Xylose fermentation by Candida shehatae and Pichia stipitis: effects of pH, temperature and substrate concentration

Construction of advanced producers of first- and second-generation ethanol in Saccharomyces cerevisiae and selected species of non-conventional yeasts (Scheffersomyces stipitis, Ogataea polymorpha)

This review summarizes progress in the construction of efficient yeast ethanol producers from glucose/sucrose and lignocellulose. Saccharomyces cerevisiae is the major industrial producer of first-generation ethanol. The different approaches to increase ethanol yield and productivity from glucose in S. cerevisiae are described. Construction of the producers of second-generation ethanol is described for S. cerevisiae, one of the best natural xylose fermenters, Scheffersomyces stipitis and the most thermotolerant yeast known Ogataea polymorpha. Each of these organisms has some advantages and drawbacks. S. cerevisiae is the primary industrial ethanol producer and is the most ethanol tolerant natural yeast known and, however, cannot metabolize xylose. S. stipitis can effectively ferment both glucose and xylose and, however, has low ethanol tolerance and requires oxygen for growth. O. polymorpha grows and ferments at high temperatures and, however, produces very low amounts of ethanol from xylose. Review describes how the mentioned drawbacks could be overcome.

Characterization of cellulolytic enzyme system of Schizophyllum commune mutant and evaluation of its efficiency on biomass hydrolysis

Schizophyllum commune is a basidiomycete equipped with an efficient cellulolytic enzyme system capable of growth on decaying woods. In this study, production of lignocellulose-degrading enzymes from S. commune mutant G-135 (SC-Cel) on various cellulosic substrates was examined. The highest cellulase activities including CMCase, FPase, and β-glucosidase were obtained on Avicel-PH101 while a wider range of enzymes attacking non-cellulosic polysaccharides and lignin were found when grown on alkaline-pretreated biomass. Proteomic analysis of SC-Cel also revealed a complex enzyme system comprising seven glycosyl hydrolase families with an accessory carbohydrate esterase, polysaccharide lyase, and auxiliary redox enzymes. SC-Cel obtained on Avicel-PH101 effectively hydrolyzed all agricultural residues with the maximum glucan conversion of 98.0% using corn cobs with an enzyme dosage of 5 FPU/g-biomass. The work showed potential of SC-Cel on hydrolysis of various herbaceous biomass with enhanced efficiency by addition external β-xylosidase.

Bioconversion of xylose, hexoses and biomass to ethanol by a new isolate of the white rot basidiomycete Trametes versicolor

Second-generation bioethanol production requires the development of economically feasible and sustainable processes that use renewable lignocellulosic biomass as a starting material. However, the microbial fermentation of xylose, which is the principal pentose sugar in hemicellulose, is a limiting factor in developing such processes. Here, a strain of the white rot basidiomycete Trametes versicolor that was capable of efficiently fermenting xylose was newly isolated and characterized. This strain, designated KT9427, was capable of assimilating and converting xylose to ethanol under anaerobic conditions with a yield of 0.44 g ethanol per 1 g of sugar consumed. In culture medium containing low yeast extract concentrations, xylose consumption and ethanol productivity were enhanced. Adjusting the initial pH between 3.0 and 5.0 did not markedly influence xylose fermentation. T. versicolor KT9427 also produced ethanol from glucose, mannose, fructose, cellobiose and maltose at yields ranging from 0.45 to 0.49 g ethanol per 1 g of sugar consumed. In addition, strain KT9427 exhibited favourable conversion of non-pretreated starch, cellulose, xylan, wheat bran and rice straw into ethanol compared to common recombinant yeast strains. Taken together, the present findings suggest that T. versicolor KT9427 is a promising candidate for environmentally friendly ethanol production directly from lignocellulosic biomass.

