Production, isolation and bioactive estimation of extracellular polysaccharides of green microalga Neochloris oleoabundans

Previous research revealed that green microalga Neochloris oleoabundans was able to excrete extracellular polysaccharides (EPS) in the media containing certain sugar, but detail researches on the production conditions and chemical and bioactive properties of the EPS are still limited. In this research, cultivation conditions, including 5 sugars (glucose, galactose, maltose, lactose and sucrose), 10–80 g/L of sugar concentrations, 9–36 mM of NaNO3 and the salinity of 0.5–1.0 g/L NaCl, were investigated their effects on the production of N. oleoabundans EPS. In order to develop a fast and convenient method for assessment of EPS concentration in algal cultivation media, an indirect quantitative technique was proposed based on the consistency coefficient of the media. An EPS (EPS1) was isolated and purified, and its immune activities were investigated in vivo and in vitro. The results showed that glucose, galactose and maltose allowed N. oleoabundans to produce substantial EPS, and glucose was the best sugar for EPS production. The results also showed that 20 g/L glucose and 12 mM NaNO3 were favorable to EPS production, whereas addition of 0.5–1.0 g/L NaCl to the medium repressed EPS production. Chemical analysis showed that the isolated EPS1 contained 0.59% (w/w) peptides and had a monosaccharide composition of glucose, mannose, galactose, xylose, ribose, arabinose and rhamnose with the molar ratio of 40.7:19.0:18.95:8.7:6.86:4.57:1.2. The bioactive tests demonstrated that EPS1 had immunomodulatory activity in vitro and in vivo. •Neochloris oleoabundans yielded massive extracellular polysaccharides under mixotrophic culture.•The effects of sugar and concentration on polysaccharide production were investigated.•High nitrogen and salinity did not promote polysaccharide yield.•The polysaccharide exhibited immuno-stimulatory and immuno-repressive activity.