Effect of Zn, Mn, and Fe on Cd accumulation in phytoplankton: implications for oceanic Cd cycling

Cd and phosphate concentrations in seawater are closely related, suggesting that Cd distributions, like those of PO4, are controlled by algal uptake and regeneration. But the factors that control Cd levels in phytoplankton and their influence on oceanic Cd versus PO4 relationships are poorly known. We examined the effect of important controlling factors (free ion concentrations of Cd, Zn, and Mn and Fe limitation of growth rate) on Cd accumulation by an oceanic diatom (Thalassiosira oceanica). More limited comparative experiments were also conducted with a coastal diatom (T. weissflogii) and an oceanic coccolithophore (Emiliania huxleyi). Cellular Cd: C ratios increased with increasing Cd ion concentrations and decreasing Zn ion concentrations in all species and with decreasing ionic Mn in the diatoms. The effects of Mn and Zn apparently are related to the uptake of Cd by the cells ’ Mn and Cd/ Co transport systems, which are under negative feedback regulation by cellular Mn and Zn. Cd: C ratios were determined in the oceanic diatom as functions of free ion concentrations of Cd, Zn, and Mn over the oceanic range of these metal ions. The data were combined with estimates of free ion concentrations of these metals in seawater to construct models of Cd: C and Cd: P ratios in oceanic phytoplankton. The modeled Cd: P ratios showed good agreement with slopes of Cd versus P relationships in different oceanic regimes. The models suggest that the high Cd versus phosphate slopes found in iron-depleted waters of the Southern Ocean and subarctic Pacific result from unusually high levels of zinc depletion in these waters. The resulting low zinc ion concentrations induce high levels of Cd uptake by phytoplankton, yielding high algal Cd: P ratios. The heavy depletion of zinc may be linked to elevated Zn: C and Zn: P ratios in iron-limited diatoms, as observed in experiments we conducted with T. oceanica. Cd is usually thought of as a toxic metal, but recent evi-dence indicates that it can nutritionally substitute for Zn in some key Zn enzymes such as carbonic anhydrase (Price an