The bioavailability and component characteristics of the aging dissolved organic matter (DOM) from the macroalgae Ulva prolifera in seawater.

The world's largest green tide, caused by Ulva prolifera, in the Yellow Sea negatively affects the social and economic development of China's coastal region. The dissolved organic matter (DOM) released from U. prolifera is a potential threat to the offshore ecological health. The bioavailability and component characteristics of the DOM from U. prolifera vary at different aging stages in marine environments and are poorly understood. Here, a 1-year incubation experiment was conducted to understand this phenomenon. The bioavailable DOM (BDOM) mainly comprised the labile bioavailable DOM (LBDOM), which accounts for 58.7% (in dissolved organic nitrogen (DON)%) in the aging stages of 0-12 days. LBDOM mainly comprised three-dimensional fluorescent tyrosine B and tryptophan T components. The semi-labile bioavailable DOM (SLBDOM) accounted for 29.2% in the aging stages of 12-80 days, which mainly comprised fulvic acid C1 and A components. Both LBDOM and SLBDOM mainly contained amine groups in their structures. Conversely, the refractory bioavailable DOM (RBDOM) accounted for only 6.50% in the aging stages of 80-365 days. RBDOM mainly comprised a humin-like acid C2 component, whereas refractory dissolved organic matter (RDOM) mainly comprised humin-like acid C2, E, and fulvic acid A components, and the structures of both mainly included functional groups, such as hydroxyl, carboxyl, alkynyl, and aromatic rings. The hydrophobic component and dissolved organic carbon (DOC) proportion increased with the aging of DOM in seawater. Thus, the aging of DOM in seawater represents the processes of aromatization, hydrophobization, and humification, wherein the SUVA, SUVA, and humification index (HIX) increased by 55.7, 42.9, and 133.0%, respectively. The results of our study contribute to a deeper understanding of the ecological effects of DOM released during the extinction process of U. prolifera in the ocean.