S57: Dynamic flux balance modeling of sugar metabolism and inhibitor detoxification by a synthetic yeast consortium

Co-culturing different species of yeast has been shown to be an effective strategy for fermentation of sugar mixtures to biofuels such as ethanol. Co-culturing a respiratory deficient mutant of Sacchromyces cerevisiae with wild-type Scheffersomyces stipitis has been shown to produce larger amounts of ethanol from glucose and xylose mixtures than either yeast in monoculture. Previous studies have also shown that S. cerevisiae and S. stipitis respond differently to furan inhibitors produced from lignocellulosic hydrolysis. Although S. stipitis can detoxify HMF at a faster rate than S. cerevisiae, furan aldehydes have a more deleterious effect on S. stipitis growth and ethanol production. In this presentation, we show that genome-scale reconstructions of S. cerevisiae and S. stipitis metabolism can be combined with batch monoculture and co-culture experiments to develop improved understanding of furaldehyde detoxification by these yeast species. Uptake kinetics and stoichiometric equations for the intracellular reduction reactions associated with each inhibitor were added to genome-scale metabolic reconstructions of the two yeasts. Inhibitory terms that captured the adverse effects of the furan aldehydes and their corresponding alcohols on cell growth and ethanol production were added to attain quantitative agreement with batch experiments. When the two yeasts were co-cultured in the presence of the furan aldehydes, inoculums that reduced the synthesis of toxic acetate produced by S. cerevisiae yielded the highest ethanol productivities. The model described here can be used to generate optimal cellular engineering and fermentation strategies for the simultaneous detoxification and fermentation of lignocellulosic hydrolysates by S. cerevisiae and S. stipitis.