EXPORT OF CADMIUM AND PHYTOCHELATIN BY THE MARINE DIATOM THALASSIOSIRA-WEISSFLOGII

Cadmium is one of the most toxic trace metals and induces high concent rations of the metal-binding polypeptide phytochelatin, (gamma-Glu-Cys )(n)Gly where n greater than or equal to 2, in the marine diatom Thala ssiosira weissflogii. Here we show that at high inorganic cadmium conc entrations there is an efflux of cadmium from T. weissflogii so large that over half the cadmium taken up by the cell is returned to the med ium. At high inorganic cadmium, there is also an efflux of phytochelat in from the cell. The efflux of both cadmium and phytochelatin stops w hen the external inorganic cadmium concentration is reduced. The efflu x of phytochelatin and cadmium occurs at a molar ratio of approximatel y 4 gamma-Glu-Cys subunits per cadmium, a stoichiometry similar to tha t measured in vivo for the cadmium-phytochelatin complex. We hypothesi ze that T. weissflogii exports the phytochelatin-cadmium complex as a detoxification mechanism. The cadmium-phytochelatin complex does not a ppear to be very stable in seawater once outside the cell since the ca dmium exported is available to T. weissflogii and induces phytochelati n synthesis. Cadmium-phytochelatin export may be an important adaptive strategy that allows phytoplankton to survive in metal-polluted water s.