Studies on the biotransformation of chromium (VI) by methane oxidising bacteria.

Bioremediation is a promising and cost-effective technology widely used to clean up waste containing organic or inorganic contaminants. Cr(VI), the highest oxidation state of the metal chromium, is widely used in various industries, and is extremely toxic to animals and humans. Utilisation of chromium (Cr) reducing microbes and their products can enhance the efficiency of the process of detoxification of Cr(VI) to Cr(III).This study was conducted to investigate the microbial reduction of chromium [Cr(VI)] in general and the potential for biological treatment of Cr(Vl)-containing wastes in particular. Cr(VI) was transformed to non-toxic Cr(III) by Escherichia coli ATCC 33456. It was observed for the first time that the type I methanotrophic bacterium Methylococcus capsulatus (Bath) is able to reduce chromium (VI) to chromium (III), using methane as the carbon and energy source. The reaction occurred over a wide range of chromium (VI) concentrations (10-1,000 mg/1). The reaction was studied by a variety of techniques, including the diphenyl carbazide assay for chromium (VI), ion chromatography coupled to inductively coupled plasma atomic absorption spectroscopy (ICP-MS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDSX), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results indicate that chromium(VI) reduction to chromium(III) is wholly or partly associated with the cells via coordination to phosphate moieties. BLAST searches using known chromate (Vl)-reducing enzymes from other bacteria indicated the presence of five potential chromate reductases in the genome sequence of M. capsulatus (Bath). Detailed bioinformatic analysis, including molecular modelling, indicates a possible mode of binding of chromium to two of these. The model type II methanotroph Methylosinus trichosporium OB3b did not reduce chromate (VI) in pure culture, and the future availability of the genome sequence of this organism may give additional clues to the origin of chromate(VI) reduction in M. capsulatus, via comparative genomics. Interestingly, however, mixed cultures of M. trichosporium and E. coli were able to reduce chromate(VI) using methane as the only source of reductant, presumably because the E. coli can scavenge nutrients from the M. trichosporium.