SIDEROPHORE‐INDEPENDENT IRON UPTAKE BY IRON‐LIMITED CELLS OF THE CYANOBACTERIUM ANABAENA FLOS‐AQUAE 1

Iron acquisition by iron‐limited cyanobacteria is typically considered to be mediated mainly by siderophores, iron‐chelating molecules released by iron‐limited cyanobacteria into the environment. In this set of experiments, iron uptake by iron‐limited cells of the cyanobacterium Anabaena flos‐aquae (L.) Bory was investigated in cells resuspended in siderophore‐free medium. Removal of siderophores decreased iron‐uptake rates by ∼60% compared to siderophore‐replete conditions; however, substantial rates of iron uptake remained. In the absence of siderophores, Fe(III) uptake was much more rapid from a weaker synthetic chelator [ N ‐(2‐hydroxyethyl)ethylenediamine‐ N , N′ , N′ ‐triacetic acid (HEDTA); log K cond  = 28.64 for Fe(III)HEDTA(OH) − ] than from a very strong chelator [ N,N′ ‐bis(2‐hydroxybenzyl)‐ethylenediamine‐ N,N′ ‐diacetic acid (HBED); log K cond  = 31.40 for Fe(III)HBED − ], and increasing chelator:Fe(III) ratios decreased the Fe(III)‐uptake rate; these results were evident in both short‐term (4 h; absence of siderophores) and long‐term (116 h; presence of siderophores) experiments. However, free (nonchelated) Fe(III) provided the most rapid iron uptake in siderophore‐free conditions. The results of the short‐term experiments are consistent with an Fe(III)‐binding/uptake mechanism associated with the cyanobacterial outer membrane that operates independently of extracellular siderophores. Iron uptake was inhibited by temperature‐shock treatments of the cells and by metabolically compromising the cells with diphenyleneiodonium; this finding indicates that the process is dependent on active metabolism to operate and is not simply a passive Fe(III)‐binding mechanism. Overall, these results point to an important, siderophore‐independent iron‐acquisition mechanism by iron‐limited cyanobacterial cells.