Microbial chromium(VI) reduction: Role of electron donors, acceptors, and mechanisms, with special emphasis on Clostridium spp

Cr(VI) has been designated as a priority pollutant by the US Environmental Protection Agency (USEPA) due to its ability to cause mutations and cancer in humans. The risk associated with soil and groundwater contamination of chromium waste generated by many industries is high, and therefore Cr(VI) remediation is of critical importance. Using chemical and biological methods conjointly can decrease the cost of remediating contaminated sites. Microbial reduction of Cr(VI), an important aspect of biological remediation, requires the knowledge of microorganisms capable of reducing Cr(VI) and the mechanisms involved in the reduction processes.; The overall objective of this study was to investigate the effect of various electron donors and acceptors on chromate reduction by indigenous Cr(VI)-reducing bacteria isolated from Cr(VI) contaminated sites and to understand the mechanism of Cr(VI) reduction by enriched bacterial consortium and the pure isolate. A series of bacterial enrichment cultures were established with a range of electron donors such as acetate, benzoate, lactate, citrate, and glucose, and electron acceptors such as Fe(III) and an humic acid analog, anthraquinone di-sulfonate (AQDS), to study their effects on the rates of Cr(VI) reduction. Results from this study demonstrated that the rates of Cr(VI) reduction in glucose and citrate enrichments were higher when compared with those of other electron donors. Enrichments amended with AQDS and Fe(III) showed enhanced rates of Cr(VI) reduction. Glucose-AQDS-Fe(III)-Cr(VI) enrichments (now on referred as GCAF) yielded the highest diversity of strains, which were distributed within the low G+C and high G+C groups of gram-positive bacteria. Phylogenetic analysis based on 16S rDNA studies revealed that isolates clustered with Bacillus, Cellulomonas, and Clostridium groups. Several strains were isolated from the consortium. Detailed kinetic studies with bacterial consortium and the pure strain GCAF-1 obtained from GCAF enrichment demonstrated an iron-promoted reduction of chromate. The presence of AQDS accelerated reduction of Cr(VI) only when Fe(III) was present in the medium. Analysis of fermentation metabolites produced by strain Clostridium spp. GCAF-1 revealed that the presence of Cr(VI) alters the acetate: butyrate and acetate: lactate ratios. Based on the overall results, direct and indirect (Fe (III) mediated) methods of reduction of Cr(VI) by Clostridium spp. GCAF-1 are proposed.