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Iron is taken up via specific sites on the cell surface in two coastal phytoplankters—the coccolithophorid Pleurochrysis carterae and the diatom Thalassiosira weissflogii. Direct uptake from the cell surface of ∼ 10−17 mol Fe cell−1 was observed in pulse‐chase experiments with cells grown under Fe limitation. This quantity corresponds to the theoretical number of transport sites needed by each cell given the slow coordination kinetics and the low seawater concentrations of Fe. The observed turnover time of Fe in the sites—6–20 min—the observed half‐saturation constants for uptake—0.7 and 3.1 nM for P. carterae and T. weissflogii—and maximal Fe complexation rates are consistent with a standard model for carrier‐mediated transport. Analysis of the reaction kinetics indicates that the sites are not at equilibrium with dissolved Fe species. Iron uptake rates are thus controlled by the complex formation kinetics of the transport site with monomeric ferric hydroxide species. Fe‐limited P. carterae and T. weissflogii can take up Fe at rates up to 14–20% of the maximum rate at which dissolved inorganic Fe can diffuse to the cell. Since oceanic phytoplankters must also be subject to the same strongly size‐dependent limit on uptake rates imposed by diffusion, large species are more likely to experience growth rate limitation under low‐Fe conditions in the ocean.

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Some trace metals such as Fe, Ni, Cu, and Zn are essential for the growth OF phytoplankton. The concentrations of these essential trace elements in seawater are so low as to limit their availability to aquatic microbiota. Trace element uptake is ultimately limited by kinetics of reaction with transport ligands or by diffusion to the cell. From what we know of the characteristics of the uptake systems of phytoplankton and their trace metal requirements we can estimate that Fe and Zn may at some times in some places limit phytoplankton productivity, which is in accord with available field data on trace metal enrichmeuts:Although the founding fathers of the field discussed many elements, including Fe, Mn, Zn, and Cu, as potentially limiting algal growth in seawater (Harvey 1945), for the past several decades biological oceanographers have focused almost exclusively on N and, to a lesser extent, on P and Si. Over that period we have learned that surface seawater concentrations of biologically interesting trace elements are much lower than previously thought (Bruland 1983). The pervasive contamination that distorted early measurements of trace elements would have also obfuscated their possible biological role. The application of so-called clean techniques to biological experiments has rekindled oceanographers' interest in the biological and ecological function of trace elements in the sea, particularly the possible role of Fe in limiting primary production in oceanic regions where surface seawater is relatively rich in N and P.Several years ago we proposed that many bioactive elements may be colimiting phytoplankton growth in oceanic waters, that the stoichiometric concepts of Redfield Acknowledgments Funding for this work was provided by grants from NSF (OCE 89-17688) and ONR (N 00014-90-J-1352). We thank the reviewers, K. W. Bruland and W. G. Sunda, for comments.