Limitations on the availability of Fe(III) as an electron acceptor are thought 
to play an important role in restricting the growth and activity of Geobacter 
species during bioremediation of contaminated subsurface environments, but the 
possibility that these organisms might also be limited in the subsurface by the 
availability of iron for assimilatory purposes was not previously considered 
because copious quantities of Fe(II) are produced as the result of Fe(III) 
reduction. Analysis of multiple Geobacteraceae genomes revealed the presence of 
a three-gene cluster consisting of homologues of two iron-dependent regulators, 
fur and dtxR (ideR), separated by a homologue of feoB, which encodes an Fe(II) 
uptake protein. This cluster appears to be conserved among members of the 
Geobacteraceae and was detected in several environments. Expression of the 
fur-feoB-ideR cluster decreased as Fe(II) concentrations increased in chemostat 
cultures. The number of Geobacteraceae feoB transcripts in groundwater samples 
from a site undergoing in situ uranium bioremediation was relatively high until 
the concentration of dissolved Fe(II) increased near the end of the field 
experiment. These results suggest that, because much of the Fe(II) is 
sequestered in solid phases, Geobacter species, which have a high requirement 
for iron for iron-sulfur proteins, may be limited by the amount of iron 
available for assimilatory purposes. These results demonstrate the ability of 
transcript analysis to reveal previously unsuspected aspects of the in situ 
physiology of microorganisms in subsurface environments.