Cadmium toxicity and resistance in Chlorella sp.

The Cd²⁺ tolerance of Chlorella sp. isolated from an urban waste water treatment plant was studied. The growth of this alga was severely inhibited at 10 μM Cd²⁺ with an LD₅₀ of 3 μM. Addition of 100 μM glutathione or 100 μM cysteine reduced the inhibitory effect of Cd²⁺. The amelioration of Cd²⁺ inhibition by GSH was not due to direct complexing of the two compounds in the growth medium, indicating that an intracellular mechanism involving GSH or its metabolites is responsible for Cd²⁺ detoxification. A Cd²⁺ resistant line (CdR-DK), with an LD₅₀ of 35 μM, was selected by growing the alga on solid medium in the presence of increasing concentrations of the ion. Phytochelatins (PCs), a family of peptides synthesized by plants in response to cadmium, were detected in extracts of the CdR-DK cells. The possible role that these peptides, which are derived from glutathione, may play in cadmium detoxification in Chlorella is discussed.