Chromium in the sixth oxidation state may easily penetrate cellular membranes via non-specific sulfate transporters due to its tetrahedral symmetry (high similarity to SO42- and HPO42-). This feature makes chromium a toxic and hazardous pollutant responsible for the deterioration of midland water quality. The aim of the study was to evaluate the capacity of two yeast species - Saccharomyces cerevisiae and Phaffia rhodozyma - and their protoplasts to reduce Cr(VI) to lower oxidation states. The study also deals with the behavior of the yeasts upon the presence of elevated sulfate ions as a competitive inhibitor of chromate transport by the sulfate transporters. The chromate-reducing activities were monitored by determination of Cr(V) free radical form with the use of L-band (1.2 GHz) EPR (electron paramagnetic resonance) spectroscopy. It was observed that both of the studied yeast strains exhibited the ability to reduce Cr(VI) applied at 4 mM. The cells of P. rhodozyma showed about 3.5 times higher reduction than S. cerevisiae. The reduction efficiency was significantly improved when the protoplasts of both strains were used and reached 100% in the first 10 minutes of the reduction process which suggests that the cellular wall may have a notable influence on the uptake and/or inhibition of chromium reduction process. The reduction effect of P. rhodozyma cells and protoplasts may be associated with the more sufficient production of metabolites (such as glutathione and cysteine), which may also be responsible for the increased tolerance of the strain towards high concentrations of toxic chromium.
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