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Behavior of Chromium in Soils: IV. Toxicity to Microorganisms¹

ABSTRACT

The toxicity of Cr to soil bacterial isolates was studied by measuring turbidity of liquid cultures supplemented with Cr(VI) or Cr(III). Ten to 12 ppm Cr(VI) was found inhibitory to most isolates growing in either a soil-extract medium and or in a semisynthetic medium. Gram negative bacteria were more affected than gram positive bacteria by Cr(VI) both at this level and at 1 ppm. Toxicity due to Cr(III) was not observed at similar levels.

Nellis loam (Typic Eutrochrept) and Hinesburg fine sandy loam (Entic Haplorthod) samples were treated with 10 and 100 ppm Cr(VI) and 100 ppm Cr(III). All three treatments significantly decreased CO₂ evolution from that of controls for the 3-week duration of the experiment. Extractable Cr(VI) in these soils decreased rapidly during this time, indicating that reduction of the added Cr(VI) was occurring.

The toxicity of low levels of Cr(VI) to gram negative bacteria in aqueous culture indicates that soil microbial transformations, such as nitrification, may be affected. The fact that both Cr(III) and Cr(VI) inhibited microbial activity in soil shows that more caution may be needed in handling Cr-containing wastes, regardless of the form of Cr.

Key Words: chromium(VI) • chromium(III) • soil bacteria

¹ Vermont Agric. Exp. Stn. J. Article no. 449. Contribution to Northeast Regional Project 96.

² Former Graduate Fellow, Dep. of Plant & Soil Science; Assoc. Professor, Dep. of Microbiology & Biochemistry; and Professor, Dep. of Plant & Soil Science; respectively, Univ. of Vermont and Agric. Exp. Stn., Burlington, VT 05405.

Received for publication March 28, 1980.