Abstract
BackgroundConsolidated bioprocessing, which combines saccharolytic and fermentative abilities in a single microorganism, is receiving increased attention to decrease environmental and economic costs in lignocellulosic biorefineries. Nevertheless, the economic viability of lignocellulosic ethanol is also dependent of an efficient utilization of the hemicellulosic fraction, which contains xylose as a major component in concentrations that can reach up to 40% of the total biomass in hardwoods and agricultural residues. This major bottleneck is mainly due to the necessity of chemical/enzymatic treatments to hydrolyze hemicellulose into fermentable sugars and to the fact that xylose is not readily consumed by Saccharomyces cerevisiae—the most used organism for large-scale ethanol production. In this work, industrial S. cerevisiae strains, presenting robust traits such as thermotolerance and improved resistance to inhibitors, were evaluated as hosts for the cell-surface display of hemicellulolytic enzymes and optimized xylose assimilation, aiming at the development of whole-cell biocatalysts for consolidated bioprocessing of corn cob-derived hemicellulose.ResultsThese modifications allowed the direct production of ethanol from non-detoxified hemicellulosic liquor obtained by hydrothermal pretreatment of corn cob, reaching an ethanol titer of 11.1 g/L corresponding to a yield of 0.328 g/g of potential xylose and glucose, without the need for external hydrolytic catalysts. Also, consolidated bioprocessing of pretreated corn cob was found to be more efficient for hemicellulosic ethanol production than simultaneous saccharification and fermentation with addition of commercial hemicellulases.ConclusionsThese results show the potential of industrial S. cerevisiae strains for the design of whole-cell biocatalysts and paves the way for the development of more efficient consolidated bioprocesses for lignocellulosic biomass valorization, further decreasing environmental and economic costs.
Highlights
Consolidated bioprocessing, which combines saccharolytic and fermentative abilities in a single microorganism, is receiving increased attention to decrease environmental and economic costs in lignocellulosic biorefineries
The most remarkable results with cell-surface display in a hemicellulosic consolidated bioprocessing (CBP) were obtained with liquors derived from hydrothermal treatment of rice straw, in which 4.04 and 8.2 g/L of ethanol were produced by engineered industrial S. cerevisiae Sun049 strain and engineered laboratorial S. cerevisiae NBRC1440/X strain, respectively [6, 8]
Corn cob processing: hydrothermal treatment for hemicellulosic liquors To evaluate the capacity of the constructed strains for the enzymatic saccharification and fermentation of hemicellulose, corn cob was selected as a representative renewable resource due to its high xylan content
Summary
Consolidated bioprocessing, which combines saccharolytic and fermentative abilities in a single microorganism, is receiving increased attention to decrease environmental and economic costs in lignocellulosic biorefineries. The economic viability of lignocellulosic ethanol is dependent of an efficient utilization of the hemicellulosic fraction, which contains xylose as a major component in concentrations that can reach up to 40% of the total biomass in hardwoods and agricultural residues This major bottleneck is mainly due to the necessity of chemical/enzymatic treatments to hydrolyze hemicellulose into fermentable sugars and to the fact that xylose is not readily consumed by Saccharomyces cerevisiae—the most used organism for large-scale ethanol production. The cell-surface display strategy exhibits several advantages, such as: (i) high localized enzyme activity, (ii) the monosaccharide release occurs close to the cell surface being instantaneously consumed by the cell, which reduces the risk of contamination or product inhibition, (iii) the immobilization of enzymes on cell surface allows their re-utilization in successive cultures, which lowers the overall process cost [3] This strategy enables the use of recombinant microorganisms as wholecell biocatalysts. The most remarkable results with cell-surface display in a hemicellulosic CBP were obtained with liquors derived from hydrothermal treatment of rice straw, in which 4.04 and 8.2 g/L of ethanol were produced by engineered industrial S. cerevisiae Sun strain and engineered laboratorial S. cerevisiae NBRC1440/X strain, respectively [6, 8]
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