Inhibition of Nitrate Reduction by Chromium(VI) in Anaerobic
Peter S. Kourtev,1* Cindy H. Nakatsu,2 and Allan Konopka1†
Department of Biology,
Central Michigan University, 228 Brooks Hall, Mt. Pleasant, MI 48858.
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.
Keywoards:Soil Microcosms, phylum Actinobacteria,Firmicutes, Bacillus cereus GNCR-4,Staphylococcus spp,plant growth.
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