Respiratory enzymes in Acidiphilium cryptum bacteria detoxify hexavalent chromium compounds even under highly acidic conditions by shuttling electrons from several sources.

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Hexavalent chromium is a known carcinogen and may also cause mutations and birth defects. The less oxidized (i.e., "reduced") trivalent form is less toxic, predominantly because it precipitates out of watery environments as a solid at normal pH levels so is more difficult to be absorbed and used by living cells. The acid-loving bacterium, Acidiphilium cryptum, can convert chromium from the hexavalent to the trivalent form in two ways using the same enzymes it uses for respiration. When oxidized iron (III) is available to the organism (the same form of iron present in rust), the organism uses respiratory enzymes to reduce iron (III) to iron (II) and then shuttles the newly added electron from iron (II) for the conversion of chromium to its less toxic form. When iron is not available, the respiratory enzymes directly reduce hexavalent chromium to the trivalent form, but it is a slower process.

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“The potential for biological reduction of Cr(VI) under acidic conditions was evaluated with the acidophilic, facultatively metal-reducing bacterium Acidiphilium cryptum strain JF-5…The reduction product…was entirely Cr(III) that was associated predominantly with the cell biomass (70-80%) with the residual residing in the aqueous phase…Starved cells could not reduce Cr(VI) when provided as sole electron acceptor, indicating that Cr(VI) reduction is not an energy-conserving process in A. cryptum. We speculate, rather, that Cr(VI) reduction is used here as a detoxification mechanism…Both of the common Cr(VI) anions, chromate (CrO42-) and dichromate (Cr2O72-), are strong oxidants, and chromate is a known carcinogen and a suspected mutagen and teratogen…it may cause oxidative or other cellular damage such as DNA breakage…The chromate oxyanion, due to its negative charge and high solubility, tends to be highly mobile and bioavailable. By contrast, Cr(III) often forms insoluble hydroxides at circum-neutral pH, and its toxicity and mobility are negligible under these conditions. Because of this decrease in solubility, toxicity, and mobility, reduction of Cr(VI) to Cr(III) is considered a beneficial reaction in many Cr-contaminated environments…Many Cr-contaminated habitats are acidic, including acid mine drainage and numerous industrial or government sites.” (Cummings 2007:146).

A. cryptum reduces Cr(VI) by at least two different mechanisms. In the first, Cr(VI) is directly reduced by cellular components…a role for cellular proteins in the process…By the second mechanism, Fe(III) is enzymatically reduced to Fe(II), which rapidly shuttles its electron to Cr(VI), reducing it to Cr(III) in three 1-electron transfers…The kinetics of Fe(III) reduction by A. cryptum appear to be much more rapid than those of direct Cr(VI) reduction, and the rate of abiotic reduction of Cr(VI) by Fe(II) is considerably faster than many enzymatic processes, so an indirect mechanism is likely to dominate the Cr(VI) reduction pathway in the presence of reducible Fe(III)…Reduction of Cr(VI) is not linked to energy conservation by A. cryptum. Instead, Cr(VI) reduction by A. cryptum may be a detoxification strategy. The toxicity of Cr(VI) is due in part to the solubility and reactivity of the chromate anion, CrO42-. At circumneutral pH, reduction to Cr(III) leads to its precipitation as insoluble Cr(OH)3. The diminished toxicity of Cr(III) is assumed to be largely the result of the limited solubility of the trivalent hydroxide. As pH drops, however, the solubility of Cr(III) increases. Yet, its toxicity appears to be greatly diminished despite the elevated aqueous concentration…although Cr(III) remains predominantly soluble (or biomass-associated) under acidic conditions, it is nonetheless considerably less toxic to A. cryptum than Cr(VI).” (Cummings 2007:150).

“The Cr(III) was, therefore, likely associated with the biomass as a complex on cellular materials and free in aqueous solution, but not as an insoluble Cr(OH)3 precipitate.” (Cummings 2007:151).

Journal article
Reduction of Cr(VI) under Acidic Conditions by the Facultative Fe(III)-Reducing Bacterium Acidiphilium cryptumEnvironmental Science & TechnologyDecember 1, 2006
David E. Cummings, Scott Fendorf, Nathan Singh, Rajesh K. Sani, Brent M. Peyton, Timothy S. Magnuson

Journal article
Genomics against flatulenceEnviron MicrobiolJuly 12, 2007
Michael Y. Galperin

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