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 μM for the wild-type strain CW-6, 125 μM for the sensitive mutant CS-6.51 and 275 μM for the tolerant mutant CT-6.66. The mutants exhibited cross-sensitivity of various patterns to Cd²⁺, Cu²⁺, Ni²⁺, Zn²⁺ and VO₄³⁻. 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 μg 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 with 75 μM 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).