Evaluation of novel xylose-fermenting yeast strains from Brazilian forests for hemicellulosic ethanol production from sugarcane bagasse

Bioconversion of hemicellulosic hydrolysates into ethanol with the desired yields plays a pivotal role for the overall success of biorefineries. This paper aims to evaluate the ethanol production potential of four native strains of Scheffersomyces shehatae (syn. Candida shehatae) viz. S. shehatae BR6-2AI, CG8-8BY, PT1-1BASP and BR6-2AY, isolated from Brazilian forests. These strains were grown in commercial d-xylose-supplemented synthetic medium and sugarcane bagasse hemicellulose hydrolysate. S. shehatae BR6-2AY showed maximum ethanol production [0.48 ± 0.019 g g⁻¹, 95 ± 3.78 % fermentation efficiency (FE)] followed by S. shehatae CG8-8BY (0.47 ± 0.016 g g⁻¹, 93 ± 3.12 % FE), S. shehatae BR6-2AI (0.45 ± 0.01 g g⁻¹, 89 ± 1.71 % FE) and S. shehatae PT1-1BASP (0.44 ± 0.02 g g⁻¹, 86 ± 3.37 % FE) when grown in synthetic medium. During the fermentation of hemicellulose hydrolysates, S. shehatae CG8-8BY and S. shehatae BR6-2AY showed ethanol production (0.30 ± 0.05 g g⁻¹, 58 ± 0.02 % FE) and (0.21 ± 0.01 g g⁻¹, 40 ± 1.93 % FE), respectively.

Production of ethanol from hemicellulose fraction of cocksfoot grass using pichia stipitis

Background

In this study, cocksfoot grass (Dactylis glomerata), an abundant lignocellulosic biomass was pretreated using different operational parameters using wet explosion (WEx) pretreatment for accessing the bioethanol potential of the hemicellulose fraction. Utilization of the hemicellulose liquid hydrolysate to ethanol is essential for economically feasible cellulosic ethanol processes. Fermentation of the separated hemicellulose liquid hydrolysates obtained after the WEx pretreatment was done by Pichia stipitis CBS 6054 (Scheffersomyces stipitis).

Results

The fermentation of the WEx liquid hydrolysate from the pretreatment at higher severity (180°C, 15 min, 87 psi oxygen and 190°C, 15 min, 0.2% sulfuric acid) was fully inhibited probable by the presence of higher concentrations of inhibitory compounds such as furfural, HMF and acetic acid. The ethanol yield among other WEx conditions was in the range of 89 to 158 mL/kg DM, with the highest yield (92% of theoretical maximum value) found for the lower pretreatment severity at 160°C, 15 min, 87 psi oxygen.

Conclusions

Our findings from this present study demonstrated that the release of hemicellulose sugars in the liquid hydrolysate is maximal when a lower pretreatment severity is applied. This is evident as the highest ethanol yields were found under the pretreatment conditions at lower severity.

Kinetics of growth and ethanol formation from a mix of glucose/xylose substrate by Kluyveromyces marxianus UFV-3

The fermentation of both glucose and xylose is important to maximize ethanol yield from renewable biomass feedstocks. In this article, we analyze growth, sugar consumption, and ethanol formation by the yeast Kluyveromyces marxianus UFV-3 using various glucose and xylose concentrations and also under conditions of reduced respiratory activity. In almost all the conditions analyzed, glucose repressed xylose assimilation and xylose consumption began after glucose had been exhausted. A remarkable difference was observed when mixtures of 5 g L(-1) glucose/20 g L(-1) xylose and 20 g L(-1) glucose/20 g L(-1) xylose were used. In the former, the xylose consumption began immediately after the glucose depletion. Indeed, there was no striking diauxic phase, as observed in the latter condition, in which there was an interval of 30 h between glucose depletion and the beginning of xylose consumption. Ethanol production was always higher in a mixture of glucose and xylose than in glucose alone. The highest ethanol concentration (8.65 g L(-1)) and cell mass concentration (4.42 g L(-1)) were achieved after 8 and 74 h, respectively, in a mixture of 20 g L(-1) glucose/20 g L(-1) xylose. When inhibitors of respiration were added to the medium, glucose repression of xylose consumption was alleviated completely and K. marxianus was able to consume xylose and glucose simultaneously.

