Abstract:Dunaliella salina is a microalgae adapted to a high-salinity seawater environment. As a major primary producer in the ocean, it produces and releases a large amount of marine autochthonous dissolved organic matter (DOM) into the environment through photosynthesis. Under suitable conditions (e.g., pH and ionic strength), the produced DOM can transform into transparent exopolymer particles (TEPs) via polymerization. Algal cells and bacteria also release large amounts of dissolved polysaccharides in the water column, and polysaccharide-rich fractions are good precursors for TEP formation. These precursors can form a large number of TEPs through coagulation, gelation, and annealing. In addition, TEPs can be generated through abiotic processes, and TEPs are formed by DOM at the microscale through adsorption on surfaces and foaming. Photochemical reactions affect TEP formation on the ocean surface. TEP formation on the surface layer promotes DOM transport from the sea surface to the deep sea. The photochemical process of DOM in the ocean can convert large molecules of DOM and TEPs into small molecules. Subsequently, gases such as carbon dioxide are released during conversion. This process is a key factor driving the changes in oceanic DOM reservoirs, cycling of matter in seawater, and sequestration of deep-sea carbon. In this study, the changes in the components of DOM, carbohydrates (polysaccharides and monosaccharides), and TEPs mediated by D. salina and their interrelationships under light conditions were investigated by conducting 60 h light irradiation experiments on algal sap during stable growth. Results showed that the photodegradation of CDOM in algal-free environments led to the cleavage of macromolecular compounds to form small-molecule compounds or their decomposition into inorganic substances, which produced larger amounts of monosaccharides. In microalgal environments, photochemical reactions facilitated DOM production due to the influence of algae, and polysaccharide production was increased. Through the three-dimensional fluorescence spectroscopy-parallel factor analysis model, fluorescent DOM, five fluorescent fractions, three protein-like fractions (C1, C2, and C3), and two humus-like fractions (C4 and C5) were identified. In both algal and algal-free environments, the tryptophan-like groups were predominant, and DOM was mostly derived from the products of algal photosynthesis and death decomposition. Although photodegradation is an important process of TEP loss, DOM still undergoes photopolymerization for spontaneous coalescence to form TEPs. Algae and microorganisms also release new TEPs, but the amount of their release and photopolymerization is smaller than the amount of photodegradation. Moreover, correlation studies revealed no significant correlation between carbohydrates (polysaccharides and monosaccharides) and TEPs in the algal-free and microalgae environments. Polysaccharides (R²=0.822, P＜0.05) and monosaccharides (R²=0.821, P＜0.05) showed a significant negative correlation with TEP concentration in the microalgae environment, whereas CDOM and TEP showed a positive correlation in the algal-free environment (R²=0.698, P＜0.05) and a weak negative correlation in the algal environment (R²=0.612, P=0.07). This result indicated that microalgae significantly affected the photochemical transformation between CDOM, carbohydrates (polysaccharides and monosaccharides), and TEP. This study may serve as a basis for elucidating the mechanisms of DOM, carbohydrate, and TEP response to light in microalgal environments, understanding the role of photochemical processes in the ocean in carbon and nutrient cycling, and revealing the complex mechanisms of marine biogeochemical cycling.
