Heterotrophic and mixotrophic cultivation of Chlorella pyrenoidosa and the enzymatic hydrolysis of its biomass for the synthesis of third generation bioethanol

The present study has been carried out with a view of evaluating a green alga Chlorella pyrenoidosa as a potential candidate for the production of reducing sugars using an enzyme cocktail of multiple carbohydrates produced on site for the fermentation into bioethanol. The ability of C . pyrenoidosa to grow similarly fast on different carbon sources and light has been studied in Fog’s medium in heterotrophic and mixotrophic cultures. The high cells densities of mixotrophic cultures demonstrated that the growth-stimulating effects of light and carbon utilization were better as compared to the effects of glucose in heterotrophic condition. Maximum biomass yield of 1.2 g/l was achieved with 1% Glucose and 0.2% KNO 3 after 7 days of incubation at 28 o C. The algal biomass was steam pretreated and hydrolyzed by a cocktail of multiple carbohydrases produced by solid state culture of a laboratory isolate belonging to Aspergillus sp. on wheat bran exhibiting the yields of 86, 35, 74, 1947, 61, 17000 and 1388 IU/g dry wheat bran for CMCase, FPase, β-glucosidase, xylanase, mannanase, α-amylase and glucoamylase respectively. The enzyme cocktail worked well in the hydrolysis of algal biomass at 50 o C and produced total reducing sugars amounting to 429 mg/g of dried biomass revealing carbohydrate conversion efficiency of 96% after 48 h of hydrolysis. The released sugars may be fermented using suitable yeast strains for the production of third generation bioethanol.