1. ¹Department of Microbiology, University of Massachusetts, Amherst, MA, USA
2. ²Department of Mathematics and Statistics, University of Massachusetts, Amherst, MA, USA

Correspondence: PJ Mouser, Department of Microbiology, Sanborn, Head and Associates Inc., 95 High Street, Portland, ME 04101, USA. E-mail: pjmouser@gmail.com

³Current address: BIOBASE Corporation, 100 Cummings Center, Suite 107A, Beverly, MA 01915, USA.

⁴Current address: Department of Biology, Washington University in St Louis, One Brookings Drive, Campus Box 1137, St Louis, MO 631410-4899, USA.

Received 2 September 2008; Revised 24 November 2008; Accepted 24 November 2008; Published online 8 January 2009.

Abstract

As part of an effort to diagnose the physiological status of Geobacter species during in situ bioremediation of uranium-contaminated groundwater, transcript levels for two genes potentially associated with oxidative stress, cydA and sodA, were quantified throughout a bioremediation field study in Rifle, CO, USA. Despite the accumulation of Fe(II) in the groundwater, which is inconsistent with the presence of dissolved oxygen, both genes were highly expressed during the bioremediation process. Therefore, the response to oxidative stress was further evaluated with Geobacter uraniireducens, an isolate from the Rifle site. When G. uraniireducens cultured with fumarate as the electron acceptor was exposed to 5% oxygen for 8 h, there was a significant increase in cydA and sodA transcripts as well as other genes associated with oxygen respiration or oxidative stress. Oxygen-exposed cells had lower transcript abundance for genes associated with anaerobic respiration, metabolism and motility. Short-term oxygen exposure had little impact on cydA transcript levels, as more than 1 h was required for increases to levels comparable to the subsurface. Abundance of cydA and sodA transcripts for the isolate G. sulfurreducens were always higher in cells cultured with Fe(III) compared with fumarate as an electron acceptor, even when fumarate-grown cells were exposed to oxygen, and Fe(III)-grown cells were grown anaerobically. These results suggest that the apparently high Geobacter cydA and sodA expression during bioremediation cannot necessarily be attributed to oxidative stress and demonstrate that diagnosis of the metabolic status of subsurface microorganisms through transcript analysis should be coupled with appropriate geochemical analyses.