Organic exudates promote Fe(II) oxidation in Fe limited cultures of Trichodesmium erythraeum

Abstract. A luminol chemiluminescence method was employed to study the oxidation kinetics of Fe(II) in both the absence and the presence of organic exudates released by the marine cyanobacterium Trichodesmium erythraeum. The apparent Fe(II) oxidation rate constant was studied for batch cultures grown with varying Fe' concentrations of 0.05–0.29 and 1.44–2.03 nmol L⁻¹ at pH ranges from 8.1–8.6, corresponding to the change in pH in the cultures during the entire growth cycle. Fe(II) oxidation was accelerated when cells were growing exponentially and gradually decreased towards the stationary phase, consistent with the presence of organic exudates. The best fit of the kinetic model to the data also demonstrated clear differences in apparent Fe(II) oxidation rate constants during different growth phases. However, no significant difference was observed in oxidation rate constants between the two Fe' treatments. These findings suggest that Trichodesmium releases organic compounds into the extracellular environment that influence Fe redox chemistry, potentially affecting Fe bioavailability, and that the nature of the Fe(II) complexes formed is not influenced by Fe limitation of the organism's growth.