Long-chain class III metallothioneins as a mechanism of cadmium tolerance in the marine diatom Phaeodactylum tricornutum Bohlin

Abstract

Cadmium tolerance of the marine diatom Phaeodactylum tricornutum Bohlin was studied. P. tricornutum growth was significantly reduced by cadmium concentrations of 5 mg l−1 or higher. The EC50 value was 22.39 mg l−1 of cadmium after four days of exposure. Therefore, P. tricornutum is more tolerant to cadmium than other microalgal cells. In response to exposure to cadmium, P. tricornutum synthesize class III metallothioneins which sequester the metal in a harmless form. However, cadmium tolerance of P. tricornutum cells is not only due to the ability of this microalga to synthesize class III metallothioneins (γ-Glu-Cys)n-Gly, but also because these metallothioneins are composed of long-chain polypeptides. The cadmium-metallothionein complexes of P. tricornutum 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 technique, the length of the separated polypeptides can also be estimated.