Effects of organic iron complexes on the growth of red tide causative phytoplankton

Iron (Fe) is a crucially essential element for the growth of phytoplankton. Various mechanisms for the biological acquisition of iron require that the dissolved inorganic or free ion species [Fe(II) and Fe(III)] are ultimate iron species taken up by phytoplankton. But, iron mostly dissolves as complexes with the organic ligands and the concentration of dissolved iron is extremely low in natural seawater. The growths of 3 species of red tide causative phytoplankton (Chattonella antiqua, Heterosigma akashiwo and Heterocapsa circularisquama) were examined using a newly developed artificial synthetic medium in the presence of different iron species complexed with organic ligands. Our study demonstrated that the organic iron of different ligands was bioavailable to red tide phytoplankton. In iron salicylate and citrate chelates media, the growths were confirmed for the raphidophyte H. akashiwo and the dinoflagellate H. circularisquama. And all red tide species examined could grow in iron ethylenediaminetetraacetate (EDTA) chelates medium. These results suggest that bioavailable Fe depends not only on dissolved inorganic iron (soluble and colloidal phase) but also on chelates with organic ligands. And further, the natural orga nic matter such as salicylic acid and citric acid may play an important role in forming red tides in coastal area as well as siderophore-like organic compounds and regeneration of iron through bacterial activity.