Changes in the metabolism of energy reserves and gene expression during different propagation strategies of tolerant xylose fermenting yeast and its effect on the bioethanol production process

Conference poster, 2013

Currently, large-scale production of bioethanol is mainly based on sugar or starch-rich feedstocks. These raw materials are also employed for animal feed and human use and seem not to be sufficient to the increasing demand for biofuels. In this context, lignocellulosic raw materials are good alternatives because they do not compete with food crops and are widely distributed. However, yeast strains to be used for lignocellulosic bioethanol production have to cope with challenging conditions that are inherent to the industrial process, such as high concentration of inhibitory products produced during pretreatment of raw material, simultaneous use of different carbon sources, and growth conditions that are not well controlled. Tolerance to these multiple stresses is likely to be a complex phenotype involving several cellular mechanisms therefore it could be difficult to perform efficient metabolic engineering. The production of inhibitor tolerant xylose-fermenting Saccharomyces cerevisiae cells during the propagation of yeast biomass will have a great effect on the following fermentation process. The traits of the produced yeast will determine the fermentation performance, ethanol yield and finally the global viability of the process. In the last years, the possibility of exposing the cells to lignocellulosic hydrolysates with inhibitors during the propagation step has given good results in terms of cell viability and high ethanol concentrations in the following fermentation. It is known that one of the general stress responses in yeast is the accumulation and mobilization of energy reserves (i.e. trehalose and glycogen). Trehalose is very important for maintaining cell viability under stress conditions because it protects cells from damage, however, when inhibitors are present in the media, the trehalose synthesis and degradation can be affected. In this work we study whether different propagation strategies have an impact on the metabolism of the energy carbohydrates trehalose and glycogen. The expression of several key genes (e.g. ALD6, ADH6, ZWF1, ERG2) is also investigated using qPCR technics in order to understand the yeast response during propagation under different conditions. Differences in trehalose and glycogen concentrations and changes in gene expression during propagation of yeast could give important insights for a successful lignocellulosic bioethanol production process