Bioprocess development on microalgae-based CO₂ fixation and bioethanol production using Scenedesmus obliquus CNW-N

A two-stage cultivation strategy was applied to achieve greater CO₂ fixation and carbohydrate productivity with an indigenous microalga Scenedesmus obliquus CNW-N, which was first cultivated using a nutrient-rich medium to promote cell growth, and was then switched to a nutrient-deficient condition to trigger carbohydrate accumulation. The optimal biomass productivity, carbohydrate productivity, and CO₂ fixation rate were 681.4, 352.9, and 1192.5mgL^-1d^-1, respectively, with a 51.8% carbohydrate content (based on dry weight). This performance is better than the results in most related studies. The microalgal carbohydrate was mainly composed of glucose, which accounts for nearly 80% of total sugars. Dilute acid hydrolysis with 2% H₂SO₄ can saccharify the wet microalgal biomass effectively, achieving a glucose yield of 96-98%. Using the acidic hydrolysate of the microalga as feedstock, the separate hydrolysis and fermentation (SHF) process gave an ethanol concentration of 8.55gL^-1, representing a theoretical yield of nearly 99.8%.