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Light-Mediated Reactive Oxygen Species Generation and Iron Redox Transformations in the Presence of Exudate from the Cyanobacterium Microcystis aeruginosa.

The photochemical properties of the organic exudate secreted by a toxic strain of Microcystis aeruginosa were studied by measuring reactive oxygen species (ROS) generation and redox transformations of iron in the presence of the organic exudate under acidic (pH 4) and alkaline (pH 8) conditions. Our results show that the organic exudate generates nanomolar concentrations of superoxide and hydrogen peroxide on irradiation with simulated sunlight in a manner consistent with that reported for terrigenous natural organic matter. The photo-generated superoxide plays an important role in Fe(III) reduction under alkaline conditions with nearly 45% of the observed Fe(II) generation on Fe(III) reduction occurring via Fe(III) reduction by superoxide while the rest of the Fe(III) reduction occurs via a ligand-to-metal charge transfer (LMCT) pathway. In contrast, under acidic conditions, 100% of the observed photochemical Fe(II) generation on Fe(III) reduction occurs via a LMCT pathway. These results suggest that steady-state dissolved Fe concentrations and hence Fe availability in natural waters will significantly increase in the presence of these algal exudates. Furthermore, significant diel variation in Fe(II) concentration is to be expected, even in acidic waters, since time scales of light-mediated Fe(III) reduction and thermal Fe(III) reduction differ markedly. A kinetic model is developed that adequately describes both the generation of ROS and the photochemical redox transformations of iron in the presence of M. aeruginosa exudate.