Multilocus phylogenetic study of the Scheffersomyces yeast clade and characterization of the N-terminal region of xylose reductase gene

Many of the known xylose-fermenting (X-F) yeasts are placed in the Scheffersomyces clade, a group of ascomycete yeasts that have been isolated from plant tissues and in association with lignicolous insects. We formally recognize fourteen species in this clade based on a maximum likelihood (ML) phylogenetic analysis using a multilocus dataset. This clade is divided into three subclades, each of which exhibits the biochemical ability to ferment cellobiose or xylose. New combinations are made for seven species of Candida in the clade, and three X-F taxa associated with rotted hardwood are described: Scheffersomyces illinoinensis (type strain NRRL Y-48827(T) = CBS 12624), Scheffersomyces quercinus (type strain NRRL Y-48825(T) = CBS 12625), and Scheffersomyces virginianus (type strain NRRL Y-48822(T) = CBS 12626). The new X-F species are distinctive based on their position in the multilocus phylogenetic analysis and biochemical and morphological characters. The molecular characterization of xylose reductase (XR) indicates that the regions surrounding the conserved domain contain mutations that may enhance the performance of the enzyme in X-F yeasts. The phylogenetic reconstruction using XYL1 or RPB1 was identical to the multilocus analysis, and these loci have potential for rapid identification of cryptic species in this clade.

Growth, death, and oxygen uptake kinetics of Pichia stipitis on xylose

Pichia stipitis NRRL Y-7124 has potential application in the fermentation of xylose-rich waste streams, produced by wood hydrolysis. Kinetic models of cell growth, death, and oxygen uptake were investigated in batch and oxygen-limited continuous cultures fed a rich synthetic medium. Variables included rates of dilution (D) and oxygen transfer (K(1)a) and concentrations of xylose (X), ethanol (E), and dissolved oxygen (C(ox)). Sustained cell growth required the presence of oxygen. Given excess xylose, specific growth rate (micro) was a Monod function of C(ox). Specific oxygen uptake rate was proportional to mu by a yield coefficient relating biomass production to oxygen consumption; but oxygen uptake for maintenance was negligible. Thus steady-state C(OX) depended only on D, while steady-state biomass concentration was controlled by both D and K(1)a. Given excess oxygen, cells grew subject to Monod limitation by xylose, which became inhibitory above 40 g/L. Ethanol inhibition was consistent with Luong's model, and 64. 3 g/L was the maximum ethanol concentration allowing growth. Actively growing cells died at a rate that was 20% of micro. The dying portion increased with E and X.

The influence of initial xylose concentration, agitation, and aeration on ethanol production by Pichia stipitis from rice straw hemicellulosic hydrolysate

Rice straw hemicellulosic hydrolysate was used as fermentation medium for ethanol production by Pichia stipitis NRRL Y-7124. Shaking bath experiments were initially performed aiming to establish the best initial xylose concentration to be used in this bioconversion process. In the sequence, assays were carried out under different agitation (100 to 200 rpm) and aeration (V(flask)/V(medium) ratio varying from 2.5 to 5.0) conditions, and the influence of these variables on the fermentative parameters values (ethanol yield factor, Y(P/S); cell yield factor, Y(X/S); and ethanol volumetric productivity, Q(P)) was investigated through a 2(2) full-factorial design. Initial xylose concentration of about 50 g/l was the most suitable for the development of this process, since the yeast was able to convert substrate in product with high efficiency. The factorial design assays showed a strong influence of both process variables in all the evaluated responses. The agitation and aeration increase caused a deviation in the yeast metabolism from ethanol to biomass production. The best results (Y(P/S) = 0.37 g/g and Q(P) = 0.39 g/l.h) were found when the lowest aeration (2.5 V(flask)/V(medium) ratio) and highest agitation (200 rpm) levels were employed. Under this condition, a process efficiency of 72.5% was achieved. These results demonstrated that the establishment of adequate conditions of aeration is of great relevance to improve the ethanol production from xylose by Pichia stipitis, using rice straw hemicellulosic hydrolysate as fermentation medium.

