Photodecomposition of natural organic metal-binding ligands from deep seawater

Abstract:

To reveal the effect of solar irradiation on the biogeochemical cycling of trace metals in the open ocean, the photodecomposition of organic iron- and copper-biding ligands was examined in an experiment using filtered seawater collected from a deep layer of the East China Sea. Six days of natural solar irradiation significantly decreased the concentrations of both ligands. The amount of decrease was similar between the two ligands, thus suggesting that the decomposed ligands were common to both iron and copper ions. The concentrations of fluorescent dissolved organic materials, which were determined fluorometrically, also decreased during six days of solar irradiation, and humic-like substances were decomposed more rapidly as compared with protein-like fluorescent compounds. The ligands may have been humic substances as these can be complexed with both ferric and cupric ions. Surprisingly, an accumulation of both iron- and copper-binding ligands was observed in the dark control bottles, but without a significant change in the concentration of humic substances. In conclusion, the upwelling of deep seawater into the sunlit surface layer can result in the rapid loss of the complexation capacity for iron and copper ions. Moreover, the competition for ligands between the two ions can exaggerate the effect of photodecomposition on the solubility, bioavailability, and toxicity of the two ions.