The effects of aeration and mixotrophy by acetate and pyruvate on the growth parameters in Scenedesmus obliquus

The mixotrophic growth of microalgae offers great potential as an efficient strategy for increasing biomass production. In this study, Scenedesmus obliquus was used to investigate the biomass growth conditions and biochemical parameters of algae cells in autotrophic and mixotrophic conditions with and without aeration. It was found that mixotrophic cultivation of algae cells by 50 mM acetate had a strong effect on increasing fresh weight, chlorophyll a, b, carotenoid, astaxanthin, protein, and lipid content (with 20, 4.6, 8.7, 60, 4.5, 4.8, and 18.4-fold, respectively) in comparison to the control cultures. Mixotrophy by pyruvate also increased the amount of all mentioned parameters. The best concentration of pyruvate was found to be 50 mM, but the effects of acetate were stronger. Aeration in a short time (2 h) had a different effect on mixotrophic cultures with acetate and pyruvate. It declined the effect of acetate on increasing the fresh weight and the amount of other biochemical parameters. Unlike the acetate treatment, aeration had an enhancement effect on increasing the fresh weight and biochemical parameters content in 50 and 100 mM pyruvate treatments. In the control cultures, the saturated acid, caprylic acid (C8:0) (40%), and the polyunsaturated eicosapentaenoic acid (EPA; C20:5 n3) (17.5%) showed higher percentages. Mixotrophy caused a decrease in the percentage of saturated fatty acids (SFAs) and, on the other hand, an increase in the percentage of unsaturated fatty acids (UFAs). EPA was the main fatty acid in the mixotrophic condition. Aeration in the phototrophic condition decreased the percentage of SFAs, while increasing that of UFAs. Aeration under mixotrophic conditions increased UFAs in both acetate and pyruvate-treated cultures. The results suggest that aeration could be an important factor in interaction with other algae growth conditions. It should be noted that acetate had generally better effects on the biomass quality of S. obliquus without aeration.