Microbes frequently navigate the environment with the help of small, excreted metabolites. Iron-binding molecules called siderophores are one such set of secondary metabolites that are commonly used by microbes to access the essential trace element iron. Although many marine microbes produce siderophores, a substantial number, including the highly abundant SAR11 clade of Pelagibacterales, do not and it has remained unclear whether such non-producers can access siderophore-bound iron. Here, we show that iron-limited SAR11 cultures fail to grow in the presence of the hydroxamate siderophore ferrichrome but exhibit robust growth in the presence of the catechol siderophore enterobactin. We confirm that this is linked to iron availability using transcriptomic and 55Fe radio tracer uptake experiments. This phenotype can be explained by the relative lability of enterobactin-bound iron in seawater, a phenomenon that has been previously observed in field studies and which we demonstrate with a simple kinetic model. Further experiments with the marine heterotrophs Phaeobacter inhibens and Vibrio harveyi suggest that enterobactin-Fe is unlikely to support the faster growth rates of these organisms without the use of biochemical uptake mechanisms. Overall, our work provides a model of siderophore use that considers bioavailability conferred through both kinetic and biochemical mechanisms and shows that some catechol-bound Fe may be widely available to small, slow growing marine organisms.