Biochemistry of chromium(VI) reduction: Formation, fate, and implications of soluble organo-chromium(III) complexes on the biogeocycle of chromium.

摘要

Chromium is widely distributed in the environment, existing in either the +3 or +6 state under environmental conditions. Trivalent chromium [Cr(III)] is the predominant oxidation state and considered relatively insoluble and non-toxic. In contrast, hexavalent chromium [Cr(VI)] forms oxyanions, i.e. CrO4-, at neutral pH, which are highly soluble and mobile in groundwater. Chromium is an important metal for use in industry based on its corrosion resistant properties. Large amounts of Cr(VI) have been introduced into the environment as industrial waste, posing a significant contamination problem requiring remediation.; Bioremediation has been proposed as a method for treating Cr(VI) contamination. Many bacteria reduce Cr(VI), but the reduction mechanism(s) and end-products are poorly understood. Research presented here focuses on understanding the biochemistry of Cr(VI) reduction and the Cr(III) end-products formed. An Escherichia coli NAD(P)H:flavin oxidoreductase was found to reduce Cr(VI) via free reduced flavins. This system rapidly reduced chromate, whereas chemical reduction by NADH and glutathione was very slow. The reduced end-product was identified as a soluble and stable complex composed of multiple Cr(III) ions bound to NAD+, NAD+-Cr(III), instead of Cr(III) precipitate as expected for microbial reduction of Cr(VI). We further demonstrated that Cr(VI) reduction in the presence of several individual intracellular organic compounds formed soluble organo-Cr(III) end-products, which remained soluble and stable upon dialysis against H2O and over a broad pH range. The fate and recalcitrance of these soluble organo-Cr(III) complexes in the environment were unknown. Microbial transformation of soluble organo-Cr(III) end-products was discovered. Two bacteria, PTX1 and PTX2, utilizing an organo-Cr(III) complex, NAD+-Cr(III), were isolated. Phylogenetic classification designated PTX1 as a Leifsonia species and PTX2 as a Rhodococcus species, both common soil bacteria. The bacteria utilize the NAD+ in the NAD+-Cr(III) complex as a carbon and energy source. Mineralization of the NAD+-Cr(III) resulted in precipitation of Cr(III) on the bacterial surface, signaling a probable long term insoluble Cr(III) form.; This work furthers the knowledge of mechanisms proposed to play a role in Cr(VI) reduction and expands the biogeocycle of Cr to include the formation of soluble organo-Cr(III) species and microbial mineralization of these species to produce an insoluble Cr(III) form.