Effects Of Salt Stress On Chemical Structure And Antioxidant Activity Of Nostoc Flagelliforme Extracellular Polysaccharide

Nostoc flagelliforme, a kind of terrestrial cyanobacteria, is also called “facai”. It is subordinate to the Cyanophyta, Nostoc Branch, Nostoc genus. It grows slowly and the yield of extracellular polysaccharide was 235(mg/g dw) in liquid culture, but the secretion of extracellular polysaccharide increased by 50.3% in salt stress culture when compared with conventional cultivation. Based on this topic, we cultured Nostoc flagelliforme under salt stress, to improve its production of extracellular polysaccharide and clarify the changes of chemical structure and antioxidant activity of Nostoc flagelliforme extracellular polysaccharide in salt stress. The main results were as follows:(1) Significant changes displayed in cell morphology. Filament homogonium were disintegrated and shorten into unicells. The majority of cells were congregated and surrounded by a transparent polysaccharide mucus layer. With the salt concentration increasing, the synthesis and secretion of Nostoc flagelliforme extracellular polysaccharide and membrane permeability were increased significantly. When the Nostoc flagelliforme were cultivated for 6 days under 0.3mol/L, the extracellular polysaccharide increased 50.3% compared with that of conventional culture sample.(2) The polysaccharide was precipitated and purified(expressed by NEPS and YEPS). Both NEPS and YEPS were containing uronic acid. They were relatively homogeneous polysaccharide with low molecular weight, and the molecular weights were 2690 Da and 2570 Da, but none of them were sulfated polysaccharide. NEPS and YEPS were consistent with mannose, rhamnose, glucose glucuronic acid, glucose, galactose, xylose, but the proportion of its corresponding undergone significant changes, mannose and galactose improves which were 0.71: 0.37: 0.44: 1.53: 1.03: 0.40 and 1.02: 0.48: 0.57: 1.47: 1.40: 0.34.(3) Both NEPS and YEPS had good moisture retention and absorption, but YEPS absorption capacity and moisturizing effects were slightly higher than the NEPS. could be divided into three stages. The weight loss of YEPS thermal degradation process was more lower than NEPS(about 20%), indicating that salt stress let Nostoc flagelliforme extracellular polysaccharide had better thermal stability. Congo red were used to study the molecular chain conformation of YEPS and NEPS. The result showed that NEPS and YEPS had ordered triple helix chain structure.(4) The containing oxygen radical absorbance capacity, ABTS radical scavenging capacity, and reducing power were investigated. Both of them showed dose-effect relationship and YEPS had higher activity, which explained that salt stress could significantly improve the antioxidant activity of exopolysaccharides.