Renewable biomass including lignocellulosic materials and agricultural residues contribute to low cost bioproduction of ethanol and fine chemicals. In order to utilize such materials by fermentative microorganisms, lignocellulosic biomass hydrolysis pretreatment, numerous chemical by-products are generated which inhibit fermentative microorganisms. Among many inhibitory compounds, 2-furaldehyde (furfural) and 5-hydroxymethyl-2-furaldehyde (5-HMF) are potent and representative inhibitors produced during biomass pretreatment, especially by economic dilute acid hydrolysis. Furfural and HMF are formed by dehydration of pentoses and hexoses released from hemicelluloses and celluloses, respectively. These inhibitors damage cell walls and membranes, inhibit cell growth, reduce enzymatic activities, break down DNA, inhibit protein and RNA synthesis, and reduce ethanol production. In fact, few strains are able to withstand the inhibitory toxicity, and additional procedures are often required to remove or reduce the toxicity level.
Saccharomyces cerevisiae is a traditional yeast used for industrial ethanol production, but susceptible to aforementioned inhibitors and other stress conditions related to lignocellulosic biomass conversion. Yeast strains tolerant to single and combined inhibitors of furfural and 5-HMF were recently developed based on the metabolic engineering strategy. A dose-dependent response of yeast to furfural and HMF has been characterized and a lag phase used to measure levels of strain tolerance. Furfural and HMF can be reduced to 2-furanmethanol and furan-2,5-dimethanol, respectively. They can further breakdown to related organic acids. Under the inhibitor challenged conditions, once furfural and HMF fell to a certain lower level of concentrations, fermentation as indicated by glucose consumption by yeast can be accelerated at a faster rate than would normally occur. Genomic adaptation is likely to happen at this stage. Glycolysis and pentose phosphate pathway are major routes for glucose metabolism that provide energy and important intermediate metabolites for biosynthesis and ethanol production. Unfortunately, important enzymes of glycolysis were inhibited by furfural. On the other hand, numerous genes and enzymes were reported to be associated with enhanced tolerance to furfural or HMF, as reported that multiple gene involved NAD(P)H-dependent aldehyde reductions is an important mechanism of the detoxification of furfural and HMF.
Genetic manipulation of one or a few genes is a common approach to improve a specific trait of yeast. However, when an integrated cell performance of quantitative trait loci or a group of balanced multiple functions is concerned, such methods often fall short in achieving satisfactory outcomes. For example, economic ethanol production and stress tolerance of yeast involving multiple genes is beyond the control of a small number of gene manipulations.
We believe they are many broad candidates available in nature, which can move beyond difficulties to out boundary so that the good platform for fermentation of lignocellulosic biomass could be built without too much genetic modification. However, it is not easy to achieve the candidates due to the unique adaptation possessed by many yeasts upon the environmental stress. In this study, we have developed the simultaneous method for screening the candidate yeasts, which capable to grow and ferment lignocellulosic lysate into ethanol and fine chemicals such as lactate. In this report, first screening on the yeast capable of growing and fermentation in the medium containing fermentation inhibitors was conducted.