The distribution, mobility, bioavailability, toxicity, bioaccumulation and biodegradability of chemical elements depend not only on their concentrations, but also on their physico-chemical associations with natural systems. [1][2][3][4] Several trace metals occur in natural water distributed between different oxidation states. At equilibrium, the species present are controlled not only by the pH, but also by the redox potential of the system that describes the progress of electron-transfer reactions. 5 The dissolved forms of trace elements are mainly present as free hydrated ions, ion pairs (where the coordinated water is retained), and inorganic and organic complexes with a covalent bound. For example, dissolved trace metals can also form complexes with a wide range of organic and inorganic compounds, depending on the pH, oxidation state, and relative abundance of the complexing agents. The main inorganic compounds are chloride, carbonate, sulfate, hydroxyl, and fluoride; the complexes formed between them and trace metals are almost always very labile. Organic ligands include both biochemical species of low molecular weight (<10 3 ), such as siderophores, carboxylic acids, aminoacids, sugars, small hydroxyacids, 6 etc., and heterogeneous compounds of unknown structure 7-9 with a wide range of relative molecular masses (10 3 -10 7 ). At any rate, there is great certitude that stronger ligands, which are able to complex metal ions, but are not yet completely identified, are present in seawater, most often at very low concentrations. These various forms of organic matters in aquatic systems come largely from living organisms, mainly as polypeptides and polysaccharides in algae exudates, 10 and from pollution sources. Estimating the ligand organic concentration in natural water has until now been a difficult problem. Generally, though the water of open-ocean contains only small amounts of organic matter.The maximum concentration of total organic carbon observed in surface waters is less than 200 µmol/L. 11 Especially prominent among organic matter compounds in aqueous systems are humic substances. These compounds, which result from living matter decay, are a mixture of organic polymers containing charged hydrophobic and hydrophilic groups that possess polyelectrolyte and multifunctional proprieties. In solution, these compounds can occur either in free form, often negatively charged, or bound to others, such as glycine, aspartic acid and alanine. The principle functional groups in humic matter are -COOH, -NH2, -OH and -SH, [12][13][14] It is well-documented that organic compounds form strong complexes with most metals in aquatic systems, and that seawater is a complex medium which contains a large variety of organic and inorganic ligands, including colloidal matter. We suggest that most trace metals are complexed in seawater and that some inorganic metals complexes are either labile or not stable. In contrast, metal-organic complexes are often stable and need various and specific treatments to be dissociated. In t...