Microbial Communities and Groundwater Chemistry in a Pristine Confined Aquifer

We consider how the structures of subsurface microbial communities from the regional Mahomet aquifer, a pristine water resource in east-central Illinois, vary with the inorganic chemical composition of groundwater there. We profiled attached and suspended microbial communities from 19 wells using terminal restriction fragment length polymorphism (T-RFLP) and compared the results using multivariate statistical analysis. The results show that among the analytes considered, community structure varies most strongly with sulfate concentration. We investigated phylogenetic variation among three wells with widely varying sulfate concentration by creating clone libraries of bacterial 16S rRNA genes.

The statistical analysis shows that variation with sulfate concentration results not from differences in the type or amount of sulfate reducers, as might be expected, but of the dissimilatory metal-reducing bacteria Geobacter, Geothrix, and Desulfuromonas. The populations of metal reducers seem to increase with decreasing sulfate concentration, but show little correlation with iron or manganese concentration. The phylogenetic analysis shows two clusters within the sulfate-reducing families Desulfobacteraceae and Desulfobulbaceae, one consisting of clones isolated from low-sulfate, and another from high-sulfate wells. Sulfate concentration appears to control the type but not the amount of sulfate reducers in the aquifer.

Regardless of sulfate concentration, however, clones of both metal reducers and sulfate reducers were found in the attached communities. This result lends support to the idea that the two groups coexist within the Mahomet, rather than being segregated into zones dominated by one or the other. The lack of correlation between some geochemical parameters and microbial community structures, such as iron content with iron reducers, suggests the need for caution when interpreting microbial community structure from groundwater composition alone.