Competitive Effects of Calcium and Magnesium Ions on the Photochemical Transformation and Associated Cellular Uptake of Iron by the Freshwater Cyanobacterial Phytoplankton Microcystis aeruginosa

Photochemical reduction of iron and iron uptake by Microcystis were investigated in a freshwater medium (pH 8) containing a range of calcium (Ca) and magnesium (Mg) ion concentrations (0.002–20 mM). In a medium containing the chelator ethylene­diamine­tetra­acetic acid (EDTA), 50-fold increases in net photochemical formation rates of unchelated ferrous iron (Fe­(II)′) were observed as the concentration of calcium or magnesium metal (Me) was increased to exceed the concentration of EDTA. Kinetic modeling of iron transformation processes indicated that the facilitated Fe­(II)′ formation is attributed to Me-promoted photo­reductive dissociation of the ferric iron-EDTA complex. In the medium containing Suwanee River fulvic acid, in contrast, the competitive effect of Me on photo­chemical Fe­(II)′ formation appears to be negligible due to the weak binding affinities of fulvic acid to Me. The cellular iron uptake rate in the EDTA-buffered system increased by ∼3-fold in the excess Me condition where the increased rate of photo­chemical Fe­(II)′ formation was observed, whereas the presence of Me resulted in a decrease in iron uptake rate in the fulvic acid system (by up to 5-fold). The decrease in iron uptake is likely caused by Me binding to iron transporters and other entities involved in intracellular iron transport. The findings of this study indicate a significant effect of Ca and Mg concentrations in natural waters on iron uptake by Microcystis, with the magnitude of effect depending strongly on ligand type.