P138: Production of ethanol from corn stalks using an engineered strain of Saccharomyces cerevisiae

Increasing concern on the oil crisis and the environmental burden are prompting society to find renewable alternative fuel. As an ideal renewable fuel for transportation, bioethanol was vastly produced from starch or molasses in the past years. However, the insufficient supply of feedstocks hampers the sustainable production of bioethanol. Cellulosic ethanol derived from corn stalks and other lignocellulosic biomass becomes an attractive choice. The major fermentable sugars in most cellulosic hydrolysate are D-glucose and D-xylose, while natural Saccharomyces cerevisiae that is widely used in industrial ethanol production cannot utilize xylose. Recently, many efforts had been made to introduce the xylose metabolic pathways into S. cerevisiae. However, most of the engineered S. cerevisiae strains cannot metabolize xylose well under anaerobic conditions. In this study, we introduced xyl1 and xyl2 genes of Pichia stipitis into a model polyploid strain of S. cerevisiae, W303, and overexpressed its own xk gene. The engineered strain was subjected to continuous evolution with dissolved oxygen concentration gradually decreased. The resulted adaptive strain, S. cerevisiae W32N55, could efficiently metabolize xylose under static conditions, and tolerate the inhibitors in cellulosic hydrolysate. To overcome the inhibitory effect of high concentration of ethanol on xylose metabolism, a fermentation-membrane pervaporation coupling process was developed. Using such a process, 150 g/L glucose and 20 g/L xylose in the hydrolysate of steam exploded corn stalks were consumed in 40 h, producing 74 g/L ethanol, with an ethanol yield of 220 kg/ton steam exploded stalks achieved.