Production of Storage Polysaccharide Paramylon in Microalga Euglena gracilis Klebs (Euglena, Euglenophyceae)

Unicellular flagellate microalga Euglena gracilis, depending on the conditions of cultivation, accumulates a large amount of reserve polysaccharide paramylon (β-l,3-glucan). This evidence provides need to consider it as a promising producer of this carbohydrate. The review presents data on the structure and synthesis of paramylon, as well as the features of its accumulation by E. gracilis cells. The degree of polysaccharide accumulation in cells depends on the conditions of cultivation including the presence of light, types of organic substrate, access to oxygen, etc. According to the literature data, the favorable conditions for stimulating the synthesis of paramylon in E. gracilis cells include periodic illumination of the culture, or lack of light, and the presence of an organic carbon source in the medium; in this capacity, glucose and fructose were the most effective. The lack of oxygen in the nutrient medium rebuilds the metabolism of E. gracilis cells from the synthesis of paramylon to the synthesis of waxes and is therefore not favorable for the accumulation of a polysaccharide. A comparison of the dynamics of paramylon accumulation in E. gracilis cells cultivated auto- and mixotrophically showed that the concentration of polysaccharide per cell was maximum in the first few days of cultivation in the presence of exogenous substrates. The addition of an organic nitrogen source additionally stimulates the accumulation of paramylon in the lag growth phase of the mixotrophic culture. During autotrophic cultivation, the content of paramylon does not noticeably change with the cultivation time. Analysis of the data on the use of by-products in the food industry as a source of nutrients for the growth of microalgae and accumulation of paramylon reveals that corn and potato extracts have the highest nutritional value for E. gracilis culture. The review also discusses the prospect of using paramylon in pharmacology and veterinary medicine as a stimulant and modulator of the immune system.