Isolation and characterization of a chromium-reducing bacterium from a chromated copper arsenate-contaminated site

A Gram-negative bacterium (CRB5) was isolated from a chromium-contaminated site that was capable of reducing hexavalent chromium to an insoluble preci pitate, thereby removing this toxic chromium species from solution. Analysi s of the 16S rRNA from the isolate revealed that it was a pseudomonad with high similarity to Pseudomaonas synxantha. CRB5 was tolerant to high concen trations of chromate (500 mg l(-1)) and can reduce Cr(VI) under aerobic and anaerobic conditions. It also exhibited a broad range of reduction efficie ncies under minimal nutrient conditions at temperatures between 4 degreesC and 37 degreesC and at pH levels from 4 to 9. As reduction increased, so di d total cellular protein, indicating that cell growth was a requirement for reduction. Under low nutrient conditions with CRB5 or when using nonsteril e contaminated groundwater from the site, reduction of Cr(VI) was followed by a increase in solution turbidity as a result of the formation of fine-gr ained Cr(III) precipitates, most probably chromium hydroxide mineral phases such as Cr(OH)(3). Chromium adsorption and precipitation, as observed by t ransmission electron microscopy coupled with energy dispersive X-ray spectr oscopy (TEM/EDS), revealed that the surfaces of the cells were uniformly st ained with bound Cr(III) and amorphous precipitates (as determined by selec ted area electron diffraction; SAED). A mass balance of chromium in a batch bioreactor revealed that up to 30% of the total Cr was as settable precipi tates or bound to cells.