Increased yield and productivity for the conversion of algal biomass carbohydrates

Microbial conversion of bio-based fuels and chemicals requires innovative strategies to achieve the best possible economic and sustainability metrics. While most studies have focused on the conversion of sugars from lignocellulosic hydrolysates, algal biomass is an appealing source of fermentable carbohydrates because of the high growth rates of algae relative to plants. The conversion of carbohydrates in algal biomass via fermentation is an important component of an overall strategy to maximize the areal productivity of fuels and chemicals from algal cultivations. There are two classic challenges to improving the cost and sustainability metrics of chemical production via fermentation: increasing product yield from substrate and increasing productivity. The goal of this project is to achieve these goals for ethanol production from algal hydrolysates. The strategies relied on the use of immobilized-cell technology in a continuous cultivation system, which has the potential to achieve significantly higher productivities than those from standard batch fermentation using free cells. Importantly, cell immobilization can also facilitate strategies for increasing the yield (carbon conversion efficiency) of sugar-to-product conversion. One such strategy, restricting biomass production, was shown to significantly improve yields. Proteome profiling was used to characterize the effects of this treatment. Hydrolysates of the alga Desmodesmus armatus were prepared using a dilute acid treatment at elevated temperature and pressure. A mock hydrolysate medium containing glucose, mannose, and galactose was also used. Saccharomyces cerevisae JAY 270 cells were immobilized in alginate to produce ethanol from algal or mock hydrolysates of D. armatus. The immobilized cells were packed in a column that was used in a system that operated either in chemostat or single-pass plug-flow mode. The rates of immobilized-cell production of ethanol were determined, along with the effects of pH, temperature, and residence time in the continuous immobilized-cell bioreactor system. No added nutrients are required for ethanol production using the algal hydrolysate of D. armatus in the continuous immobilized-cell bioreactor system. pH 4 and 35 °C are the optimum conditions for the immobilized yeast fermentation. The productivity of the chemostat-like continuous immobilized-cell bioreactor system could be more than ten times that of free-cell bioreactors. Furthermore, shorter residence times led to higher ethanol productivities but lower glucose conversion rates. A sequence of a continuous well-mixed bioreactor and a plugflow bioreactor is shown to achieve both goals. The increase in productivity can benefit the economics and sustainability of the overall process for production of algal biofuels, and the strategies described here are applicable to any extracellular metabolite production process