Physicochemical properties and antioxidant activities of Desmodesmus armatus polysaccharides (DAP) were studied. They were extracted by microwave‐assisted constant temperature extraction and purification by DEAE‐cellulose 52. Four eluents of water (DAP1), 0.25 mol/L NaCl (DAP2), 0.5 mol/L NaCl (DAP3), and 1.0 mol/L NaCl (DAP4) were collected. Four polysaccharides fractions were analyzed, and they were all composed of mannose, rhamnose, glucuronic acid, galacturonic acid, arabinose, and fucose. Gel Permeation Chromatography (GPC) analysis showed that the four polysaccharides fractions have a uniform molecular weight distribution. Scanning electron microscope showed that DAP1 had a dense structure and a smooth but uneven surface, while DAP2, DAP3, and DAP4 were amorphous solids in sheets. Oxidation in vitro experiments showed that DAP2 and DAP3 had scavenging effects on ABTS, DPPH, and hydroxyl radicals. Practical applications In the determination of the antioxidant activity, it was found that the antioxidative activity of the polysaccharide of Desmodesmus armatus measured was significantly stronger than the crude polysaccharide of other microalgae. After the polysaccharide was purified, two polysaccharide fractions (DAP2 and DAP3) of Desmodesmus armatus were found to have strong scavenging ability to ABTS, DPPH, and hydroxyl radicals. They can be regarded as a new type of antioxidant, and the differences in the physicochemical properties between the parts can provide a preliminary explanation for the differences in antioxidant activity. But the connection between them needs further analysis. The Desmodesmus armatus used in the experiment is easy to cultivate and easy to obtain, which greatly increases its applicability. This research opens up new possibilities for the development of antioxidants and provides favorable evidence for the use of Desmodesmus armatus in food and feed.

In order to optimize the optimal cultivation conditions of freshwater algae Desmodesmus armatus for biomass production. In this study, the optimum source of carbon, nitrogen and intensity of light were investigated. Particularly, the variables which are affect the biomass of D. armatus was screened by the Plackett‐Burman (PB) method. Furthermore, the optimized medium composition using central composite design (CCD) of response surface method (RSM) central for D. armatus was reconstituted accordingly to have 0.93 g/L nitrate, 0.04 g/L phosphate, 0.15 g/L magnesium sulphate and 0.07 g/L bicarbonate, and optimum growth conditions of temperature at 27°C, light intensity of 108 μmol m−2 s−1, pH 7.00 and air flow of 0.50 L/min. After 12 days, the biomass, protein and polysaccharose content were 1.65 ± 0.15 g/L, 53.61 ± 1.25% and 6.15 ± 0.43%, respectively. Finally, the optimized conditions were applied to the outdoor 800‐L photobioreactor for scale‐up cultivation.

Microalga is a general term for autotrophic single-cell microalgae that can perform photosynthesis. It has various species, wide distribution, rich resources, rapid growth and strong adaptability. Chlorella is a unicellular green microalga rich in protein, polysaccharides, carotenoids, vitamins and other nutrients. It has broad application prospects and has been gradually applied in the fields of food (Matos,