High Density Cultivation of a Heat-Resistant Microalga Chlorella Sorokiniana for Biodiesel Production

High density cultivation of a heat-resistant microalga Chlorella sorokiniana

for biodiesel production

Tingting Li, Yubin Zheng, Shulin Chen

Department of Biological Systems Engineering,

Washington State University, Pullman, WA 99164

Abstract

Microalgae are considered to be outstanding potential sources for biofuel production and thus have attracted growing interest. Heterotrophic algae cultures have the advantages of fast growth without subjection to light limitation, high biomass concentration for easily harvesting and high lipid content for biodiesel production. The green alga Chlorella sorokiniana has the capability to accumulate a high content of lipid heterotrophically, however, the high cell density cultivation in fermentor has not been well examined yet.  In this study, we firstly made a comparison between phototrophic and heterotrophic culture of C. sorokiniana, and the data showed the heterotrophic cultured algae had twice higher lipid production. Then, effects of temperature, carbon source, nitrogen source and C/N ratio on the biomass and lipid production of C. sorokiniana were evaluated. The results stated that the biomass and lipid reached 20.3 g/L and 5.4 g/L after 72 hours cultivation with 40 g/L glucose and 4 g/L potassium nitrate at 37^oC, which were approximately 4 and 6 folds respectively higher compared with the culture without optimization. Interestingly, the best culture temperature for C. sorokiniana was 37^oC since most Chlorella species preferred the environmental temperature in the range of 25 to 30^oC. Moreover, the temperature had significant effects on the fatty acid profile and a higher content of oleic acid (C18:1) but a lower content of linolenic acid (C18:3) was observed at higher temperature. The low temperature led the algae to accumulate unsaturated fatty acid, especially polyunsaturated fatty acid, because the low melting point was very important for the fluidity of the cell membrane at low temperature. Finally, fed-batch cultivation in fermentor was conducted with the optimal conditions and the dry biomass and lipid production for C. sorokiniana was significantly increased. These results demonstrated that heterotrophic culture of the microalga C. sorokiniana had the great capability for biodiesel production. The finding is very significant because a special microalga was presented in this study with the characteristics of high lipid content and heat-resistant.