Iron acquisition by cyanobacteria: siderophore production and iron transport by Anabaena

The cyanobacterium Anabaena sp. (PCC #6411) is known to produce the siderophore, schizokinen, in response to iron limitation. Environmental factors which influence schizokinen production have been examined utilizing a bioassay method based upon the stimulation of growth of the siderophore-requiring bacterium Arthrobacter flavescens JG-9. Schizokinen production by Anabaena increased in low-iron media and was stimulated approximately five-fold media containing one millimolar citric acid. Growth of Anabaena under nitrogen-fixing conditions reduced extracellular schizokinen concentrations 3-4 fold, in late growth phase, compared to cells grown on nitrate. Cells grown in ammonium-containing medium yielded intermediate concentrations of schizokinen. The Csaky assay, which is often used to detect hydroxamate siderophores, was found to be subject to interferences that can yield erroneously high values. The problems associated with detection and quantitation of cyanobacterial siderophores are discussed. Anabaena was found to utilize schizokinen to accumulate \u3e 90% of the ((\u2755)Fe)-ferric iron added to the medium. Iron transport capability was increased in iron-starved cells. The transport system appears to be fairly specific for schizokinen, in that an acetylated derivitive of schizokinen, also supported iron transport, but the structurally related siderophore, aerobactin, and the trihydroxamate siderophore, ferrioxamine B, did not support iron uptake by Anabaena. The uptake of ferric schizokinen displayed saturation-type kinetics with an apparent K(,M) of 35 nM, and required the input of metabolic energy. Lightdriven transport was blocked by uncouplers and ATPase inhibitors. Transport in dark-adapted cells was additionally blocked by inhibitors of respiration. We conclude that ATP serves as an energy source for the cellular uptake of schizokinen. Two other kinds of Anabaena were examined for siderophore production. Anabaena sp. (PCC #7120) was found to produce a substance which stimulates the growth of the Arthrobacter JG-9. This substance binds iron and has a similar absorbance maximum to schizokinen in ferric-perchlorate solutions. Anabaena 7120 also utilized ferric schizokinen and ferric acetyl-schizokinen for iron uptake, but not aerobactin or ferrioxamine B. These results suggest that the iron transports systems of Anabaena 6411 and 7120 may be the same. Anabaena cylindrica Lemm. apparently does not produce or utilize a schizokinen-type siderophore. No Arthrobacter JG-9 stimulating material could be detected in low-iron filtrates, even if concentrated 25-fold