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The Active Bacterial Community in a Pristine Confined Aquifer
Citation:
Flynn, T. M., R. A. Sanford, J. W. SANTO-DOMINGO, N. J. Ashbolt, A. D. Levine, AND C. M. Bethke. The Active Bacterial Community in a Pristine Confined Aquifer. WATER RESOURCES RESEARCH. American Geophysical Union, Washington, DC, 48(W09510):1-10, (2012).
Impact/Purpose:
To inform the public.
Description:
This study of the active bacteria residing in a pristine confined aquifer provides unexpected insights into the ecology of iron-reducing and sulfate-reducing bacteria in the subsurface. At 18 wells in east-central Illinois, we trapped the microbes that attached to aquifer sediment and used molecular techniques to examine the composition of the bacterial community. We examined using multivariate statistics how the composition of bacterial communities varied among wells with respect to the chemical composition of the groundwater. We found groundwater at each well was considerably richer in ferrous iron than sulfide, indicating iron-reducing bacteria should by established criteria dominate the sulfate reducers. Molecular analyses of the bacterial 16S rRNA genes present, however, revealed significant populations of sulfate reducers colonized the aquifer sediment alongside iron reducers. As a result, the concentration of sulfate appears to be the key discriminant of bacterial communities in the Mahomet. In areas where groundwater contains a negligible amount of sulfate (<0.03 mM), bacterial populations closely related to the iron reducers Geobacter, Geothrix, and Desulfuromonas comprise about a third of the bacterial community while putative sulfate reducers of the genera Desulfobacter and Desulfobulbus comprise < 5% of the total. In high sulfate areas (> 0.2 mM) where sulfate reducers likely gain a competitive advantage, however, sequences belonging to these two functional groups are of nearly equal abundance. In contrast to sulfate, we observed no statistical relationship between the distribution of bacterial populations and the concentration of either ferrous iron or dissolved sulfide. These results call into question the validity of using the relative concentration of these two ions to predict the nature of bacterial activity in an aquifer. Sulfate reducers and iron reducers do not appear to be segregated into discrete zones in the aquifer through competitive exclusion. Instead, we find the two groups co-existing in the subsurface in a relationship that is at times either competitive or mutualistic, depending upon the geochemical environment.
