CHROMATE TOLERANCE IN STRAINS OF RHODOSPORIDIUM TORULOIDES MODULATED BY THIOSULFATE AND SULFUR AMINO-ACIDS

Cr(VI) tolerance was studied in four strains of Rhodosporidium toruloi des and compared with that of a fifth strain, DBVPG 6662, isolated fro m metallurgical wastes and known to be Cr(VI) resistant, Tolerance was studied in relation to different species of sulfur (sulfates, thiosul fates, methionine, cysteine) at different concentrations. Djenkolic ac id, a poor source of sulfur and an activator of sulfate transport, was also considered. In synthetic medium all strains except the Cr(VI)-re sistant one started to be inhibited by 10 mu g ml-l (0.2 mM) Cr(VI) as K2Cr2O7. DBVPG 6662 was inhibited by 100 mu g ml(-1) (2.0 mM) Cr(VI), In Yeast Nitrogen Base without amino acids (minimal medium), suppleme nted with varying concentrations of chromate, all Cr(VI)-sensitive str ains accumulated concentrations of total chromium (from 0.8 to 1.0 mu g mg-l cell dry wt) after 18 h of incubation at 28 degrees C, In minim al medium supplemented with 10 pg ml-l Cr(VI), the addition of sulfate did not significantly improve the yeast growth, Cysteine at mu M leve ls increased tolerance up to 10 mu g ml(-1), whereas methionine only r educed the Cr(VI) toxicity in the strain DBVPG 6739, Additions of djen kolic acid resulted in increased Cr(VI) sensitivity in all strains, Th e best inorganic sulfur species for conferring high tolerance was thio sulfate at concentrations up to 1 mM, In all cases increased Cr(VI) to lerance was due to a significantly reduced uptake in the oxyanion by t he cells and not to the chemical reduction of Cr(TT) to Cr(III) by sul fur compounds.