Stimulating microbial reduction of soluble U(VI) to insoluble U(IV) shows 
promise as a strategy for immobilizing uranium in uranium-contaminated 
subsurface environments. In order to learn more about which microorganisms might 
be involved in U(VI) reduction in situ, the changes in the microbial community 
when U(VI) reduction was stimulated with the addition of acetate were monitored 
in sediments from three different uranium-contaminated sites in the floodplain 
of the San Juan River in Shiprock, N.Mex. In all three sediments U(VI) reduction 
was accompanied by concurrent Fe(III) reduction and a dramatic enrichment of 
microorganisms in the family Geobacteraceae, which are known U(VI)- and 
Fe(III)-reducing microorganisms. At the point when U(VI) reduction and Fe(III) 
reduction were nearing completion, Geobacteraceae accounted for ca. 40% of the 
16S ribosomal DNA (rDNA) sequences recovered from the sediments with bacterial 
PCR primers, whereas Geobacteraceae accounted for fewer than 5% of the 16S rDNA 
sequences in control sediments that were not amended with acetate and in which 
U(VI) and Fe(III) reduction were not stimulated. Between 55 and 65% of these 
Geobacteraceae sequences were most similar to sequences from Desulfuromonas 
species, with the remainder being most closely related to Geobacter species. 
Quantitative analysis of Geobacteraceae sequences with most-probable-number PCR 
and TaqMan analyses indicated that the number of Geobacteraceae sequences 
increased from 2 to 4 orders of magnitude over the course of U(VI) and Fe(III) 
reduction in the acetate-amended sediments from the three sites. No increase in 
Geobacteraceae sequences was observed in control sediments. In contrast to the 
predominance of Geobacteraceae sequences, no sequences related to other known 
Fe(III)-reducing microorganisms were detected in sediments. These results 
compare favorably with an increasing number of studies which have demonstrated 
that Geobacteraceae are important components of the microbial community in a 
diversity of subsurface environments in which Fe(III) reduction is an important 
process. The combination of these results with the finding that U(VI) reduction 
takes place during Fe(III) reduction and prior to sulfate reduction suggests 
that Geobacteraceae will be responsible for much of the Fe(III) and U(VI) 
reduction during uranium bioremediation in these sediments.