Monosaccharide composition of particulate hydrolysable sugar fraction in surface microlayers from brackish and marine waters

Twenty-nine sea-surface microlayer samples, associated with their related bulk water, were collected in coastal waters of the northwestern Mediterranean Sea, and examined for the concentration and composition of their particulate sugar fraction. The monosaccharide content was assayed by high-performance liquid chromatography (HPLC) and fluorescence detection after dansylhydrazine precolumn derivatization. Samples were collected in both marine areas influenced by fresh water. As compared with bulk waters, accumulation of particulate organic matter in the surface microlayer was higher in marine than in brackish waters (about three-fold higher for particulate organic carbon (POC) and ten-fold higher for sugars). However, in brackish bulk waters, the concentrations of organic compounds (POC, particulate organic nitrogen (PON), chlorophyll a and sugars) were higher than in marine bulk waters. In all the samples the sugar content appeared to be directly linked to POC. All the surface microlayers were enriched in bacteria but phytoplanktonic pigments were less represented in microlayers than in other particulate organic fractions. In brackish waters, chlorophyll a concentrations were correlated with the percentage of monosaccharide carbon in POC, but the microbiomass could explain only a small part (5–10%) of the sugar concentrations recorded in surface microlayers. In all cases, glucose was the major monosaccharide form. In brackish waters, no change was noted in monosaccharide composition of the sugar fraction in surface microlayers as compared with their related bulk waters. In marine samples, a slight but significant depletion of glucose was recorded in the microlayers, and three forms (mannose, arabinose and rhamnose) exhibited higher percentages (+4%, +4% and +15%, respectively) in marine sea-surface microlayers. During mild acid hydrolysis of the particulate organic matter, the release of the ·Dubois-reactive compounds’ was faster in marine microlayer samples than in related bulk waters, and the relative concentration of glucose increased with time of hydrolysis in the two types of water. When hydrolysis was not complete, a compound which eluted at the beginning of the chromatogram and remained at a constant level throughout the hydrolysis, was detected.