Natural organic ligands controlling iron concentration in the water column in the North Pacific Ocean and important role of the organic ligands on the primary production.

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Vertical distributions of Fe(III) hydroxide solubilities and dissolved Fe concentrations, which are strongly related to the concentration and affinity of natural organic Fe(III) chelators in seawater, were measured at three typical stations (0-3500 m) and inside (HP) and outside (LP) a phytoplankton spring bloom patch area (0-250 m) in the northwestern North Pacific Ocean. Iron(III) hydroxide solubility in the surface mixed layer was generally high and variable, corresponding with the depth of high chlorophyll a concentrations. The high Fe(III) solubility in the surface layer was probably due to a high concentration or strong affinity of natural organic Fe(III) chelators, which were possibly released by phytoplankton or bacteria. In culture experiments, the dissolved siderophore-Fe(III) organic complex in seawater supplied biologically available inorganic Fe(III) species, which may determine the iron uptake rate in culture media by providing bioavailable Fe as the siderophore decomposes. The therma, photochemical and microbial decomposition of organic chelators complexed with Fe(III) in aquatic environments is probably one of the most important mechanisms for providing bioavailable inorganic Fe(III) species into thse environments. In middepth and deep waters, the Fe(III) solubility levels appeared to increase with increasing nutrient concentration. The increase in Fe(III) solubility with nutrient concentrations is likely related to the decomposition and transformation of sinking organic matter and is due to the formation of organic Fe(III) chelators through the decomposition of biogenic organic matter. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters are controlled primarily by the Fe(III) complexation with natural organic ligands.