15-12: Algal starch levels typically found in nutrient deprived cultures produced under replete conditions in an engineered strain of Chlamydomonas reinhardtii

The saccharification of starch coupled with fermentation to ethanol is a mature technology that currently provides the majority of biofuels in the United States. Traditional food-based starch feedstocks, such as corn, however, cannot meet our current fuel demands and their use remains controversial. Microalgae have been of recent interest for use as a biofuel feedstock because they can produce large quantities of carbohydrates, contain little recalcitrant biomass, and do not impact the food supply. The primary carbohydrate found in many green algae, such as Chlamydomonas reinhardtii, is starch. High yields of this bioenergy carrier, however, are usually only found under conditions of nutrient stress (N, S, P). We have engineered strains of C. reinhardtii that overexpress a key enzyme in starch biosynthesis, isoamylase, that has resulted in a 3 to 4 fold increase in total glucan production under nutrient replete conditions. These algae divert metabolic flux into starch biosynthesis in nutrient replete medium at the expense of cell division and protein synthesis, and obtain the highest levels of intracellular starch observed to date in this organism. Possible mechanisms responsible for this phenotype and the role of isoamylase in starch biosynthesis are discussed.