MONOSACCHARIDE COMPOSITION OF PARTICULATE HYDROLYZABLE SUGAR FRACTIONIN SURFACE MICROLAYERS FROM BRACKISH AND MARINE WATERS

Twenty-nine sea-surface microlayer samples, associated with their rela ted bulk water, were collected in coastal waters of the northwestern M editerranean Sea, and examined for the concentration and composition o f their particulate sugar fraction. The monosaccharide content was ass ayed by high-performance liquid chromatography (HPLC) and fluorescence detection after dansylhydrazine precolumn derivatization. Samples wer e collected in both marine areas influenced by fresh water. As compare d 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 h igher for sugars). However, in brackish bulk waters, the concentration s of organic compounds (POC, particulate organic nitrogen (PON), chlor ophyll a and sugars) were higher than in marine bulk waters. In all th e 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 particula te 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 su gar concentrations recorded in surface microlayers. In all cases, gluc ose was the major monosaccharide form. In brackish waters, no change w as noted in monosaccharide composition of the sugar fraction in surfac e microlayers as compared with their related bulk waters. In marine sa mples, a slight but significant depletion of glucose was recorded in t he microlayers, and three forms (mannose, arabinose and rhamnose) exhi bited higher percentages (+4%, +4% and +15%, respectively) in marine s ea-surface microlayers. During mild acid hydrolysis of the particulate organic matter, the release of the 'Dubois-reactive compounds' was fa ster in marine microlayer samples than in related bulk waters, and the relative concentration of glucose increased with time of hydrolysis i n the two types of water. When hydrolysis was not complete, a compound which eluted at the beginning of the chromatogram and remained at a c onstant level throughout the hydrolysis, was detected.