CONTROL OF CD CONCENTRATIONS IN A COASTAL DIATOM BY INTERACTIONS AMONG FREE IONIC CD, ZN, AND MN IN SEAWATER

Cadmium and phosphate concentrations in seawater are closely correlate d, suggesting that Cd distributions, like those of PO4, are controlled by algal uptake and regeneration. But the factors controlling Cd leve ls in phytoplankton are poorly known. Experiments in metal ion buffer systems with a coastal diatom Thalassiosira pseudonana revealed that c ellular Cd:C ratios within the Cd ion concentration ([Cd2+]) range (10 (-13) to 10(-10) M) in, seawater were generally proportional to [Cd2+] and inversely related to concentrations of Zn and Mn ions ([Zn2+] and [Mn2+]) and specific growth rate. The effects of Mn and Zn reflect ce llular uptake of Cd by two inducible transport systems: the Mn system whose capacity (V-max) is enhanced at low [Mn2+] and a separate system induced at low cellular zinc. At the low [Zn2+] of surface oceanic wa ters (less than or equal to 10(-11.0) M), Cd uptake is controlled by t his latter system and, therefore, is inversely related to ionic zinc l evels. However, at the higher [Zn2+] range of coastal waters, Cd uptak e by this system is strongly suppressed and Cd instead is taken up by the Mn system; as a result it is inversely related to [Mn2+] and large ly independent of variations in [Zn2+]. Because of the suppression of Cd uptake by high [Zn2+] and [Mn2+] in coastal waters, algal Cd concen trations may be lower in these waters than in the ocean despite the pr esence of higher coastal [Cd2+].