Utilization of enzymatically hydrolyzed wood hemicelluloses by microorganisms for production of liquid fuels

Synthesis, Antimicrobial and Mutagenic Activity of a New Class of d-Xylopyranosides

Eight N-[2-(2',3',4'-tri-O-acetyl-α/β-d-xylopyranosyloxy)ethyl]ammonium bromides, a new class of d-xylopyranosides containing a quaternary ammonium aglycone, were obtained. Their complete structure was confirmed using NMR spectroscopy (¹H, ¹³C, COSY and HSQC) and high-resolution mass spectrometry (HRMS). An antimicrobial activity against fungi (Candida albicans, Candida glabrata) and bacteria (Staphylococcus aureus, Escherichia coli) and a mutagenic Ames test with Salmonella typhimurium TA 98 strain were performed for the obtained compounds. The greatest activity against the tested microorganisms was shown by glycosides with the longest (octyl) hydrocarbon chain in ammonium salt. None of the tested compounds exhibited mutagenic activity in the Ames test.

Differential Engagement of Fermentative Taxa in Gut Contents of the Earthworm Lumbricus terrestris

The earthworm gut is an anoxic, saccharide-rich microzone in aerated soils. The apparent degradation of diverse saccharides in the alimentary canal of the model earthworm Lumbricusterrestris is concomitant with the production of diverse organic acids, indicating that fermentation is an ongoing process in the earthworm gut. However, little is known about how different gut-associated saccharides are fermented. The hypothesis of this investigation was that different gut-associated saccharides differentially stimulate fermentative microorganisms in gut contents of L. terrestris This hypothesis was addressed by (i) assessing the fermentation profiles of anoxic gut content microcosms that were supplemented with gut-associated saccharides and (ii) the concomitant phylogenic analysis of 16S rRNA sequences. Galactose, glucose, maltose, mannose, arabinose, fucose, rhamnose, and xylose stimulated the production of fermentation products, including H2, CO2, acetate, lactate, propionate, formate, succinate, and ethanol. Fermentation profiles were dependent on the supplemental saccharide (e.g., glucose yielded large amounts of H2 and ethanol, whereas fucose did not, and maltose yielded large amounts of lactate, whereas mannose did not). Approximately 1,750,000 16S rRNA sequences were affiliated with 37 families, and phylogenic analyses indicated that a respective saccharide stimulated a subset of the diverse phylotypes. An Aeromonas-related phylotype displayed a high relative abundance in all treatments, whereas key Enterobacteriaceae-affiliated phylotypes were stimulated by some but not all saccharides. Collectively, these results reinforce the likelihood that (i) different saccharides stimulate different fermentations in gut contents of the earthworm and (ii) facultative aerobes related to Aeromonadaceae and Enterobacteriaceae can be important drivers of these fermentations.IMPORTANCE The feeding habits of earthworms influence the turnover of elements in the terrestrial biosphere. The alimentary tract of the earthworm constitutes an anoxic saccharide-rich microzone in aerated soils that offers ingested microbes a unique opportunity for anaerobic growth. The fermentative activity of microbes in the alimentary tract are responsible for the in situ production of (i) organic compounds that can be assimilated by the earthworm and (ii) H2 that is subject to in vivo emission by the earthworm and can be trophically linked to secondary microbial events in soils. To gain insight on how fermentative members of the gut microbiome might respond to the saccharide-rich alimentary canal, this study examines the impact of diverse gut-associated saccharides on the differential activation of fermentative microbes in gut contents of the model earthworm L. terrestris.

Enhanced rates of enzymatic saccharification and catalytic synthesis of biofuel substrates in gelatinized cellulose generated by trifluoroacetic acid

BACKGROUND

The crystallinity of cellulose is a principal factor limiting the efficient hydrolysis of biomass to fermentable sugars or direct catalytic conversion to biofuel components. We evaluated the impact of TFA-induced gelatinization of crystalline cellulose on enhancement of enzymatic digestion and catalytic conversion to biofuel substrates.

RESULTS

Low-temperature swelling of cotton linter cellulose in TFA at subzero temperatures followed by gentle heating to 55 °C dissolves the microfibril structure and forms composites of crystalline and amorphous gels upon addition of ethanol. The extent of gelatinization of crystalline cellulose was determined by reduction of birefringence in darkfield microscopy, loss of X-ray diffractability, and loss of resistance to acid hydrolysis. Upon freeze-drying, an additional degree of crystallinity returned as mostly cellulose II. Both enzymatic digestion with a commercial cellulase cocktail and maleic acid/AlCl₃-catalyzed conversion to 5-hydroxymethylfurfural and levulinic acid were markedly enhanced with the low-temperature swollen cellulose. Only small improvements in rates and extent of hydrolysis and catalytic conversion were achieved upon heating to fully dissolve cellulose.

CONCLUSIONS

Low-temperature swelling of cellulose in TFA substantially reduces recalcitrance of crystalline cellulose to both enzymatic digestion and catalytic conversion. In a closed system to prevent loss of fluorohydrocarbons, the relative ease of recovery and regeneration of TFA by distillation makes it a potentially useful agent in large-scale deconstruction of biomass, not only for enzymatic depolymerization but also for enhancing rates of catalytic conversion to biofuel components and useful bio-products.