The effect of initial cell concentration on xylose fermentation by Pichia stipitis

Xylose was fermented using Pichia stipitis CBS 6054 at different initial cell concentrations. A high initial cell concentration increased the rate of xylose utilization, ethanol formation, and the ethanol yield. The highest ethanol concentration of 41.0 g/L and a yield of 0.38 g/g was obtained using an initial cell concentration of 6.5 g/L. Even though more xylitol was produced when the initial cell concentrations were high, cell density had no effect on the final ethanol yield. A two-parameter mathematical model was used to predict the cell population dynamics at the different initial cell concentrations. The model parameters, a and b correlate with the initial cell concentrations used with an R(2) of 0.99.

Fermentation of sunflower seed hull hydrolysate to ethanol by Pichia stipitis

Ethanol production from sunflower seed hull hydrolysate was evaluated using Pichia stipitis NRRL Y-7124. The hydrolysate prepared with 0.7 M H2SO4 at 90 degrees C was fermented as substrate in shaking bath experiments at 30 degrees C. In a group of experiments, the influence of various detoxification methods on the fermentability of hydrolysate was investigated at pH 6. Even though the ability of all employed pretreatments to enhance fermentation performance was close, the sequential application of overliming with sodium sulfite addition was the best detoxification method. Additional experiments were performed with detoxified hydrolysate to investigate the effect of shaking rate (70-130 rpm) and initial pH (5.5-7) on the fermentation. The highest ethanol level 11 gL(-1) was achieved at initial pH of 6 and 100 rpm shaking rate from a hydrolysate containing 48 gL(-1) total reducing sugar. The corresponding alcohol yield and volumetric productivity were 0.32 gg(-1) and 0.065 gL(-1)h(-1).

L-Arabinose metabolism in Candida arabinofermentans PYCC 5603T and Pichia guilliermondii PYCC 3012: influence of sugar and oxygen on product formation

L-Arabinose utilization by the yeasts Candida arabinofermentans PYCC 5603(T) and Pichia guilliermondii PYCC 3012 was investigated in aerobic batch cultures and compared, under similar conditions, to D-glucose and D-xylose metabolism. At high aeration levels, only biomass was formed from all the three sugars. When oxygen became limited, ethanol was produced from D-glucose, demonstrating a fermentative pathway in these yeasts. However, pentoses were essentially respired and, under oxygen limitation, the respective polyols accumulated--arabitol from L-arabinose and xylitol from D-xylose. Different L-arabinose concentrations and oxygen conditions were tested to better understand L-arabinose metabolism. P. guilliermondii PYCC 3012 excreted considerably more arabitol from L-arabinose (and also xylitol from D-xylose) than C. arabinofermentans PYCC 5603(T). In contrast to the latter, P. guilliermondii PYCC 3012 did not produce any traces of ethanol in complex L-arabinose (80 g/l) medium under oxygen-limited conditions. Neither sustained growth nor active metabolism was observed under anaerobiosis. This study demonstrates, for the first time, the oxygen dependence of metabolite and product formation in L-arabinose-assimilating yeasts.

Plate ethanol-screening assay for selection of the Pichia stipitis and Hansenula polymorpha yeast mutants with altered capability for xylose alcoholic fermentation

A new method for the selection of Pichia stipitis and Hansenula polymorpha yeast mutants with altered capability to ferment xylose to ethanol was developed. The method is based on the ability of P. stipitis and H. polymorpha colonies to grow and produce ethanol on agar plates with xylose as the sole carbon and energy source. Secreted ethanol, in contrast to xylose, supports growth of cells of the indicator xylose-negative strains (the wild-type strain of Saccharomyces cerevisiae or Deltaxyl1 mutant of H. polymorpha) mixed with agar medium. The size of the tester culture-growth zone around xylose-grown colonies appeared to be dependent on the amount of secreted ethanol. Mutants with altered (decreased or elevated) ethanol production in xylose medium have been isolated using this method. The mutants exhibited pleiotropic alterations in enzymatic activities of the intermediary xylose metabolism.

