Cadmium and phosphate concentrations in seawater are closely correlated, suggesting that Cd distributions, like those of PO4, are controlled by algal uptake and regeneration. But the factors controlling Cd levels in phytoplankton are poorly known. Experiments in metal ion buffer systems with a coastal diatom Thalassiosira pseudonana revealed that cellular 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 cellular 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 waters (less than or equal to 10(-11.0) M), Cd uptake is controlled by this latter system and, therefore, is inversely related to ionic zinc levels. However, at the higher Zn2+ range of coastal waters, Cd uptake 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 largely independent of variations in Zn2+. Because of the suppression of Cd uptake by high Zn2+ and Mn2+ in coastal waters, algal Cd concentrations may be lower in these waters than in the ocean despite the presence of higher coastal Cd2+. References: 35
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