IN the oceans, many trace metals show a surface depletion relative to deep waters that is typical of the principal algal nutrients, N, P and Si, and is therefore presumed to result from biological uptake at the sea surface and regeneration at depth. Among trace metals, cadmium has an especially acute surface depletion1,2, and shows the best correlation with a major algal nutrient (P)1-3. But the biological reason for Cd surface depletion is particularly puzzling, because unlike other surface-depleted trace elements (for example, Fe, Ni, Co, Zn and Cu), Cd is not known to be required by organisms. However, because Cd can substitute for Zn in some metalloenzymes in vitro4 and in vivo5, we hypothesized that Cd might promote the growth of Zn-limited phytoplankton. Marine phytoplankton are limited by a free Zn ion activity of 10-11.5 M (refs 6, 7), which is similar to the activity estimated for ocean surface waters8 as a result of the low concentration and organic complexation of Zn in the oceans. We now report that, in sea water with low Zn concentration, mimicking conditions of the ocean surface waters, Cd stimulates the growth of the marine diatom Thalassiosira weissflogii by substituting for Zn in certain macromolecules. The substitution is highly effective, in that Zn-deficient cells can grow at 90% of their maximum rate when supplied with Cd. We also find that Co can substitute for Zn (although less efficiently than Cd), indicating that Co could be an important nutrient for algal growth for reasons other than its role in vitamin B 12 (ref. 9).
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