Redox Transformations of Iron in the Presence of Exudate from the Cyanobacterium Microcystis aeruginosa under Conditions Typical of Natural Waters.

Interaction of the exudate secreted by a toxic strain of the cyanobacterium Microcystis aeruginosa with Fe(II) and Fe(III) was investigated here under both acidic (pH 4) and alkaline (pH 8) conditions. At the concentrations of iron and exudate used, iron was present as dissolved iron (<0.025 μm) at pH 4 but principally as small (<0.45 μm) iron oxyhydroxide particles at pH 8 with only ∼3-27% present in the dissolved form as a result of iron binding by the organic exudate. The formation of strong Fe(III) exudate and relatively weak Fe(II) exudate complexes alters the reduction potential of the Fe(III)-Fe(II) redox couple, facilitating more-rapid oxidation of Fe(II) at pH 4 and 8 than was the case in the absence of exudate. Our results further show that the organic exudate contains Fe(III)-reducing moieties, resulting in the production of measurable concentrations of Fe(II). However, these reducing moieties are short-lived (with a half-life of 1.9 h) and easily oxidized in air-saturated environments. A kinetic model was developed that adequately describes the redox transformation of Fe in the presence of exudate both at pH 4 and pH 8.