ABSTRACT
Ethanol for biofuels is currently produced from the fermentation of starches or cellulose-derived sugars. The predominant energy storage polysaccharide in Chlorophyta (green algae), Dinophyta (dinoflagellates), Glaucophyta and Rhodophyta (red algae) is starch, while Phaeophyceae (brown algae) and Bacillariophyceae (diatoms) store glucans in laminarian and chrysolaminarin, respectively. In industry, the algal polysaccharides are firstly hydrolyzed and then fermented to ethanol by other organisms such as yeast. However, an approach that would couple ethanol production directly to photosynthetic carbon fixation in situ may be preferred. Numerous microalgae have fermentative metabolic pathways to ethanol, but the coupling of ethanol production to photoautotrophic metabolism would require changes in regulatory pathways or the insertion of new metabolic pathways (Radakovits et al. 2010a). Increasing carbohydrate production in algae would help biofuel production. Proposed strategies to that end include overexpression of key enzymes in starch biosynthesis (e.g., ADP-glucose pyrophosphorylase or isoamylase), knockout of starch degrading enzymes (e.g., glucan-water dikinases and amylases), and secretion of soluble carbohydrates (Work et al. 2012).
