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Applied and Environmental Microbiology, October 2009, p. 6249-6257, Vol. 75, No. 19
0099-2240/09/$08.00+0     doi:10.1128/AEM.00347-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Inhibition of Nitrate Reduction by Chromium(VI) in Anaerobic Soil Microcosms{triangledown}

Peter S. Kourtev,1* Cindy H. Nakatsu,2 and Allan Konopka1,{dagger}

Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907,1 Department of Agronomy, Purdue University, West Lafayette, Indiana 479072

Received 11 February 2009/ Accepted 5 August 2009

Chromium is often found as a cocontaminant at sites polluted with organic compounds. For nitrate-respiring microbes, Cr(VI) may be not only directly toxic but may also specifically interfere with N reduction. In soil microcosms amended with organic electron donors, Cr(VI), and nitrate, bacteria oxidized added carbon, but relatively low doses of Cr(VI) caused a lag and then lower rates of CO2 accumulation. Cr(VI) strongly inhibited nitrate reduction; it occurred only after soluble Cr(VI) could not be detected. However, Cr(VI) additions did not eliminate Cr-sensitive populations; after a second dose of Cr(VI), bacterial activity was strongly inhibited. Differences in microbial community composition (assayed by PCR-denaturing gradient gel electrophoresis) driven by different organic substrates (glucose and protein) were smaller than when other electron acceptors had been used. However, the selection of bacterial phylotypes was modified by Cr(VI). Nine isolated clades of facultatively anaerobic Cr(VI)-resistant bacteria were closely related to cultivated members of the phylum Actinobacteria or Firmicutes. In Bacillus cereus GNCR-4, the nature of the electron donor (fermentable or nonfermentable) affected Cr(VI) resistance level and anaerobic nitrate metabolism. Our results indicate that carbon utilization and nitrate reduction in these soils were contingent upon the reduction of added Cr(VI). The amount of Cr(VI) required to inhibit nitrate reduction was 10-fold less than for aerobic catabolism of the same organic substrate. We speculate that the resistance level of a microbial process is directly related to the diversity of microbes capable of conducting it.

* Corresponding author. Present address: Department of Biology, Central Michigan University, 228 Brooks Hall, Mt. Pleasant, MI 48858. Phone: (989) 774-2388. Fax: (989) 774-3462. E-mail: kourt1ps{at}cmich.edu

{triangledown} Published ahead of print on 14 August 2009.

{dagger} Present address: Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352.

Applied and Environmental Microbiology, October 2009, p. 6249-6257, Vol. 75, No. 19
0099-2240/09/$08.00+0     doi:10.1128/AEM.00347-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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