To compensate for low levels of available iron, cyanobacteria may produce siderophores to assist in the scavenging of iron from the environment. In this paper we examine the role of catechol-type siderophores produced by the halo-tolerant cyanobacterium Synechococcus sp. PCC 7002 in the acquisition of iron from a chelated source. To inhibit catechol-type siderophore mediated iron transport, bovine serum albumin (BSA) was added to iron-deficient and replete cultures. Batch culture growth rates and cellular photosynthetic pigments decreased markedly in iron-limited populations in the presence of BSA, with no apparent decreases in growth rate in the iron-replete cultures. These results are supported by experiments with continuous culture chemostats where the addition of BSA to steady-state cultures leads to the washout of cells from low-iron chemostats, indicating that the cellular growth rate was reduced. The addition of BSA to short-term iron assimilation experiments further demonstrates that the presence of BSA can induce uptake kinetics consistent with the activity of an 'iron-shuttle,' while BSA itself has no affinity for iron. These results demonstrate that catechol-type siderophores associated with the surface of the cell play an important role as 'iron custodians.' While the presence of these catechols introduces complexity in the iron-transport mechanism and decreases the maximum velocity of iron uptake during episodic pulses of iron, the presence of the catechol associated with the cell surface functions to increase the overall cellular affinity for iron in low-iron environments.