Optimized membrane process to increase hemicellulosic ethanol production from pretreated rice straw by recombinant xylose-fermenting Saccharomyces cerevisiae

Shared Molecular Targets Confer Resistance over Short and Long Evolutionary Timescales

Pre-existing and de novo genetic variants can both drive adaptation to environmental changes, but their relative contributions and interplay remain poorly understood. Here we investigated the evolutionary dynamics in drug-treated yeast populations with different levels of pre-existing variation by experimental evolution coupled with time-resolved sequencing and phenotyping. We found a doubling of pre-existing variation alone boosts the adaptation by 64.1% and 51.5% in hydroxyurea and rapamycin, respectively. The causative pre-existing and de novo variants were selected on shared targets: RNR4 in hydroxyurea and TOR1, TOR2 in rapamycin. Interestingly, the pre-existing and de novo TOR variants map to different functional domains and act via distinct mechanisms. The pre-existing TOR variants from two domesticated strains exhibited opposite rapamycin resistance effects, reflecting lineage-specific functional divergence. This study provides a dynamic view on how pre-existing and de novo variants interactively drive adaptation and deepens our understanding of clonally evolving populations.

Effective usage of sorghum bagasse: Optimization of organosolv pretreatment using 25% 1-butanol and subsequent nanofiltration membrane separation

We investigated the use of low concentrations of butanol (<40%, all v/v) as an organosolv pretreatment to fractionate lignocellulosic biomass into cellulose, hemicellulose, and lignin. The pretreatment conditions were optimized for sorghum bagasse by focusing on four parameters: butanol concentration, sulfuric acid concentration, pretreatment temperature, and pretreatment time. A butanol concentration of 25% or higher together with 0.5% or higher acid was effective for removing lignin while retaining most of the cellulose in the solid fraction. The highest cellulose (84.9%) and low lignin (15.3%) content were obtained after pretreatment at 200 °C for 60 min. Thus, pretreatment comprising 25% butanol, 0.5% acid, 200 °C, and 60 min process time was considered optimal. Enzymatic saccharification and fermentation by Saccharomyces cerevisiae produced 61.9 g/L ethanol from 200 g/L solid fraction obtained following pretreatment, and 10.2 g/L ethanol was obtained from the liquid fraction by xylose-utilizing S. cerevisiae following membrane nanofiltration to remove butanol.

Development of combined nanofiltration and forward osmosis process for production of ethanol from pretreated rice straw

A membrane process combining nanofiltraion (NF) and forward osmosis (FO) was developed for the sugar concentration with the aim of high bio-ethanol production from the liquid fraction of rice straw. The commercial NF membrane, ESNA3, was more adequate for removal of fermentation inhibitors (such as acetic acid) than the FO membrane, whereas the commercial FO membrane, TFC-ES, was more adequate for concentration of the sugars than the NF membrane. The liquid fraction was subjected to the following process: NF concentration with water addition (NF_(+H2O))→enzymatic hydrolysis→FO concentration. This NF_(+H2O)-FO hybrid process generated a total sugar content of 107g·L^-1. Xylose-assimilating S. cerevisiae produced 24g·L^-1 ethanol from the liquid fraction that was diluted 1.5-fold and then concentrated by the NF_(+H2O)-FO hybrid process. The NF_(+H2O)-FO hybrid process has the potential for optimized ethanol production from pretreated lignocellulosic biomass.

Cell surface engineering of Saccharomyces cerevisiae combined with membrane separation technology for xylitol production from rice straw hydrolysate

Xylitol, a value-added polyol deriving from D-xylose, is widely used in both the food and pharmaceutical industries. Despite extensive studies aiming to streamline the production of xylitol, the manufacturing cost of this product remains high while demand is constantly growing worldwide. Biotechnological production of xylitol from lignocellulosic waste may constitute an advantageous and sustainable option to address this issue. However, to date, there have been few reports of biomass conversion to xylitol. In the present study, xylitol was directly produced from rice straw hydrolysate using a recombinant Saccharomyces cerevisiae YPH499 strain expressing cytosolic xylose reductase (XR), along with β-glucosidase (BGL), xylosidase (XYL), and xylanase (XYN) enzymes (co-)displayed on the cell surface; xylitol production by this strain did not require addition of any commercial enzymes. All of these enzymes contributed to the consolidated bioprocessing (CBP) of the lignocellulosic hydrolysate to xylitol to produce 5.8 g/L xylitol with 79.5 % of theoretical yield from xylose contained in the biomass. Furthermore, nanofiltration of the rice straw hydrolysate provided removal of fermentation inhibitors while simultaneously increasing sugar concentrations, facilitating high concentration xylitol production (37.9 g/L) in the CBP. This study is the first report (to our knowledge) of the combination of cell surface engineering approach and membrane separation technology for xylitol production, which could be extended to further industrial applications.