Enhanced Xylitol Production by Precultivation of Candida guilliermondii Cells in Sugarcane Bagasse Hemicellulosic Hydrolysate

The present work evaluated the key enzymes involved in xylitol production (xylose reductase [XR] and xylitol dehydrogenase [XDH]) and their correlation with xylose, arabinose, and acetic acid assimilation during cultivation of Candida guilliermondii FTI 20037 cells in sugarcane bagasse hemicellulosic hydrolysate. For this purpose, inocula previously grown either in sugarcane bagasse hemicellulosic hydrolysate (SBHH) or in semidefined medium (xylose as a substrate) were used. The highest xylose/acetic acid consumption ratio (1.78) and the lowest arabinose consumption (13%) were attained in the fermentation using inoculum previously grown in semidefined medium (without acetic acid and arabinose). In this case, the highest values of XR (1.37 U mg prot(-1)) and XDH (0.91 U mg prot(-1)) activities were observed. The highest xylitol yield (approximately 0.55 g g(-1)) and byproducts (ethanol and glycerol) formation were not influenced by inoculum procedure. However, the cell previously grown in the hydrolysate was effective in enhancing xylitol production by keeping the XR enzyme activity at high levels (around 0.99 U.mg(prot) (-1)), reducing the XDH activity (34.0%) and increasing xylitol volumetric productivity (26.5%) with respect to the inoculum cultivated in semidefined medium. Therefore, inoculum adaptation to SBHH was shown to be an important strategy to improve xylitol productivity.

Ethanol and thermotolerance in the bioconversion of xylose by yeasts

The mechanisms underlying ethanol and heat tolerance are complex. Many different genes are involved, and the exact basis is not fully understood. The integrity of cytoplasmic and mitochondrial membranes is critical to maintain proton gradients for metabolic energy and nutrient uptake. Heat and ethanol stress adversely affect membrane integrity. These factors are particularly detrimental to xylose-fermenting yeasts because they require oxygen for biosynthesis of essential cell membrane and nucleic acid constituents, and they depend on respiration for the generation of ATP. Physiological responses to ethanol and heat shock have been studied most extensively in S. cerevisiae. However, comparative biochemical studies with other organisms suggest that similar mechanisms will be important in xylose-fermenting yeasts. The composition of a cell's membrane lipids shifts with temperature, ethanol concentration, and stage of cultivation. Levels of unsaturated fatty acids and ergosterol increase in response to temperature and ethanol stress. Inositol is involved in phospholipid biosynthesis, and it can increase ethanol tolerance when provided as a supplement. Membrane integrity determines the cell's ability to maintain proton gradients for nutrient uptake. Plasma membrane ATPase generates the proton gradient, and the biochemical characteristics of this enzyme contribute to ethanol tolerance. Organisms with higher ethanol tolerance have ATPase activities with low pH optima and high affinity for ATP. Likewise, organisms with ATPase activities that resist ethanol inhibition also function better at high ethanol concentrations. ATPase consumes a significant fraction of the total cellular ATP, and under stress conditions when membrane gradients are compromised the activity of ATPase is regulated. In xylose-fermenting yeasts, the carbon source used for growth affects both ATPase activity and ethanol tolerance. Cells can adapt to heat and ethanol stress by synthesizing trehalose and heat-shock proteins, which stabilize and repair denatured proteins. The capacity of cells to produce trehalose and induce HSPs correlate with their thermotolerance. Both heat and ethanol increase the frequency of petite mutations and kill cells. This might be attributable to membrane effects, but it could also arise from oxidative damage. Cytoplasmic and mitochondrial superoxide dismutases can destroy oxidative radicals and thereby maintain cell viability. Improved knowledge of the mechanisms underlying ethanol and thermotolerance in S. cerevisiae should enable the genetic engineering of these traits in xylose-fermenting yeasts.

