Response to copper and cadmium stress in wild-type and copper tolerant strains of the lichen alga Trebouxia erici:: metal accumulation, toxicity and non-protein thiols

Cysteine, glutathione (GSH) and phytochelatins were determined in the cells of both wild and copper tolerant strains of the lichen alga Trebouxia erici following short-term (24 h) exposure to copper and cadmium and long-term (4 weeks) exposure to copper. Both metals caused concentration dependent synthesis of phytochelatins (PC2-PC5), but cadmium was a more potent activator of phytochelatin synthesis, even inducing synthesis of PC5. The copper-tolerant strain did not reveal a higher degree of phytochelatin synthesis than the wild strain, and at 5 mu M Cu production of phytochelatins was in fact significantly lower. Lower levels of phytochelatin correlated with significantly decreased intracellular copper content in the copper-tolerant strain. Both strains maintained high GSH levels even at a high copper concentration of 5 mu M, and only the highest copper concentration (10 mu M) was toxic for both strains, causing a decrease of GSH and PC content in algal cells. Cadmium had less effect on GSH in the cells of both tested strains. In the long term experiments, only relatively small amounts of PC2 were detected in both strains, but the copper-tolerant strain retained significantly higher levels of reduced glutathione, probably due to the lesser degree of oxidative stress caused by Cu. The significant increase of cysteine synthesis in the copper-tolerant strain found in the present study may be related to copper tolerance in T erici, while decreased intracellular Cu uptake, detoxification by PCs and increased free proline levels for protection of chloroplast membranes may also be implicated.