Benthic mucilaginous aggregates in the Mediterranean Sea: Origin, chemical composition and polysaccharide characterization

Benthic mucilaginous aggregates are frequently formed in various parts of the Mediterranean basin, as in the Tyrrhenian and Adriatic Seas. Notwithstanding their wide spatial distribution, the role played by these aggregates in the biogeochemical cycling of organic matter is still largely unknown. The characteristics of the benthic aggregates examined in the present study showed that they are different from other mucilaginous aggregates, such as the “pelagic” ones that can form in the same areas in the water column and subsequently sediment to the seafloor. The aggregates are usually formed of structures of the filamentous macroalgae Acinetospora crinita, Chrysonephos lewisii and Nematochrysopsis marina. The elemental composition of the aggregates showed a marine macrophyte origin on the basis of the bulk organic matter content of the aggregates. Carbohydrates and proteins account for 26.6 to 55.9% of the organic carbon in the mucilage, respectively. Monosaccharide composition of exopolysaccharides in the mucilage aggregates revealed a characteristic pattern, with galactose, xylose or mannose and fucose as the major components. The relatively high content of deoxysugars is another distinctive feature. The abundant sulphate and uronic groups present in the polysaccharides in addition to their macromolecular dimensions and elongation contribute to inter-chain aggregation. Electron microscopic observations suggest that the polysaccharide fraction is the main macromolecular component in the formation of the persistent gel network in the aggregates.