Significance Iron limits marine production across one third of the surface ocean. The chemical form of iron in these regions is unknown, but it is well established that molecular speciation affects microbial competition for iron uptake. Here we show that the abundance and identity of siderophores, strong iron-binding compounds secreted by microbes to enhance iron uptake, changes across iron-replete and iron-deficient regions of the South Pacific Ocean. In low-iron regions, amphiphilic siderophores are particularly abundant, suggesting a microbial strategy designed to minimize diffusive loss of metabolically expensive compounds while facilitating iron acquisition. Phylogenetic analysis further suggests that the ability to produce amphiphilic siderophores has been transferred across multiple bacterial lineages, suggesting a possible mechanism of adaptation.