Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands

Iron is a limiting nutrient for primary production in large areas of the oc eans(1-4). Dissolved iron(III) in the upper oceans occurs almost entirely i n the form of complexes with strong organic ligands(5-7) presumed to be of biological origin(8,9). Although the importance of organic ligands to aquat ic iron cycling is becoming clear, the mechanism by which they are involved in this process remains uncertain. Here we report observations of photoche mical reactions involving Fe(III) bound to siderophores-high-affinity iron( III) ligands produced by bacteria to facilitate iron acquisition(10-12). We show that photolysis of Fe(III)-siderophore complexes leads to the formati on of lower-affinity Fe(III) ligands and the reduction of Fe(III), increasi ng the availability of siderophore-bound iron for uptake by planktonic asse mblages. These photochemical reactions are mediated by the alpha -hydroxy a cid moiety, a group which has generally been found to be present in the mar ine siderophores that have been characterized(13-15). We suggest that Fe(II I)-binding ligands can enhance the photolytic production of reactive iron s pecies in the euphotic zone and so influence iron availability in aquatic s ystems.