Genetic engineering for improved xylose fermentation by yeasts

Xylose utilization is essential for the efficient conversion of lignocellulosic materials to fuels and chemicals. A few yeasts are known to ferment xylose directly to ethanol. However, the rates and yields need to be improved for commercialization. Xylose utilization is repressed by glucose which is usually present in lignocellulosic hydrolysates, so glucose regulation should be altered in order to maximize xylose conversion. Xylose utilization also requires low amounts of oxygen for optimal production. Respiration can reduce ethanol yields, so the role of oxygen must be better understood and respiration must be reduced in order to improve ethanol production. This paper reviews the central pathways for glucose and xylose metabolism, the principal respiratory pathways, the factors determining partitioning of pyruvate between respiration and fermentation, the known genetic mechanisms for glucose and oxygen regulation, and progress to date in improving xylose fermentations by yeasts.

Factors that affect the biosynthesis of xylitol by xylose-fermenting yeasts. A review

Xylitol is a sweetener with important technological properties like anticariogenicity, low caloric value, and negative dissolution heat. Because it can be used successfully in food formulations and pharmaceutical industries, its production is in great demand. Xylitol can be obtained by microbiological process, since many yeasts and filamentous fungi synthesize the xylose reductase enzyme, which catalyses the xylose reduction into xylitol as the first step in the xylose metabolism. The xylitol production by biotechnological means has several economic advantages in comparison with the conventional process based on the chemical reduction of xylose. The efficiency and the productivity of this fermentation chiefly depends upon the microorganism and the process conditions employed. In this mini-review, the most significant upstream parameters on xylitol production by biotechnological process are described.

Cloning and characterization of two pyruvate decarboxylase genes from Pichia stipitis CBS 6054

In Pichia stipitis, fermentative and pyruvate decarboxylase (PDC) activities increase with diminished oxygen rather than in response to fermentable sugars. To better characterize PDC expression and regulation, two genes for PDC (PsPDC1 and PsPDC2) were cloned and sequenced from P. stipitis CBS 6054. Aside from Saccharomyces cerevisiae, from which three PDC genes have been characterized, P. stipitis is the only organism from which multiple genes for PDC have been identified and characterized. PsPDC1 and PsPDC2 have diverged almost as far from one another as they have from the next most closely related known yeast gene. PsPDC1 contains an open reading frame of 1,791 nucleotides encoding 597 amino acids. PsPDC2 contains a reading frame of 1,710 nucleotides encoding 570 amino acids. An 81-nucleotide segment in the middle of the beta domain of PsPDC1 codes for a unique segment of 27 amino acids, which may play a role in allosteric regulation. The 5' regions of both P. stipitis genes include two putative TATA elements that make them similar to the PDC genes from S. cerevisiae, Kluyveromyces marxianus, and Hanseniaspora uvarum.

The influence of pH and aeration rate on the fermentation of D-xylose by Candida shehatae

The effects of the initial pH and air supply on the production of ethanol from D-xylose using the yeast Candida shehatae in a batch reactor were investigated. The initial pH was altered within the range of 2.5-6.5 and the specific aeration rate from 0.0-0.3 vv-1 min-1. The results showed that the most favorable initial pH for ethanol production was 4.5 and aeration via the stirring vortex of the bioreactor was sufficient. Under these conditions, the maximum specific growth rate (mu(m)) was 0.329 h-1; biomass production rate (b), 0.024 kg m-3 h-1; overall biomass yield (YGx/s), 0.036 kg kg-1; the specific uptake rate of D-xylose (qs), 2.0 kg kg-1 h-1; and the specific ethanol production rate (qE), 0.72 kg kg-1 h-1 (both at 20 h culture time). The average xylitol yield (Yxy/s) was 0.078 kg kg-1 and the overall ethanol yield (YGE/s), 0.41 kg kg-1. Both qs and qE diminished once the exponential growth phase was over.