Nanofiltration concentration of extracellular glutathione produced by engineered Saccharomyces cerevisiae

This study aimed to optimize extracellular glutathione production by a Saccharomyces cerevisiae engineered strain and to concentrate the extracellular glutathione by membrane separation processes, including ultrafiltration (UF) and nanofiltration (NF). Synthetic defined (SD) medium containing 20 g L(-1) glucose was fermented for 48 h; the fermentation liquid was passed through an UF membrane to remove macromolecules. Glutathione in this permeate was concentrated for 48 h to 545.1 ± 33.6 mg L(-1) using the NF membrane; this was a significantly higher concentration than that obtained with yeast extract peptone dextrose (YPD) medium following 96 h NF concentration (217.9 ± 57.4 mg L(-1)). This higher glutathione concentration results from lower cellular growth in SD medium (final OD600 = 6.9 ± 0.1) than in YPD medium (final OD600 = 11.0 ± 0.6) and thus higher production of extracellular glutathione (16.0 ± 1.3 compared to 9.2 ± 2.1 mg L(-1) in YPD medium, respectively). Similar fermentation and membrane processing of sweet sorghum juice containing 20 g L(-1) total sugars provided 240.3 ± 60.6 mg L(-1) glutathione. Increased extracellular production of glutathione by this engineered strain in SD medium and subsequent UF permeation and NF concentration in shortend time may help realize industrial recovery of extracellular glutathione.

Precipitate obtained following membrane separation of hydrothermally pretreated rice straw liquid revealed by 2D NMR to have high lignin content

BACKGROUND

Hydrothermal pretreatment of lignocellulosic biomass such as rice straw can dissolve part of the lignin and hemicellulose into a liquid fraction, thus facilitating enzyme accessibility to cellulose in bioethanol production process. Lignin is awaited to be recovered after hydrothermal pretreatment for utilization as value-added chemical, and lignin recovery also means removal of fermentation inhibitors. To recover lignin with high content from the liquid fraction, it is necessary to separate lignin and hemicellulose-derived polysaccharide. Therefore, the following processes were applied: membrane separation with nanofiltration (NF) and enzymatic hydrolysis by hemicellulase. To clarify lignin-concentrated fraction obtained during these processes, the fates of lignin and polysaccharide components were pursued by a solution NMR method and confirmed by compositional analysis of each fraction.

RESULTS

After hydrothermal pretreatment of rice straw, the NF concentrate of the supernatant of liquid fraction was hydrolyzed by hemicellulase and the resulting black precipitate was recovered. In this black precipitate, the intensity of NMR spectra related to lignin aromatic regions increased and those related to polysaccharides decreased, compared to rice straw, the solid fraction after hydrothermal pretreatment, and the NF concentrate. The lignin content of the black precipitate was 65.8 %. Lignin in the black precipitate included 52.9 % of the acid-insoluble lignin and 19.4 % of the soluble lignin in the NF concentrate of supernatant of liquid fraction.

CONCLUSION

A precipitate with high lignin content was obtained from supernatants of the liquid fraction. These results suggested that precipitation of lignin was enhanced from concentrated mixtures of lignin and hemicellulosic polysaccharides by hydrolyzing the polysaccharides. Precipitation of lignin can contribute to lignin recovery from lignocellulosic biomass and, at the same time, allow more efficient ethanol production in the subsequent fermentation process.

Mechanical milling and membrane separation for increased ethanol production during simultaneous saccharification and co-fermentation of rice straw by xylose-fermenting Saccharomyces cerevisiae

Mechanical milling and membrane separation were applied to simultaneous saccharification and co-fermentation from hydrothermally pretreated rice straw. Mechanical milling with minimized 4 cycles enabled 37.5±3.4gL(-1) and 45.3±4.4gL(-1) of ethanol production after 48h by xylose-fermenting Saccharomyces cerevisiae from solid fractions (200 and 250gL(-1)) of pretreated rice straw with 5 filter paper unitg-biomass(-1) cellulase (respectively, 77.3±7.1% and 74.7±7.3% of theoretical ethanol yield). Use of a membrane-based process including nanofiltration and ultrafiltration increased the sugar concentrations in the liquid fraction of pretreated rice straw and addition of this liquid fraction to 250gL(-1) solid fraction increased ethanol production to 52.0±0.4gL(-1) (73.8±0.6% of theoretical ethanol yield). Mechanical milling was effective in increasing enzymatic hydrolysis of the solid fraction and membrane separation steps increased the ethanol titer during co-fermentation, leading to a proposal for combining these processes for ethanol production from whole rice straw.

Challenges for the production of bioethanol from biomass using recombinant yeasts

Lignocellulose biomass, one of the most abundant renewable resources on the planet, is an alternative sustainable energy source for the production of second-generation biofuels. Energy in the form of simple or complex carbohydrates can be extracted from lignocellulose biomass and fermented by microorganisms to produce bioethanol. Despite 40 years of active and cutting-edge research invested into the development of technologies to produce bioethanol from lignocellulosic biomass, the process remains commercially unviable. This review describes the achievements that have been made in generating microorganisms capable of utilizing both simple and complex sugars from lignocellulose biomass and the fermentation of these sugars into ethanol. We also provide a discussion on the current "roadblocks" standing in the way of making second-generation bioethanol a commercially viable alternative to fossil fuels.