Secretion of iron-complexing ligands from Closterium aciculare (Charophyceae, Chlorophyta) under iron-deficient conditions

Abstract

Siderophores, ferric iron-specific ligands, are produced by many species of prokaryotes, fungi and higher plants under iron-limited conditions. We report the possibility of siderophore production by a freshwater eukaryotic microalga Closterium aciculare under iron-deficient conditions. Closterium aciculare secreted a substance that is reactive in the chrome azurol S (CAS) assay in an iron-deficient, chemically defined medium. The final concentrations of the CAS-reactive substance were 0.17 and 0.35 μM for some 0.1 g 1−1 of C. aciculare in the medium with noniron and 10 nM iron addition, respectively. The amount of the CAS-reactive substance increased along with the increase in cell density of C. aciculare, whereas the addition of iron with nonlimiting concentration (100 nM) to the medium completely repressed the production of the CAS-reactive substance. Because C. aciculare did not produce the CAS-reactive substance in the iron-replete medium, the CAS-reactive substance was considered to be a siderophore.