Metal–organic matter interaction: Ecological roles of ligands in oceanic DOM

The ecological roles of dissolved organic matter (DOM) in seawater have not been well understood. One definite function of DOM stems from its complexation ability with trace metals under the conditions of seawater. A chemical complexation model of the marine system was introduced in order to clarify the ecological roles of strong organic ligands in DOM related to the acquisition of bioactive metals (Cu, Fe and Zn) by phytoplankton, assuming that two types of strong organic ligands coexist in oceanic DOM and complexes with bioactive metals. The results reveal that the weaker organic ligand, rather than the stronger one, plays a significant role in the reduction of Cu toxicity for phytoplankton growth. It is suggested that the presence of reactions with Cu that are competitive to the strong organic ligand causes extremely low Fe concentrations in seawater and leads to Fe deficiency for phytoplankton growth. Therefore, it is concluded that the strong ligands in DOM play a chemical role in controlling free ion concentration levels of bioactive metals in the marine environment.