Photodegradation of algae and macrophyte-derived dissolved organic matter: A multi-method assessment of DOM transformation

Due to the frequent occurrence of algal bloom and eutrophication, algae and macrophyte-derived dissolved organic matter (DOM) play a more important role in the lake environment. In the study, algae and macrophyte-derived DOM were exposed for 30 days either in the dark or light in order to develop a broader understanding of their photochemical and microbial transformation. Changes of quantification and quality of DOM were characterized by Uv-vis spectroscopy, fluorescence spectroscopy, Fourier infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Simulated solar UV exposure (30 d) without sterilization removed 49.7% and 42.4% of the dissolved organic carbon (DOC) from chlorella and water chestnut DOM, respectively. In contrast, 31.6% and 22.2% of DOC were removed under dark conditions accordingly. Compared with humic OM, autochthonous DOM was more biolabile and less photolabile, with less photochemical removal of DOC in this study. Molecular structures of studied DOM exhibited selective alterations during incubation. Fluorescence spectroscopy revealed that fulvic acid-like substances were susceptible to photodegradation, while protein-like substances were susceptible to both photodegradation and biodegradation. Furthermore, analytic results of carbon functional groups suggested that polysaccharides were more biolabile; aromatics were more photolabile; and phenols/aromatic proteins were both biolabile and photolabile. The higher proportion of aliphatics and lower proportion of aromatics in autochthonous DOM might be the primary cause of their greater biodegradation and weaker photodegradation.