Molecular tools that can provide an estimate of the in situ growth rate of 
Geobacter species could improve understanding of dissimilatory metal reduction 
in a diversity of environments. Whole-genome microarray analyses of a subsurface 
isolate of Geobacter uraniireducens, grown under a variety of conditions, 
identified a number of genes that are differentially expressed at different 
specific growth rates. Expression of two genes encoding ribosomal proteins, rpsC 
and rplL, was further evaluated with quantitative reverse transcription-PCR 
(qRT-PCR) in cells with doubling times ranging from 6.56 h to 89.28 h. 
Transcript abundance of rpsC correlated best (r(2) = 0.90) with specific growth 
rates. Therefore, expression patterns of rpsC were used to estimate specific 
growth rates of Geobacter species during an in situ uranium bioremediation field 
experiment in which acetate was added to the groundwater to promote 
dissimilatory metal reduction. Initially, increased availability of acetate in 
the groundwater resulted in higher expression of Geobacter rpsC, and the 
increase in the number of Geobacter cells estimated with fluorescent in situ 
hybridization compared well with specific growth rates estimated from levels of 
in situ rpsC expression. However, in later phases, cell number increases were 
substantially lower than predicted from rpsC transcript abundance. This change 
coincided with a bloom of protozoa and increased attachment of Geobacter species 
to solid phases. These results suggest that monitoring rpsC expression may 
better reflect the actual rate that Geobacter species are metabolizing and 
growing during in situ uranium bioremediation than changes in cell abundance.