An Analysis of the Genotoxicity of Mining Effluent in Toad Tadpoles (Bufo boreas)

Abstract

Amphibian populations are declining across the world. Implicated in this decline have been environmental pollutants and genotoxic agents such as pesticides, fertilizers, UV radiation, sewage contaminants, emissions of internal combustion engines, landfill run-offs, and industrial effluents. In Montana, abandoned mines have been shown to be a source of mine tailings in ponds and streams, resulting in low survival rates of certain frogs and toads. I examined survivorship, growth, and DNA fragmentation in Bufo boreas tadpoles after exposure to mining sediment at different pH levels. I used an alkaline single-cell gel (SCG) “comet” assay to quantify DNA fragmentation. This technique detects DNA fragments which, when electrophoresed, migrate from the nucleus of erythrocytes forming a “comet-with-tail.” When exposed to mining sediment, B. boreas showed significantly lower rates of survival and lower rates of growth compared to those exposed to control sediments (sand). In addition, I found a statistically significant increase in “comet” length-to-width ratios from 1.06 for those maintained in sand to 1.34 for those maintained in mining sediment. Moreover, in mining sediment treatments, the average length-to-width ratio was 1.29 for those at pH 5.0 compared to 1.39 for those at pH 7.0, which was also statistically significant. There was no difference in length-to-width ratio for different pHs in sand treatments. The results of the SCG assay indicate that this technique is sensitive to DNA damage resulting from mining sediment and that it may be used as an indicator of the genotoxicity of such environmental pollutants. The results also suggest that DNA damage could be a reason for declining amphibian populations in Montana ponds and streams that receive mine tailings.