is to design an integrated biomonitoring approach using chemical analyses and a suite of biomarkers to characterize the bioavailability and genotoxicity of complex chemical mixtures in sediments. In collaboration with Dr. Bruce Duncan (USEPA Region 10), the Bioassay Network conducted a monitoring study using caged Coho Salmon in a freshwater lake near a PAH-contaminated site in Region 10.
Advances: Four approaches have been used separately to assess exposure and genotoxicity of chemical mixtures in aquatic organisms:
- In situ studies to monitor body burden of various contaminants in caged fish.
- Quantification of erythrocyte micronuclei, which are small DNA-containing bodies found near the cell nucleus as a consequence of chromosome breakage and spindle dysfunction.
- Detection of DNA changes using flow cytometry (FCM), which simultaneously assays numerous cellular and molecular endpoints.
- Quantification of formation of DNA adducts, a key process in early carcinogenesis, using 32P-postlabeling.
Dr. Donnelly's group used all four measures, along with chemical analyses, in a field study designed to mimic the exposure scenario experienced by migratory juvenile Coho Salmon. Caged salmon were suspended in the water column at sampling stations in a freshwater lake contaminated by a former manufactured gas plant (MGP), or in a reference (control) lake. USEPA SCUBA divers collected sediment samples at each station the day before placement of salmon cages. They also collected lake water samples at each sediment station at the beginning and end of the one week long fish exposure study.
The researchers analyzed the sediment and water samples for the seven PAHs identified by EPA as probable human carcinogens. They also measured liver PAH concentrations to assess PAH uptake and bioavailability from sediments and lake water, and to assess correlations between liver PAH concentrations and in vivo biomarker responses.
Briefly, they found that:
- The test locations all had levels of PAH contamination significantly above typical anthropogenic levels, in proportions expected at sites impacted by MGPs.
- Liver PAH concentrations showed no correlation to concentrations at cage locations, supporting earlier studies that liver PAH concentration is not a good biomarker of exposure or effect (possibly due to rapid metabolism of PAHs in fish liver).
- Erythrocyte micronucleus counts did not show a dose-response relationship in the Coho Salmon. There was no difference in micronuclei between the hatchery fish, reference lake fish, and fish at any location in the MGP site lake.
- FCM data revealed a clear trend in genotoxic response that corresponded with the PAH concentration gradient observed in sediments.
- 32P-postlabeling assays showed a dose-response effect, suggesting there was an increase in salmon hepatic DNA adducts from exposure to MGP site lake PAHs. Liver adducts were found to be a more sensitive measurement of exposure than gill adducts (which is unusual for fish). The 32P-postlabeling data revealed a DNA adduct response gradient that corresponds to the sediment, but not water column, PAH concentration gradient.
This exposure scenario study demonstrated that PAHs in sediments and lake water are bioavailable and can have a genotoxic impact on migratory juvenile Coho Salmon.
Significance: When incorporated into a well-designed in situ biomonitoring plan, the biomarkers used in this research appear to provide sensitive measurements of the bioavailability and genotoxicity of PAH-contaminated sediments to aquatic species. Such biomarkers of exposure and effect can be used to establish quantitative correlations of the cause and effect relationship of a toxicant at any level of biological organization, from an individual organism up to a population or ecosystem. In situ biomonitoring has a great potential for linking biomarker data with community and ecosystem level responses.
Biomarkers respond to toxicant exposures in a time-dependent manner and have varying levels of longevity and stability. Thus, using multiple biomarkers of exposure and effect should improve the quality of the risk assessment. It may be useful to combine the measurements of genotoxicity (e.g., DNA adduct formation), as a molecular dosimeter, with an analysis of carcinogen metabolism, exposure dose, and the determination of tumor formation in order to provide insight into the dose-response relationship and the mechanisms involved in chemical carcinogenesis.
For More Information Contact:
Kirby C Donnelly
Texas A&M University
Environmental & Occupational Health
The School of Rural Public Health Bldg.
College Station, Texas 77843-1266
To learn more about this research, please refer to the following sources:
- Barbee GC, Barich J, Duncan B, Bickham JW, Matson CW, Hintze CJ, Autenrieth RL, Zhou GD, McDonald TJ, Cizmas LH, Norton D, Donnelly KC. 2008. In situ biomonitoring of PAH-contaminated sediments using juvenile coho salmon (Oncorhynchus kisutch). Ecotoxicol Environ Saf 71(2):454-464. doi:10.1016/j.ecoenv.2008.01.001 PMID:18304636
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