Physiological and proteomic analysis of Nostoc flagelliforme in response to alkaline pH shift for polysaccharide accumulation

Nostoc flagelliforme is a terrestrial cyanobacterium whose exopolysaccharides have been found to have antioxidant, antivirus, and antitumor properties. Alkaline conditions could effectively increase the polysaccharide yield of N. flagelliforme. In this study, to explore the potential mechanism underlying this effect, the physiological and proteomic changes in N. flagelliforme under alkaline conditions were determined. The polysaccharide accumulations of N. flagelliforme were examined in alkaline and neutral cultures, and proteomic analysis was carried out to identify candidate proteins related to polysaccharide accumulation under alkali stress. Comparison of proteins from cells grown in pH 7.5 and 9.0 media, using two-dimensional electrophoresis (2-DE), enabled the identification of seven proteins related to polysaccharide secretion, which showed significant differences in abundance. The differential expression of those proteins led to metabolic changes, including glycolipid and peptidoglycan accumulation and reduced chlorophyll synthesis. Meanwhile, photosynthesis and Rubisco activity were increased in high-pH conditions, with variable fluorescence/maximal fluorescence (Fv/Fm) and Rubisco activity being 1.24- and 2.50-fold those of the control, respectively. The activities of enzymes related to polysaccharide production ranged from 1.15- to 1.83-fold those of the control, suggesting that the mechanism by which alkaline conditions promote polysaccharide accumulation in N. flagelliforme is to redirect carbon flow into the sugar nucleotide synthesis pathway. Furthermore, the activities of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), in N. flagelliforme increased in the pH 9.0 treatment, reaching 3.0-, 3.9- and 2.7-fold of the control, respectively. These results provide a new foundation for study of the mechanism of polysaccharide accumulation and the response of N. flagelliforme to alkaline conditions. •The polysaccharide production of N. flagelliforme was increased under alkaline conditions.•Seven proteins related to polysaccharide secretion were identified.•Photosynthesis and Rubisco activity increased under high-pH conditions.•The activities of enzymes involved in polysaccharide production were enhanced.•Oxidoreductase activity in N. flagelliforme was increased under alkaline conditions.