CO2 fixation and ethanol production with microalgal photosynthesis and intracellular anaerobic fermentation

Microalgae were screened from seawater. More than 250 strains were isolated, and some of the isolated strains and two strains from culture collections were tested to examine ethanol productivity. Some strains had high growth rate of 20–30 g dry biomass/m2/day and high starch content of more than 20% (dry base). A strain Chlorella vulgaris (IAM C-534) had a high starch content of 37%. Starch was extracted from the cells of the Chlorella, saccharified and fermented with yeasts; 65% of the ethanol-conversion rate was obtained as compared to the theoretical rate from starch. The algal starch proved to be a good source for ethanol production using the conventional process. As an example of another type of ethanol production process, intracellular starch fermentation under dark and anaerobic conditions was examined. All of the tested strains showed intracellular starch degradation and ethanol production, but the levels of ethanol production were significantly different from each other. Higher ethanol productions were obtained with Chlamydomonas reinhardtii (UTEX2247) and Sak-1 isolated from seawater. These showed a maximum ethanol concentration of 1 (w/w)%. The characteristics of intracellular ethanol production were examined with the Chlamydomonas. These results indicate that intracellular ethanol production is simpler and less energy intensive than the conventional ethanol-fermentation process.