Most iron (Fe) dissolved in seawater is complexed to organic ligands of unknown composition.  Here we investigate the distribution of siderophores, small organic compounds synthesized by microbes to bind environmental Fe and facilitate its transport into the cell.  Siderophores help relieve Fe-stress and enhance microbial production under Fe-deficient conditions.  Siderophores were measured at all stations sampled across a section of the Pacific Ocean visited by the GEOTRACES GP15 expedition. In the chronically Fe-limited subpolar gyre, Fe-marinobactin and Fe-amphibactin siderophore concentrations reached 66 pM and accounted for up to 80% of dissolved Fe. Ferrioxamine/ferrichrome-like siderophores, including ferricrocin, were abundant near the Alaskan coast. Metal-free and Al-siderophores were also abundant across the section indicating rapid and active Fe-siderophore uptake.  Fe-acquisition via siderophores is a common strategy for microbes inhabiting the upper ocean and likely supplies a large fraction of bioavailable-Fe in Fe-deficient regions.

siderophores, Fe, LC-MS