Synthesis and stability of phytochelatins induced by cadmium and lead in the marine diatom Phaeodactylum tricornutum

The synthesis of phytochelatins (PC), intracellular metal-binding polypeptides characterized by a repeating sequence of γ-glutamic acid- cysteine (γ-Glu-Cys) pairs, has been studied in laboratory cultures of the marine diatom Phaeodactylum tricornutum exposed to Cd, Pb or Zn. Cd and Pb were able to induce PC of different degree of polymerization. The accumulation of the peptides follows a direct relationship with the metal exposure. No PC induction was observed in Zn-treated cultures, although the intracellular concentration of Zn increased during exposure. Both in short-term (7 h exposure, 10 μM Cd or Pb) and 3-day experiments (metal concentration less than 0.5 μM), the major fraction of total PC γ-Glu-Cys subunits synthesized was polymerized as PC2 when cells were exposed to Pb, but as PC4 when cells were exposed to Cd. In short-term experiments about 50% of the γ-Glu-Cys residues of the cellular pool of glutathione was quickly and almost quantitatively converted into PC. Recovery experiments, in which metal-stressed cells are suspended in a metal-free medium, showed a decrease of the PC pool and a concomitant increase of glutathione, suggesting a mechanism of degradation and release of metal-phytochelatin complexes.