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

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posted on 2015-08-04, 00:00 authored by Manabu Fujii, Anna C. Y. Yeung, T. David Waite
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.

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