Mechanism of Cr(VI) reduction by Aspergillus niger: enzymatic characteristic, oxidative stress response, and reduction product.

Bioremediation of hexavalent chromium by Aspergillus niger was attributed to the reduction product (trivalent chromium) that could be removed in precipitation and immobilized inside the fungal cells and on the surface of mycelium. The site location of reduction was conducted with assays of the permeabilized cells, cell-free extracts, and cell debris, which confirmed that the chromate reductase was mainly located in the soluble fraction of cells. The oxidation-reduction process was accompanied by the increase of reactive oxygen species and antioxidant levels after hexavalent chromium treatment. Michaelis-Menten constant (K(m)) and maximum reaction rate (V(max)), obtained from the Lineweaver-Burk plot were 14.68 μM and 434 μM min(-1) mg(-1) of protein, respectively. Scanning electron microscopy and Raman spectra analyses manifested that both Cr(VI) and Cr(III) species were present on the mycelium. Fourier transform-infrared spectroscopy analysis suggested that carboxyl, hydroxide, amine, amide, cyano-group, and phosphate groups from the fungal cell wall were involved in chromium binding by the complexation with the Cr(III) and Cr(VI) species. A Cr(VI) removal mechanism of Cr(VI) reduction followed by the surface immobilization and intracellular accumulation of Cr(III) in living A. niger was present.