Exopolysaccharide production by Anabaena sp. PCC 7120: physicochemical parameter optimization and two-stage cultivation strategy to maximize the product yield

Cyanobacterial exopolysaccharides are recently gaining attention due to their high charge density and the ability to modify the rheological properties of water. This property makes them potential flocculating and thickening agents for industrial purposes. In this study, the effects of physicochemical parameters on biomass and exopolysaccharide production using Anabaena sp. PCC7120 were studied, and accordingly, mathematical models were developed. Interaction between the different parameters, such as sodium nitrate concentration, initial pH, and temperature, was studied using the Box-Behnken design for response surface methodology. Optimum values of the parameters (sodium nitrate having a concentration of 2 g/L, pH 8.5, and temperature 30 °C) were decided by analyzing the statistical design, which led to a biomass concentration of 1.58 g/L. A rise in exopolysaccharide production was observed at high temperatures up to 35 °C and in nitrate-limited conditions. The airlift reactor was found suitable for cyanobacterial growth. However, the maximum amount of exopolysaccharide was obtained in the bubble column reactor. A two-stage cultivation strategy was visualized for a simultaneous increase in biomass concentration and exopolysaccharide content. The maximum biomass concentration and carbohydrate content were found to be 1.96 g/L and 27.55% (g/g), respectively. The crude exopolysaccharide extracted had a carbohydrate content of 67.59% (g/g) and a protein content of 23.89% (g/g). It was used to flocculate different algal cells and showed the maximum flocculating efficiency of 86.24% with Chlorella sorokiniana. In addition, after exopolysaccharide extraction, the nutritional properties of the leftover biomass were evaluated. It can be used as fodder and in biofuel production mainly due to its high protein and carbohydrate content.