The influence of light on the availability of photoreactive and photostable iron(III)–ligand complexes to diatoms

Abstract

AbstractUptake of iron (Fe) by eukaryotic phytoplankton is suggested to proceed by a reductive pathway in which Fe(III) bound to inorganic and organic ligands is reduced prior to Fe internalization. Photochemical reduction of the Fe–ligand complexes can increase Fe bioavailability, but the role of light in regulating physiological processes related to Fe acquisition is not well known. Here, we report how model Fe–ligand complexes differing in photolability affected growth, cellular Fe reduction, and Fe uptake rates of two marine diatoms as a function of irradiance. Growth rates of Thalassiosira oceanica and Phaeodactylum tricornutum in media amended with Fe complexed with ethylenediaminetetraacetic acid (Fe‐EDTA) and desferrioxamine B (FeDFB) increased linearly with light between 50 and 400 μmol photons m−2 s−1 under Fe‐limiting conditions. Steady‐state Fe uptake rates (ρSS) were also light dependent, increasing proportionally with growth irradiance for both Fe–ligand complexes. Photolysis of the Fe‐EDTA complex could explain the increase in ρSS in Fe‐EDTA‐amended medium because it increased [Fe(III)’], but it could not account for the faster rate of Fe uptake from FeDFB at high light because FeDFB is photostable. Short‐term Fe uptake rates measured in the absence of photochemically mediated Fe(III) reduction showed that cells preconditioned to high growth irradiance took up Fe faster than cells grown at low light. Thus, growth at high light induced a physiological response that increased Fe uptake. High growth irradiance also increased the rate of cellular Fe(III)’ reduction in the dark, similar to its stimulatory effect on short‐term Fe uptake. These results identify a previously unrecognized Fe–light interaction that could be important in enhancing Fe availability to diatoms. We suggest that the light effect could be the result of a light‐dependent regulation of cellular Fe(III) reduction.