Extracellular carbohydrate polymers from five desert soil algae with different cohesion in the stabilization of fine sand grain

Extracellular polymeric substances (EPS) from four filamentous cyanobacteria Microcoleus vaginatus, Scytonema javanicum, Phormidium tenue and Nostoc sp. and a coccoid single-cell green alga Desmococcus olivaceus that had been separated from desert algal crusts of Tegger desert of China, were investigated for their chemical composition, structure and physical properties. The EPS contained 7.5–50.3% protein (in polymers ranging from 14 to more than 200 kD, SDS-PAGE) and 16.2–40.5% carbohydrate (110–460 kD, GFC). 6–12 kinds of monosaccharides, including 2-O-methyl rhamnose, 2-O-methyl glucose, and N-acetyl glucosamine were found. The main carbohydrate chains from M. vaginatus and S. javanicum consisted mainly of equal proportion of Man, Gal and Glc, that from P. tenue consisted mainly of arabinose, glucose and rhamnose. Arabinose was present in pyranose form, mainly α-l 1→3 linked, with branches on C4 of almost half of the units. Glucose was responsible for the terminal units, in addition of having some units as β1→3 and some as β1→4 linked. Rhamnose was mainly 1→3 linked with branches on C2 on half of the units. The carbohydrate polymer from D. olivaceus was composed mainly of β-1→4 linked xylose, galactose and glucose. The galactose part was present both in β-pyranose and -furanose forms. Arabinose in α-l-furanose form was mainly present as 1→2 and 1→2, 5 linked units, rhamnose only as α 1→3 and xylose as β 1→4. The backbone of the polysaccharide from Nostoc sp. was composed of β-1→4 linked xylose, galactose and glucose. Most of the glucose was branched on position C6, terminal glucose and 2-O-methyl glucose units are also present. The relationship between structure, physical properties and potential biological function is discussed.