LONG-CHAIN CLASS-III METALLOTHIONEINS AS A MECHANISM OF CADMIUM TOLERANCE IN THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM BOHLIN

Cadmium tolerance of the marine diatom Phaeodactylum tricornutum Bohli n was studied. P. tricormutum growth was significantly reduced by cadm ium concentrations of 5 mg l(-1) or higher. The EC50 value was 22.39 m g l(-1) of cadmium after four days of exposure. Therefore, P. tricornu tum is more tolerant to cadmium than other microalgal cells. In respon se to exposure to cadmium, P. tricornutum synthesize class III metallo thioneins which sequester the metal in a harmless form. However, cadmi um tolerance of P. tricornutum cells is not only due to the ability of this microalga to synthesize class III metallothioneins (gamma-Glu-Cy s)(n)-Gly, but also because these metallothioneins are composed of lon g-chain polypeptides. The cadmium-metallothionein complexes of P. tric ornutum cells contain predominantly polypeptides of n=4-9. Long-chain metallothioneins are more efficient in binding heavy metals. Capillary electrophoresis was used to separate class III metallothioneins. This is a recent technique used to separate many molecules, With this tech nique, the length of the separated polypeptides can also be estimated. (C) 1997 Elsevier Science B.V.