Effect of Fe(II) and Fe(III) Transformation Kinetics on Iron Acquisition by a Toxic Strain of Microcystis aeruginosa

We have investigated the mechanism of Fe uptake by a toxic strain of the freshwater cyanobacterium Microcystis aeruginosa (PCC7806) with particular attention given to the effect of Fe(II) and Fe(III) transformation kinetics on Fe uptake. Chemiluminescence analysis revealed that M. aeruginosa produces extracellular superoxide (a moderate Fe reducing agent) at rates of 0.4-1.2 amol cell(-1) h(-1) depending on initial Fe concentration in the culture medium. Short-term assimilation assays using Fe-55 showed that reduction of Fe(III) in both organic and inorganic forms by cell-generated superoxide or ascorbate facilitated Fe uptake via formation of unchelated Fe(II), when Fe availability was low because of the use of the strong Fe chelator ethylenediaminetetra acetate (EDTA) as a ligand. In contrast, Fe reduction was unimportant for Fe uptake in the presence of low concentrations (<= 100 mu M) of the weak Fe-binding ligand citrate because of a high concentration of unchelated Fe(III), indicating that the contribution of reduction to Fe uptake depends on the nature of Fe binding and availability of unchelated Fe(III) in the external medium. A kinetic model incorporating uptake of both unchelated Fe(II) and Fe(III) and based on similar models developed for marine microalgae successfully described Fe uptake rates by M. aeruginosa PCC7806.