Neutral monosaccharides in surface sediments of the northwestern Mediterranean Sea

This paper is focused on carbohydrates, an abundant chemical class in marine organic matter, mostly found as polysaccharides. An experiment showed that the use of H2SO4 1 M (95 °C, 4 h) yielded a good compromise between recovery and degradation of all neutral monosaccharides from two kinds of organic material: surface sediments and trapped particles. Twenty samples of surface sediments (0–0.5 cm) were collected in various physiographic and bathymetric locations on the Northwestern Mediterranean margin. Neutral monosaccharides were determined by an ionic chromatography system with electrochemical detection (HPAEC–PAD). Differences between total neutral monosaccharides (NMTOT) and total carbohydrates, determined by a spectrophotometric method on the same samples, brought further assumptions on the nature of the carbohydrate pool. Total neutral monosaccharides contents varied from 0.46 to 1.04 mg g−1 and their proportions donated to the organic carbon pool (NMTOT–C/OC) barely reached 5%. The spatial distribution of NMTOT and organic carbon (OC) clearly showed that organic inputs, originating from the shallower epicontinental area, spread along canyon axis while undergoing dilution processes. In spite of the high physiographic and bathymetric diversity, surface sediments from most stations were characterized markedly by the homogeneity in the neutral monosaccharide compositions. Two stations, E21 (Ebro prodelta) and EC36 (Balearic slope) showed, however, distinct signatures with high relative abundances of glucose and galactose, respectively. Glucose was found to trace the continental inputs, whereas galactose characterized the marine inputs. The relative “freshness” of the organic material at station EC36, assumed by the high proportion of hexose neutral monosaccharides, confirms the dominance of phytoplankton remains at this station. This work showed that the use of monosaccharides as tracers of the biological source and fate of organic compounds is deeply linked to the quality of the organic material. The monosaccharide signature is only preserved in samples of surface sediments where inputs of fresh organic material are higher than the refractory one.