High-efficiency transformation of Pichia stipitis based on its URA3 gene and a homologous autonomous replication sequence, ARS2

This paper describes the first high-efficiency transformation system for the xylose-fermenting yeast Pichia stipitis. The system includes integrating and autonomously replicating plasmids based on the gene for orotidine-5'-phosphate decarboxylase (URA3) and an autonomous replicating sequence (ARS) element (ARS2) isolated from P. stipitis CBS 6054. Ura- auxotrophs were obtained by selecting for resistance to 5-fluoroorotic acid and were identified as ura3 mutants by transformation with P. stipitis URA3. P. stipitis URA3 was cloned by its homology to Saccharomyces cerevisiae URA3, with which it is 69% identical in the coding region. P. stipitis ARS elements were cloned functionally through plasmid rescue. These sequences confer autonomous replication when cloned into vectors bearing the P. stipitis URA3 gene. P. stipitis ARS2 has features similar to those of the consensus ARS of S. cerevisiae and other ARS elements. Circular plasmids bearing the P. stipitis URA3 gene with various amounts of flanking sequences produced 600 to 8,600 Ura+ transformants per micrograms of DNA by electroporation. Most transformants obtained with circular vectors arose without integration of vector sequences. One vector yielded 5,200 to 12,500 Ura+ transformants per micrograms of DNA after it was linearized at various restriction enzyme sites within the P. stipitis URA3 insert. Transformants arising from linearized vectors produced stable integrants, and integration events were site specific for the genomic ura3 in 20% of the transformants examined. Plasmids bearing the P. stipitis URA3 gene and ARS2 element produced more than 30,000 transformants per micrograms of plasmid DNA. Autonomously replicating plasmids were stable for at least 50 generations in selection medium and were present at an average of 10 copies per nucleus.

Effects of pH and acetic acid on glucose and xylose metabolism by a genetically engineered ethanologenic Escherichia coli

Efficient utilization of the pentosan fraction of hemicellulose from lignocellulosic feedstocks offers an opportunity to increase the yield and to reduce the cost of producing fuel ethanol. The patented, genetically engineered, ethanologen Escherichia coli B (pLOI297) exhibits high-performance characteristics with respect to both yield and productivity in xylose-rich lab media. In addition to producing monomer sugar residues, thermochemical processing of biomass is known to produce substances that are inhibitory to both yeast and bacteria. During prehydrolysis, acetic acid is formed as a consequence of the deacetylation of the acetylated pentosan. Our investigations have shown that the acetic acid content of hemicellulose hydrolysates from a variety of biomass/waste materials was in the range 2-10 g/L (33-166 mM). Increasing the reducing sugar concentration by evaporation did not alter the acetic acid concentration. Acetic acid toxicity is pH dependent. By virtue of its ability to traverse the cell membrane freely, the undissociated (protonated) form of acetic acid (HAc) acts as a membrane protonophore and causes its inhibitory effect by bringing about the acidification of the cytoplasm. With recombinant E. coli B, the pH range for optimal growth with glucose and xylose was 6.4-6.8. With glucose, the pH optimum for ethanol yield and volumetric productivity was 6.5, and for xylose it was 6.0 and 6.5, respectively. However, the decrease in growth and fermentation efficiency at pH 7 is not significant. At pH 7, only 0.56% of acetic acid is undissociated, and at 10 g/L, neither the ethanol yield nor the maximum volumetric productivity, with glucose or xylose, is significantly decreased. The "uncoupling" effect of HAc is more pronounced with xylose and the potency of HAc is potentiated in a minimal salts medium. Controlling the pH at 7 provided an effective means of circumventing acetic acid toxicity without significant loss in fermentation performance of the recombinant biocatalyst.

