Photodecomposition of natural organic metal-binding ligands from deep seawater

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-binding 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 caused a significant decrease in the concentrations of both ligands, along with the loss of fluorescent dissolved organic matters. The relative decrease of iron-binding ligands, which are considered to be mainly composed of humic substances, was larger than that of copper-binding ligands or humic-like fluorescence. This may be attributable to relatively high photoreactivity of hydrophilic fluvic acids, which have higher affinity to ferric 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 fluorescence of humic-like substances. In conclusion, the upwelling of deep seawater into the sunlit surface layer can result in the rapid loss of the complexation capacity for both iron and copper ions. The stronger ligands for both metals, which were readily decomposed by sunlight irradiation, appears to have a similar composition, suggesting that competition could be important to determine the solubility and bioavailability of iron and copper