Nutrient limitation is an environmental stress that may reduce the effectiveness 
of bioremediation strategies, especially when the contaminants are organic 
compounds or when organic compounds are added to promote microbial activities 
such as metal reduction. Genes indicative of phosphate-limitation were 
identified by microarray analysis of chemostat cultures of Geobacter 
sulfureducens. This analysis revealed that genes in the pst-pho operon, which is 
associated with a high-affinity phosphate uptake system in other microorganisms, 
had significantly higher transcript abundance under phosphate-limiting 
conditions, with the genes pstB and phoU upregulated the most. Quantitative PCR 
analysis of pstB and phoU transcript levels in G. sulfurreducens grown in 
chemostats demonstrated that the expression of these genes increased when 
phosphate was removed from the culture medium. Transcripts of pstB and phoU 
within the subsurface Geobacter species predominating during an in situ 
uranium-bioremediation field experiment were more abundant than in chemostat 
cultures of G. sulfurreducens that were not limited for phosphate. Addition of 
phosphate to incubations of subsurface sediments did not stimulate dissimilatory 
metal reduction. The added phosphate was rapidly adsorbed onto the sediments. 
The results demonstrate that Geobacter species can effectively reduce U(VI) even 
when experiencing suboptimal phosphate concentrations and that increasing 
phosphate availability with phosphate additions is difficult to achieve because 
of the high reactivity of this compound. This transcript-based approach 
developed for diagnosing phosphate limitation should be applicable to assessing 
the potential need for additional phosphate in other bioremediation processes.