Production of ethanol from pulp mill hardwood and softwood spent sulfite liquors by genetically engineered E. coli

Although lignocellulosic biomass and wastes are targeted as an attractive alternative fermentation feedstock for the production of fuel ethanol, cellulosic ethanol is not yet an industrial reality because of problems in bioconversion technologies relating both to depolymerization and fermentation. In the production of wood pulp by the sulfite process, about 50% of the wood (hemicellulose and lignin) is dissolved to produce cellulose pulp, and the pulp mill effluent ("spent sulfite liquor" SSL) represents the only lignocellulosic hydrolysate available today in large quantities (about 90 billion liters annually worldwide). Although softwoods have been the traditional feedstock for pulping operations, hardwood pulping is becoming more popular, and the pentose sugars in hardwood SSL (principally xylose) are not fermented by the yeasts currently being used in the production of ethanol from softwood SSL. This study assessed the fermentation performance characteristics of a patented (US Pat. 5,000,000), recombinant Escherichia coli B (ATCC 11303 pLOI297) in anaerobic batch fermentations of both nutrient-supplemented soft and hardwood SSL (30-35 g/L total reducing sugars). The pH was controlled at 7.0 to maximize tolerance to acetic acid. In contrast to the high-performance characteristics exhibited in synthetic media, formulated to mimic the composition of softwood and hardwood SSL (yield approaching theoretical maximum), performance in SSL media was variable with conversion efficiencies in the range of 67-84% for hardwood SSL and 53-76% for softwood SSL. Overlimiting treatment of HSSL, using Ca(OH)2, improved overall volumetric productivity two- to sevenfold to a max of 0.42 g/L/h at an initial cell loading of 0.5 g dry wt/L. A conversion efficiency of 92% (6.1 g/L ethanol) was achieved using diluted Ca(OH)2-treated hardwood SSL. The variable behavior of this particular genetic construct is viewed as a major detractant regarding its candidacy as a biocatalyst for SSL fermentations.

Genetic transformation of xylose-fermenting yeast Pichia stipitis. Scientific note

A plasmid-mediated transformation system has been developed for the xylose-fermenting yeast Pichia stipitis. We found that plasmid vectors containing the Saccharomyces cerevisiae 2 mu replicon and the kanamycin resistance gene (KmR) could be introduced into the Pichia cells and maintained as extrachromosomal elements. Pichia transformants containing such vectors will be resistant to the antibiotic geneticin that can be inactivated by the protein product of KmR. Plasmids identical to those used for transformation can be recovered from the Pichia transformants. Protocols for transformation of P. stipitis by the CaCl2-polyethylene glycol-protoplast process or by direct electroporation of intact Pichia cells have both been developed.

Thermoanaerobacter ethanolicus Growth and Product Yield from Elevated Levels of Xylose or Glucose in Continuous Cultures

The performance of Thermoanaerobacter ethanolicus was evaluated in continuous culture with media containing concentrations of xylose (8 to 20 g/liter) greater than those previously reported. The ethanol yield declined from to 0.42 to 0.29 g of ethanol per g of xylose consumed when input xylose was increased from 4 to 20 g/liter. Yields of both total C 2 and C 3 products from consumed xylose and of cell biomass from ATP produced declined as the input xylose concentration was increased, which was not the case when glucose was the substrate. This suggested that yeast extract functioned as a significant energy and carbon source for cells in fermentations of xylose but not of glucose. The feasibility of this interpretation was confirmed by (i) the calculation of the products theoretically obtainable from yeast extract and (ii) the observation of significant quantities of fermentation products in inoculated sugar-free media. Markedly different patterns of metabolism for the two sugar substrates were also evidenced by the cell yield for glucose being twice that of xylose at elevated sugar concentrations. It was noted that caution must be exerted when results obtained at low xylose concentrations are extrapolated to predict those which can be obtained at higher concentrations.