5-16: Bioconversion of lipid-extracted Nannochloropsis salina biomass into ethanol

Until now, the primary focus of algal biofuels has been on the production of biodiesel from the lipid fraction and using the non-lipid fraction of biomass for production of biogas, electricity or animal feed. Since the non-lipid fraction, consisting of mainly carbohydrates and proteins, comprises approximately half of the algal biomass, our approach is to biologically convert the lipid-extracted algal (LEA) biomass into fuels. We have focused on a fermentation process with either a chemical or enzymatic pretreatment step. In this work, we used LEA from Nannochloropsis salina, and ethanol is the model fermentation product.  The first stage of this process is the deconstruction of cell wall polysaccharides into fermentable sugars.  To accomplish this, we have compared several enzyme mixtures as well as hydrochloric and sulfuric acids at different concentrations, temperatures, and reaction times. The two-step sulfuric acid hydrolysis released sugars at the highest yield and rate. One-step treatments using either sulfuric or hydrochloric acid released sugars at moderately high yields and rates, but a combination of four enzymes also released sugars relatively quickly. Three ethanologenic fermenting microorganisms were used:  two strains of Saccharomyces cerevisiae (JAY270 and JAY238) and Zymomonas mobilis ATCC 10988.  All three microorganisms were capable of growth on the enzyme-deconstructed LEA with no added nutrients, and protein deconstruction didn’t appear to accelerate growth but improved the ethanol yield for JAY270.  However, anaerobic growth on two-step sulfuric acid-deconstructed LEA was inhibited.  All three microbes produced ethanol from the enzyme-deconstructed LEA.  Yields, productivities, and process comparisons will be presented.