Cobalt and zinc interreplacement in marine phytoplankton: Biological and geochemical implications

Zinc is used extensively in the metabolism of higher organisms; cobalt's usage is minimal. We found an unusual pattern of requirement for these metals in marine phytoplankton in which the cyanobacterium Synechococcus bacillaris needed Co but not Zn for growth, the coccolithophore Emiliania huxleyi had a Co requirement that could be partly met by Zn, and the diatoms Thalassiosira pseudonana and Thalassiosira oceanica had Zn requirements that could be largely met by Co. These results indicate that Co and Zn can replace one another metabolically in the eucaryotic species. Associated with this replacement, there was up to a 700‐fold increase in cellular Co uptake rates with decreasing Zn ion concentration, indicating that Zn should have a major influence on biological scavenging of Co. This hypothesis is consistent with Zn and Co distributions within the oceanic nutricline which show Co depletion only after Zn has become depleted. Zn ion concentrations and Co : Zn ratios vary widely in the ocean, and these variations could influence the relative growth of diatoms and coccolithophores, with potential effects on global carbon cycles.