lron transport in marine phytoplankton: Kinetics of cellular and medium coordination reactions

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Abstract
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.