Photochemical, microbial and metal complexation behavior of fluorescent dissolved organic matter in the aquatic environments

Chemical properties and reactivity of fluorescent dissolved organic matter (FDOM) are examined in this paper. They are key issues to understand the biogeochemical processes in the aquatic environments. Typically, FDOM undergoes photochemical transformation and is recalcitrant to microbial degradation, except for the aromatic amino acids that are microbiologically degraded under dark conditions. Experimental results demonstrate that the fluorescence intensity of various FDOM components is depleted upon irradiation (in the hours to 70 days time scale), approximately by 20-85% for fulvic acid, by 12-95% for fluorescent whitening agents (FWAs) or commercial detergents, and by 5-60% for trytophan. Microbial degradation is able to decompose the amino acid tryptophan and similar compounds, by approximately 13-24% in unfiltered river waters, 67% in unfiltered sewerage samples, and 11% in filtered river samples. The photoreactivity of FDOM is greatly decreased when passing from freshwater (river and lakes) to marine waters, but deep waters in lakes or marine environments are often more sensitive to photodegradation processes than surface waters. The high reactivity of FDOM toward photodegradation could be understood on the basis of its (however complex) chemical structure, considering that many FDOM components can undergo photoionization or otherwise photosensitized oxidation under sunlight. The controlling factors to the photochemical and microbial degradation of FDOM for a variety of waters are extensively discussed. One of the important functions of FDOM is the formation of complexes with transition metals in the aquatic environments, and this review discusses the mechanisms by which FDOM interacts with metals. Further investigations on FDOM, namely the identification of still unknown FDOM components, the metal-FDOM interactions as well as the photochemical and microbial reactivity will give invaluable information on the DOM dynamics in the aquatic environments.