Cr (VI) reduction by sulfidogenic and nonsulfidogenic microbial consortia

In time course experiments, bacterial community compositions were compared between a sulfidogenic and two nonsulfidogenic Cr(VI)-reducing consortia enriched from metal-contaminated sediments. The consortia were subjected to 0 and 0.85 mM or 1.35 mM Cr(VI), and Cr(VI) reduction, growth, and denaturing gradient gel electrophoresis profiles of PCR products of small-subunit (16S) ribosomal genes were compared. Results showed that although Cr(VI) was completely reduced by the three consortia, Cr(VI) inhibited cell growth, with sulfate-reducing bacteria being particularly sensitive to Cr(VI) toxicity relative to other bacteria in the consortia. Chromium is a metal contaminant that, in nature, exists primarily as the soluble, highly toxic Cr(VI) anion and the less soluble, less toxic Cr(III) species. Many facultative and strictly anaerobic bacteria commonly found in soils and marine sedi-ments are capable of reducing Cr(VI) to Cr(III) (10, 14, 20, 34, 35). Sulfate- and iron-reducing bacteria can indirectly reduce Cr(VI) via their anaerobic metabolic end products, hydroge