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Iron-limited cyanobacterial cells are generally considered to acquire extracellular iron through a siderophore-dependent system, although evidence has started to accumulate that other, as yet poorly characterized, iron acquisition systems may also play a role. Iron-limited cells of the cyanobacterium Anabaena flos-aquae (Lyng.) Breb. are well known to produce the relatively low Fe(III) affinity dihydroxamate siderophore schizokinen. In this set of experiments we show that iron-limited A. flos-aquae cells (i) acquired iron at substantial rates in the absence of the schizokinen and (ii) acquired iron from a bacterial siderophore (the trihydroxamate molecule desferrioxamine B (DFB)), and also a synthetic chelator (N, N-bis(2-(bis(carboxymethyl)amino)ethyl)glycine (DTPA)), with substantially higher affinities for Fe(III) than schizokinen, indicating that a schizokinen-independent iron acquisition pathway was operating. We suggest that there exists a siderophoreindependent iron acquisition system that is able to acquire Fe(III) from high stability Fe(III)-chelates, which are not accessible to iron-limited cells via the schizokinen-based system. As well, we present two possible models for iron acquisition by iron-limited A. flos-aquae cells. Both of these models suggest that there are two major routes for Fe(III) entry into the periplasm of iron-limited cells: (1) the well-characterized siderophore (schizokinen) dependent process and (2) a siderophore-independent process that is able to access Fe(III) sources not biologically available to the schizokinen system. Key words: Anabaena, cyanobacteria, iron limitation, iron uptake, outer membrane, schizokinen, siderophores, siderophoreindependent. On considere generalement que les cellules cyanobacteriennes limitees par le fer acquierent du fer extracellulaire par un systeme dependant de siderophores, bien que la preuve commence a s'accumuler a l'effet que d'autres systemes d'acquisition du fer encore peu documentes puissent egalement jouer un role. On sait tres bien que les cellules de la cyanobacterie Anabaena flos-aquae (Lyng.) Breb. limitees par le fer, produisent la schizochinene sous forme de siderophore a dihydroxamate a relativement faible affinite pour le Fe(III). Dans cet ensemble d'experiences, les auteurs montrent que les cellules de l'A. flos-aquae limitees par le fer (i) acquierent du fer a des taux substantiels en absence de la schizochinene, et (ii) acquierent du fer a partir d' un siderophore bacterien (la molecule trihydroxamate desferrioxamine B (DFB), et aussi d' un chelateur synthetique (N,N-bis(2-(bis(carboxymethyl)amino)ethyl) glycine (DTPA)), ayant une affinite substantielle plus forte pour le Fe(III) que pour la schizokinene, indiquant qu' un sentier d' acquisition du fer independante de la schizochinene est intervenu. Les auteurs suggerent qu'il existe un systeme d'acquisition du fer independant, capable d'acquerir le Fe(III) a partir de chelates de Fe(III) a haute stabilite N'etant pas accessible aux cellules via le systeme base sur la schizochinene. De plus, les auteurs presentent deux modeles possibles pour l'acquisition du fer par les cellules de l'A. flos-aquae. Les deux modeles suggerent qu'il y aurait deux routes principales pour permettre l'entree du Fe(III) dans le periplasme des cellules limitees par le fer : (1) le processus bien caracterise du processus dependant d'un siderophore (Schizokinene) et (2) un processus independant du siderophore capable d'acceder a des sources de Fe(III) non biologiquement accessibles au systeme impliquant la schizokinene. Mots-cles : Anabaena, cyanobacterie, limitation par le fer, absorption du fer, membrane externe, schizokinene, siderophore, independant de siderophores. [Traduit par la Redaction], Introduction Iron is an essential element for almost all organisms and photosynthetic organisms have an especially high iron requirement, mainly owing to the high iron content of the photosynthetic apparatus. [...]

The x-axis labels for Figs. 2 and 3 should be micromolar (μM) rather than millimolar (mM). Ref.: Botany, 90(3): 181-190 (2012). doi: 10.1139/B2012-104 Mathew B. Sonier, Daniel A. Contreras, Ron [...]