Abstract
Iron is an especially essential element for the growth of phytoplankton and its deficiency is well known to suppress primary productivity in both freshwater and marine ecosystems. We discovered iron-complexing ligands secreted by a freshwater eukaryotic phytoplankton, Closterium aciculare in an iron-deficient chemically defined medium. To investigate the character of the siderophore-like substance secreted from C. aciculare, growth experiments were carried out for several phytoplankton species using the culture filtrate (<0.4 μm) of C. aciculare after incubation under iron-deficient conditions. Addition of the culture filtrate of C. aciculare enhanced growth in the green algae C. aciculare, Pediastrum simplex, Staurastrum paradoxum and the diatom Aulacoseria granulata. However, addition of the filtrate suppressed that of the green alga Cosmocladium constrictum, and did not affect growth in the cyanobacteria Anabaena spiroides or Microcystis wesenbergii. These results suggest that the substances secreted from C. aciculare have species-specific growth-promoting effects and may control phytoplankton growth under iron-deficient conditions.
