Cadmium and zinc uptake,growth, and primary production inCoscinodiscus granii cultures containing low levels of cells and dissolved organic carbon

Coscinodiscus granii Gough was cultivated at low cell densities in aged Atlantic sea water containing very little dissolved organic carbon; the water was enriched with low levels of nutrients but no chelators were added. Cadmium additions provided final concentrations of 0.1 to 26.5 µg Cd l^?1, zinc being kept constant at a level of 38 µg Zn l^?1. Carrier-free¹⁰⁹Cd and⁶⁵Zn were used as tracers for the two metals. Growth in terms of cell numbers and primary productivity capacity, using the¹⁴C uptake rate, was followed during the exponential growth phase for the first 5 days of the experiment and then for a further 3 days during the stationary phase. On each day, the metal contents of the cells were determined. Cadmium concentrations of 20 µg Cd l^?1 and more resulted in growth inhibition whereas 17.5 µg Cd l^?1 reduced the growth only slightly. The physiological state of the cells influenced the heavy-metal uptake per cell at sublethal Cd levels. Dead cells had a higher heavy-metal concentration than living cells. Microscopical observations revealed that cells just about to divide were less sensitive to a given toxic heavy-metal concentration than cells which had recently divided. This might have been due to different surface/volume ratios.