Enrichment of Geobacter species in response to stimulation of Fe(III) reduction in sandy aquifer sediments

Engineered stimulation of Fe(III) has been proposed as a strategy to enhanc e the immobilization of radioactive and toxic metals in metal-contaminated subsurface environments. Therefore, laboratory and field studies were condu cted to determine which microbial populations would respond to stimulation of Fe(III) reduction in the sediments of sandy aquifers. In laboratory stud ies, the addition of either various organic electron donors or electron shu ttle compounds stimulated Fe(III) reduction and resulted in Geobacter seque nces becoming important constituents of the Bacterial 16S rDNA sequences th at could be detected with PCR amplification and denaturing gradient gel ele ctrophoresis (DGGE). Quantification of Geobacteraceae sequences with a PCR most-probable-number technique indicated that the extent to which numbers o f Geobacter increased was related to the degree of stimulation of Fe(III) r eduction. Geothrix species were also enriched in some instances, but were o rders of magnitude less numerous than Geobacter species. Shewanella species were not detected, even when organic compounds known to be electron donors for Shewanella species were used to stimulate Fe(III) reduction in the sed iments. Geobacter species were also enriched in two field experiments in wh ich Fe(III) reduction was stimulated with the addition of benzoate or aroma tic hydrocarbons. The apparent growth of Geobacter species concurrent with increased Fe(III) reduction suggests that Geobacter species were responsibl e for much of the Fe(III) reduction in all of the stimulation approaches ev aluated in three geographically distinct aquifers. Therefore, strategies fo r subsurface remediation that involve enhancing the activity of indigenous Fe(III)-reducing populations in aquifers should consider the physiological properties of Geobacter species in their treatment design.