EVIDENCE FOR THE IMPORTANCE OF CATECHOL-TYPE SIDEROPHORES IN THE IRON-LIMITED GROWTH OF A CYANOBACTERIUM

To compensate for low levels of available iron, cyanobacteria may prod uce siderophores to assist in the scavenging of iron from the environm ent. In this paper we examine the role of catechol-type siderophores p roduced by the halotolerant 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 decrease s in growth rate in the iron-replete cultures. These results are suppo rted by experiments with continuous culture chemostats where the addit ion of BSA to steady-state cultures leads to the washout of cells from low-iron chemostats, indicating that the cellular growth rate was red uced. The addition of BSA to shea-term iron assimilation experiments f urther demonstrates that the presence of BSA can induce uptake kinetic s consistent with the activity of an ''iron-shuttle,'' while BSA itsel f has no affinity for iron. These results demonstrate that catechol-ty pe siderophores associated with the surface of the cell play an import ant role as ''iron custodians.'' While the presence of these catechols introduces complexity in the iron-transport mechanism and decreases t he maximum velocity of iron uptake during episodic pulses of iron, the presence of the catechol associated with the cell surface functions t o increase the overall cellular affinity for iron in low-iron environm ents.