In order to elucidate the potential mechanisms of U(VI) reduction for the 
optimization of bioremediation strategies, the structure-function relationships 
of microbial communities were investigated in microcosms of subsurface materials 
cocontaminated with radionuclides and nitrate. A polyphasic approach was used to 
assess the functional diversity of microbial populations likely to catalyze 
electron flow under conditions proposed for in situ uranium bioremediation. The 
addition of ethanol and glucose as supplemental electron donors stimulated 
microbial nitrate and Fe(III) reduction as the predominant terminal 
electron-accepting processes (TEAPs). U(VI), Fe(III), and sulfate reduction 
overlapped in the glucose treatment, whereas U(VI) reduction was concurrent with 
sulfate reduction but preceded Fe(III) reduction in the ethanol treatments. 
Phyllosilicate clays were shown to be the major source of Fe(III) for microbial 
respiration by using variable-temperature Mössbauer spectroscopy. Nitrate- and 
Fe(III)-reducing bacteria (FeRB) were abundant throughout the shifts in TEAPs 
observed in biostimulated microcosms and were affiliated with the genera 
Geobacter, Tolumonas, Clostridium, Arthrobacter, Dechloromonas, and Pseudomonas. 
Up to two orders of magnitude higher counts of FeRB and enhanced U(VI) removal 
were observed in ethanol-amended treatments compared to the results in 
glucose-amended treatments. Quantification of citrate synthase (gltA) levels 
demonstrated a stimulation of Geobacteraceae activity during metal reduction in 
carbon-amended microcosms, with the highest expression observed in the glucose 
treatment. Phylogenetic analysis indicated that the active FeRB share high 
sequence identity with Geobacteraceae members cultivated from contaminated 
subsurface environments. Our results show that the functional diversity of 
populations capable of U(VI) reduction is dependent upon the choice of electron 
donor.