Characterization of macroaggregates and photodegradation of their water soluble fraction

The analysis of macroaggregates of phytoplankton originating from the northern Adriatic using 13C and 1H nuclear magnetic resonance (NMR) spectroscopy showed the presence of long polymethylene chains, heteropolysaccharides and carboxyl groups. It is our belief that the polymethylene chains compose a part of bound carboxylic acids. Although it is possible that they are bonded in some other way, we favour this structure because it is commonly encountered in lipids. The water soluble fraction of macroaggregates were photolyzed using a photochemical reactor and exposure to sunlight. The photochemical transformations of aqueous solutions of natural macroaggregates and model mono- (glucose), di- (maltose) and polysaccharides (pullulan, laminarin) were followed colorimetrically at regular time intervals during light exposure using 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) and also using 1H NMR spectroscopy. Concentrations of monosaccharides produced from macroaggregates, pullulan and laminarin were increased under artificial light and sunlight. The photochemical degradation of macroaggregates in sunlight exhibits similar first order degradation kinetics to that under artificial light, but with a lower reaction rate. The 1H NMR studies of the irradiation products of pullulan and laminarin revealed a decrease in the signal of polysaccharide anomeric protons, suggesting degradation of the glycoside linkage. The prolonged irradiation of macroaggregates leads to decreasing monomer concentration and also suggests the degradation of produced monomers. The irradiation experiments on glucose confirmed this conclusion. Maltose, according to 1H NMR spectra, degrades to monomers which successively degrade. Photopolymerization can also occur after cleavage of glycoside linkages. These experiments showed that the northern Adriatic macroaggregates, mostly composed of heteropolysaccharides and polymethylene chains and originating from the cell wall, are photo-labile and their degradation proceeds through the cleavage of glycoside linkage and the subsequent degradation of monosaccharides produced. This photochemical degradation is significant and competitive with microbial degradation and could be important in reducing macroaggregate concentrations in shallow water columns of the northern Adriatic during summertime. Moreover, the lower molecular weight photoproducts can be subsequently used as substrates by microbes.