Superfund Research Program
Development of a Biomarker to Detect Arsenic in Aquatic Ecosystems
Arsenic is a widespread environmental contaminant that enters rivers, streams and lakes from both natural and anthropogenic sources. Weathering of arsenic-containing rocks, metal processing, burning of fossil fuels, and using arsenic pesticides all contribute to arsenic contamination in the country's freshwater systems. This contamination is a source of concern because of arsenic's potent toxicity.
At high levels, arsenic can be lethal to many freshwater organisms. At the lower concentrations typical of most contaminated aquatic ecosystems, arsenic produces more subtle effects that can nonetheless threaten the survival of aquatic life. Relatively small amounts of arsenic also pose cancer and other health risks to humans. Thus, there is a need for ways to monitor low levels of this metal in aquatic environments.
Researchers at Dartmouth College have been developing a biomarker to detect arsenic stress in freshwater systems. In general, stress from the environment induces cells to increase production of proteins called "heat shock proteins." This biological response helps cells maintain their integrity when they encounter stressors such as chemical pollutants, high temperatures and ultraviolet light.
In the freshwater zooplankton Daphnia pulex researchers found that expression of heat shock protein 83 (HSP 83) is sensitive to sublethal levels of arsenic. Specifically, the method uses a molecular technique to measure the protein's mRNA in Daphnia pulex, providing for a reliable molecular indicator of arsenic exposure. The ultimate goal is to find a response that would provide early warning of arsenic stress in aquatic environments. As the Dartmouth studies show, the heat shock response may be a useful endpoint for detecting chemical stress in ecosystems.
To develop this biomarker, the Dartmouth researchers carried out experiments evaluating HSP 83 mRNA levels in cultured Daphnia pulex exposed to arsenic. The toxicities of two inorganic forms of arsenic - arsenite and arsenate - were compared because both are present in aquatic systems, yet their relative effects are not well understood. In addition, both the juvenile and adult life stages of Daphnia pulex were studied to determine whether arsenic-induced expression of HSP 83 varies with the age of this organism. Finally, the heat shock response of arsenic-exposed Daphnia pulex was compared to several demographic responses - namely, survival, reproduction and development - because these are known sensitive early indicators of chemical stress in ecosystems.
The researchers found that HSP 83 mRNA expression was significantly higher in arsenic-exposed organisms in comparison to controls and was sustained over relatively long-term arsenic exposures. Just as important, the heat shock protein response was well correlated with the demographic responses to arsenic that were investigated.
Interestingly, this study suggests that the heat shock response of Daphnia pulex varies by age. The study also identified differences in the toxicities of arsenite and arsenate that have not been previously described. Arsenite was slightly more toxic at lethal levels than arsenate, but arsenate was more toxic at sublethal levels as shown by reduced reproduction and increased resting egg production.
Altogether, these results show that measurement of HSP 83 is a promising biomarker of arsenic stress. Further research is being conducted to develop its future application to detect arsenic levels typical of many contaminated aquatic ecosystems.
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To learn more about this research, please refer to the following sources:
- Chen CY, Sillett KS, Folt CL, Whittemore S, Barchowsky A. 1999. Molecular and demographic measures of arsenic stress in D. pulex. Hydrobiologia 410:229-238.
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