Training of Rhizopus stolonifer and Cunninghamella blakesleeana to copper: cotolerance to cadmium, cobalt, nickel, and lead

Rhizopus stolonifer and Cunninghamella blakesleeana trained after five passages to tolerate elevated concentrations of copper simultaneously acquired a tolerance to elevated levels of cadmium, cobalt, nickel, and lead. The acquired tolerance to copper was not stable in the absence of the metal, as serial transfers to a copper-free medium progressively reduced the level of tolerance, and after the sixth passage on a copper-free medium, the tolerance was equivalent to that of the nontrained parentals ("untraining"). However, the untrained fungi regained the same tolerance to copper as the original copper-trained fungi after only three passages on copper-amended medium ("retraining"). The tolerance to copper was apparently the result of physiological adaptation rather than of the selection of resistant cells or the induction of mutation. The metal may have activated genes that coded for biochemical processes that conferred tolerance to copper as well as to other heavy metals. Furthermore, the tolerance to copper of the mycelia was transferred to the sporangiospores. The copper-trained fungi did not appear to produce extracellular metabolites that complexed with and, hence, excluded and detoxified the copper. However, the mycelia of the trained fungi removed approximately twice as much copper from solution than those of the nontrained parentals, suggesting that the tolerance to copper resulted from the binding of the metal to the cell wall or from an intracellular detoxification mechanism.