Trichodesmium colonies contain an abundant microbial consortium that is likely to play a role in nutrient cycling within the colony. This study used laboratory cultures of Trichodesmium and two genome-sequenced strains of bacteria typical of Trichodesmium-associated microbes to develop an understanding of the cycling of iron, a potentially limiting micronutrient, within Trichodesmium colonies. We found that the ferric siderophores desferrioxamine B and aerobactin were not readily bioavailable to Trichodesmium, relative to ferric chloride or citrate-associated iron. In contrast, the representative bacterial strains we studied were able to acquire iron from all of the iron sources, implying that naturally occurring Trichodesmium-associated bacteria may be capable of utilizing a more diverse array of iron sources than Trichodesmium. From the organism-specific uptake data collected in this study, a theoretical Trichodesmium colony was designed to model whole colony iron uptake. The bacteria accounted for most (> 70%) of the iron acquired by the colony, highlighting the importance of determining organism-specific uptake in a complex environment. Our findings suggest that, although they may share the same micro-environment, Trichodesmium and its colony-associated microbial cohort may differ substantially in terms of iron acquisition strategy. © 2011 Society for Applied Microbiology and Blackwell Publishing Ltd.