12-06: Bioconversion of Nannochloropsis salina lipid-extracted biomass into ethanol

To date, research, development, and commercialization within the algal biofuels industry has focused on the production of diesel-like fuels from the lipid fraction, with any unconverted biomass used for production of electricity, biogas, animal feed, or fertilizer.  However, since approximately half of the algal biomass is non-lipid, additional production of fuels from this fraction would improve the process economics and environmental impact.  We are developing a process for the biological conversion of lipid-extracted algal (LEA) biomass into fuels.  To date, the process development research has 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.  Hydrochloric acid treatment released sugars at the highest yield and rate, but a combination of four enzymes also released sugars relatively quickly.  Proteins were deconstructed using a commercially available protease.  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 did not appear to accelerate growth.  However, anaerobic growth on acid-deconstructed LEA was inhibited.  All three microbes produced ethanol from the enzyme-deconstructed LEA.  Yields, productivities, and process comparisons will be presented.