BIOCONVERSION OF LIPID-EXTRACTED ALGAL BIOMASS INTO ETHANOL

Energy security, high atmospheric greenhouse gas levels, and issues associated with fossil fuel extraction are among the incentives for developing alternative and renewable energy resources. Biofuels, produced from a wide range of feedstocks, have the potential to reduce greenhouse gas emissions. In particular, the use of microalgae as a feedstock has received a high level of interest in recent years. 

Microalgal biofuels are promising replacement for fossil fuels and have the potential to displace petroleum-based fuels while decrease greenhouse gas emissions. The primary focus of research and development toward algal biofuels has been on the production of biodiesel or renewable diesel from the lipid fraction, with use of the non-lipid biomass fraction for production of biogas, electricity, animal feed, or fertilizer. 

Since the non-lipid fraction, consisting of mainly carbohydrates and proteins, comprises approximately half of the algal biomass, our approach is biological conversion of the lipid-extracted algal biomass (LEAB) into fuels. We used LEAB from Nannochloropsis salina, and ethanol was the model product. The first step in conversion of LEAB to ethanol was deconstruction of the cell wall into fermentable substrates by using different acids or enzymes. Sugar release yields and rates were compared for different treatments. One-step sulfuric acid hydrolysis had the highest yield of released sugars, while the one-step hydrochloric acid treatment had the highest sugar release rate. Enzymatic hydrolysis produced acceptable sugar release rates and yields but enzymes designed for algal biomass deconstruction are still needed. Proteins were deconstructed using a commercially available protease. 

The hydrolysate, containing the released sugars, peptides, and amino acids, was used as a fermentation medium with no added nutrients. Three ethanologenic microorganisms were used for fermentation: two strains of Saccharomyces cerevisiae (JAY270 and ATCC 26603) and Zymomonas mobilis ATCC 10988. Ethanol yields and productivities were compared. Among the studied microorganisms, JAY270 had the highest ethanol yield while Z. mobilis had the lowest yield for most of the studied conditions. A protease treatment improved the biomass and ethanol yields of JAY270 by providing more carbon and nitrogen. 

To increase ethanol productivity, a continuous fermentation approach was adapted. Continuous stirred tank reactors have increased productivity over batch systems due to lower idle time. The downtime associated with batch fermentation is the time it takes for empting, cleaning, and filling the reactor. Productivity in the continuous fermentation was limited by the growth characteristics of the microorganism since at high flow rates, with washout occurring below a critical residence time. To overcome the washout problem, the use of an immobilized cell reactor was explored. The performance (ethanol productivity) of free and immobilized cells was compared using an enzymatic hydrolysate of LEAB. Higher ethanol productivities were observed for the continuous immobilized cell reactor compared to the stirred tank reactor.