Recent studies demonstrate that the dissolved iron in seawater is bound to strong organic complexes that have stability constants comparable to those of microbial iron chelates (siderophores). We investigated iron acquisition by 7 strains of heterotrophic marine bacteria using siderophores as a model for the natural iron-binding ligands. Siderophores were detected in the supernatants of 4 strains. All strains utilized iron bound to siderophores regardless of whether they produced their own. The majority took up iron bound to the fungal siderophore desferrioxamine B (dfoB). Over half also utilized iron bound to strain Neptune's siderophore, nep-L, while iron bound to pwf-L was available solely to the producing strain, Pwf3. Uptake rates of Fe-siderophores were similar among iron-limited strains and among ligands. Transport of Fe-dfoB in Neptune was enhanced 20 times by iron limitation. The half-saturation constant of Fe-dfoB transport was 15 nM, the lowest reported for Fe-siderophore transport in microorganisms. In contrast, uptake of inorganic iron (Fe' ) by iron-limited Neptune did not saturate at the highest concentration tested and was not upregulated under iron stress. This suggests that Fe ' uptake occurs by simple diffusion through the outer membrane.