Characterization of artificially induced cadmium-tolerant yeast mutants

Bioremediation of heavy metals by using microorganisms is an effective strategy in regions that have low and wide-ranging metal concentrations and in situations where physical and chemical techniques are not suitable. Because of their higher capacity to remove a wide range of metals by biosorption, yeasts are useful for the bioremediation of heavy metals. In this study, identification of yeast mutants (CdRs) was focused, which have strong resistance to cadmium (Cd), a representative heavy metal. Yeast cells were sequentially adapted to gradually increasing the Cd concentration up to 30 mM. The resultant mutant, CdR30 cells survived a final Cd concentration of 30 mM, while the control cells failed to survive at 0.5 mM in 7 d. It was analyzed whether the increased Cd tolerance of the mutants was associated with sensitivity toward other metals. Compared to control cells, CdR20 cells showed increased tolerance to Cu, decreased tolerance to Ni, and comparable tolerance to Zn. However, these tolerances were not reproducible, because CdRs isolated in a second round of induction showed different metal sensitivities. The increase in Ni sensitivity in CdR20 cells was overcome by performing a second adaptation to Ni stress. Thus, CdR20 cells that were tolerant to both Cd and Ni were generated. These data presented in this study may be useful for the application of microorganisms to the bioremediation of heavy metals.