Hexavalent chromium uptake by sensitive and tolerant mutants of Schizosaccharomyces pombe

Lysine and leucine auxotrophic, heterothallic (h(+), h(-)) strains of Schizosaccharomyces pombe were used to obtain chromium(VI)-sensitive and -tolerant mutants by ultraviolet radiation-induced and nitrosoguanidine-induced mutagenesis. The minimal inhibitory concentrations of K2Cr2O7 on YEA media were 225 micromolar for the wild-type strain CW-6, 125 micromolar for the sensitive mutant CS-6.51 and 275 micromolar for the tolerant mutant CT-6.66. The mutants exhibited cross-sensitivity of various patterns to Cd(2+), Cu(2+), Ni(2+), Zn(2+) and VO4(3-). Cr(VI) was added to the actively growing cultures and the total chromium (TOCr) content of the cells was determined. The sensitive mutant exhibited a high bioaccumulation ability, with a dry biomass of 810 microgram g(-1) after 30 min, while the tolerant mutant had a significantly lower ability than the wild-type strain. In PIPES buffer, washed, lysine-starved biomasses were treated 75 micromolar Cr(VI) and after 2 h, the TOCr and the organically bound chromium (OBCr) were determined. Under these conditions, the sensitive and tolerant mutants had the same TOCr content, 50% of which was OBCr. The wild-type strain exhibited a lower TOCr content than that of its mutants and only 35% of this was OBCr. The Cr(VI)-sensitivity was due to a significantly increased uptake of Cr(VI).