Correlations between Arsenic in Maine Groundwater and Microbial Populations as Determined by Fluorescent In-Situ Hybridization

Arsenic is known to cause serious health effects when consumed in drinking water. In the state of Maine, approximately forty percent of the population relies on private groundwater wells for their drinking water. As many as 13% of the private groundwater wells may contain arsenic levels above the current EPA maximum contaminant level of 10 µg/L. Microorganisms can potentially contribute to arsenic release into groundwater through several mechanisms. Some can reduce arsenate to arsenite, which is more toxic and may be more mobile. Sulfurospirillum species NP4, which was isolated from well water, respires arsenate and could act in this way. Microorganisms can also act indirectly by reducing bedrock surface coatings, such as iron oxyhydroxides, that adsorb arsenic in the groundwater environment. The genus Geobacter contains many species that are capable of iron reduction that could play a role in the indirect release of arsenic into groundwater. Water samples from Northport, ME and the Branch Lake region of Ellsworth, ME, which both have elevated groundwater arsenic levels, have been enumerated using fluorescence in situ hybridization (FISH), to determine the percentage of the population that is NP4 and the percentage that are Geobacter species. Geobacter abundance correlates well with the total arsenic concentration indicating that indirect mechanisms could be important in releasing arsenic. NP4 appears to be reducing arsenate since its prevalence correlates well with arsenite, the end product of arsenate respiration