Lake sediment microbial populations are key participants in biogeochemical processes and form the base of many food webs.  The actions of microbial populations often influence the lake system chemistry, and therefore have direct impacts on lake system stability and on the macroorganisms that inhabit the lake.  However, little information is currently available about the impact of metal contamination to the microbial populations, particularly in anaerobic freshwater environments.  We are studying the interaction between metal contaminants and bacterial communities in freshwater lake sediments in a collaborative project with Jean-François Gaillard of Northwestern University and his students Amy Dahl, Sam Webb, and Melissa Nolan (now at an analytical lab in Henderson, NV).  Our research is focused on Lake DePue, Illinois, USA.  Heavy metals migrated to Lake DePue from an historic zinc smelting operation in the adjacent town of DePue, Illinois.

To conduct our research we have collected samples from the lake along a heavy metal gradient.  Samples have been collected simultaneously for molecular work at the Stahl laboratory, and for geochemical work in the Gaillard laboratory.

We have found a dramatic decrease in microbial biomass with increased metal concentration.  Correlation studies have found that this decrease was best explained by the dissolved concentrations of zinc and arsenic, implicating these two species as the controlling species in this lake sediment system.  Current studies are underway to determine to identify the microbial populations associated with this decreased biomass.  This information will provide a first approximation of the potential community level functional impacts.

Additionally, we are specifically working to more fully characterize the sulfate-reducing bacteria (SRB) in these lake sediments.  There is ample evidence documenting the capacity of this group to reduce a variety of toxic and non-toxic metals (e.g., uranium, selenium, iron, technetium, arsenate, chromium), generally converting them to less mobile forms.  An end product of their metabolism (sulfide) is well known to immobilize many toxic metals as insoluble metal sulfides.  To do this we have measured the sulfate reduction rates in the sediments, and will use molecular techniques to identify important SRB populations from different locations in the lake.

The information obtained from this study will be crucial for designing bioremedial strategies not only for this site, but also at other metal contaminated sites.  Additionally, by focusing on the ecology of a population not often considered in environmental toxicological field studies or in environmental regulations, this information may help to identify appropriate contaminant mitigation levels.

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