REPLACEMENT OF ZINC BY CADMIUM IN MARINE-PHYTOPLANKTON

The concentration of cadmium varies like that of a nutrient in the ope n ocean. Detailed studies of the marine diatom Thalassiosira weissflog ii have shown that cadmium can act as an algal nutrient under conditio ns of zinc Limitation. We show here that cadmium can also enhance the growth of a variety of species, including a chlorophyte and some prymn esiophytes, when they are zinc limited. The replacement of zinc by cad mium occurs at environmentally relevant inorganic cadmium and zinc con centrations. Very low concentrations of inorganic cadmium that are ben eficial under conditions of moderate zinc-limitation become toxic in c ultures severely limited by zinc. The role of cadmium as an algal nutr ient is thus observable in a narrow, species-specific range of inorgan ic zinc and cadmium concentrations.