Competitive
Effects of Calcium and Magnesium Ions
on the Photochemical Transformation and Associated Cellular Uptake
of Iron by the Freshwater Cyanobacterial Phytoplankton Microcystis
aeruginosa
posted on 2015-08-04, 00:00authored byManabu 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.