Production of 2,3-butanediol from acid hydrolysates of Jatropha hulls with Klebsiella oxytoca

Jatropha hulls were successfully for the first time used as raw materials for the production of 2,3-butanediol via dilute sulfuric acid hydrolysis and fermentation with Klebsiella oxytoca. Two-step hydrolysis was used to effectively hydrolyze the hulls at 150°C after pretreatment. In the first-step, hemicellulose was hydrolyzed under mild conditions (0.5h, 1.5% acid) to avoid secondary decomposition. The remained cellulose was further hydrolyzed in the second-step under severer conditions (1h, 4% acid). After hydrolysis, total hydrolysis yield was 64%, which was much higher than that (37%) from the first-step. Maximum yields of 2,3-butanediol and acetoin in flask experiments were 35.6% and 41.4% from the hydrolysates of the first- and second-step hydrolysis, equivalent to 71.2% and 82.8% of the theoretical values, respectively. Similar yields were obtained in a controlled bioreactor but with higher productivities. Jatropha hulls are attractive raw materials for the production of 2,3-butanediol with high yield.

Improvement of straw surface characteristics via thermomechanical and chemical treatments

This study focused on effects of thermomechanical treatment, acid hydrolysis and enzymatic hydrolysis of rice straw fibre on its water resistant. Xylose and arabinose yields were 19.97% and 2.55%, respectively with thermomechanical treatment, 24.35% and 3.18% with acid hydrolysis, and 20.11% and 2.73% with enzymatic hydrolysis. The acid treatment dissolved hemicellulose significantly, leading to more fines and more voids in the surface of the rice straw fibre. The fines showed a higher water retention value (WRV) of 137.41% and exhibited higher swelling capacity. An increase in acid loading resulted in the increased WRV. The enzymatic hydrolysis increased the crystallinity of cellulose, but no significant correlation could be found between the chemical component and the water resistance of the rice straw fibre.

Bioconversion of xylan to triglycerides by oil-rich yeasts

A series of lipid-accumulating yeasts was examined for their potential to saccharify xylan and accumulate triglyceride. Of the genera tested, including Candida, Cryptococcus, Lipomyces, Rhodosporidium, Rhodotorula, and Trichosporon, only Cryptococcus and Trichosporon isolates saccharified xylan. All of the strains could assimilate xylose and accumuate triglyceride under nitrogen-limiting conditions. Strains of Cryptococcus albidus were found to be especially useful for a one-step saccharification of xylan coupled to triglyceride synthesis. Cryptococcus terricolus, a strain constitutive for lipid accumulation, lacked extracellular xylanase, but did assimilate xylose and xylobiose and was able to continuously convert xylan to triglyceride if the culture medium was supplemented with xylanase.

Aerobic microbial degradation of glucoisosaccharinic Acid

alpha-Glucoisosaccharinic acid (GISA), a major by-product of kraft paper manufacture, was synthesized from lactose and used as the carbon source for microbial media. Ten strains of aerobic bacteria capable of growth on GISA were isolated from kraft pulp mill environments. The highest growth yields were obtained with Ancylobacter spp. at pH 7.2 to 9.5. GISA was completely degraded by cultures of an Ancylobacter isolate. Ancylobacter cell suspensions consumed oxygen and produced carbon dioxide in response to GISA addition. A total of 22 laboratory strains of bacteria were tested, and none was capable of growth on GISA. GISA-degrading isolates were not found in forest soils.

Equations and calculations of product yields and preferred pathways for butanediol and mixed-acid fermentations

Using the available information of fermentation biochemistry, fermentation (stoichiometric) equations are derived for anaerobic saccharolytic fermentations of butanediol and mixed acids. The equations describe the interrelations among the fermentation products, biomass, and consumed substrate (glucose). The validity of the equations is tested using a variety of batch data from the literature. The validity of the equations is expected to extend to steady-state and transient fermentations, as well. Uses, improvements, and extensions of the equations are also discussed in detail. Among others, it is shown that the equations are useful for checking the consistency of experimental data, for calculating maximal yields and selectivities for the fermentation products, and calculating the extent of utilization of the Embden-Meyerhof-Parnas pathway versus the Hexose Monophosphate pathway of glucose utilization.

The release of fermentable carbohydrate from peat by steam explosion and its use in the microbial production of solvents

Steam treatment of peat at 200 degrees C for 3 min, followed by instantaneous decompression (steam explosion), solubilized up to 28% of the dry matter. Seventy-five percent of the solubilized material was carbohydrate, 33% of which was composed of mono- and disaccharides, including galactose, glucose, xylose, mannose, arabinose, and cellobiose, in order of decreasing concentration. The solubilized materials served as the sole source of carbohydrate for growth and solvent production by Clostridium acetobutylicum and C. butylicum which utilized up to 40% of the carbohydrate. Of the saccharides in this mixture, galactose was the least readily utilized. Approximately 30% of the fermentable carbohydrate used was converted to fatty acids and solvents, with the primary fermentation product being butyrate. Clostridium thermohydrosulfuricum was able to utilize ca. 50% of the carbohydrate, and simultaneously produced slightly more than 1 mol ethanol/mol saccharide metabolized. This organism, like other strains tested, used galactose less readily than the other sugars. The residue from the steam explosion process contained 24% cellulose, but it could not serve as a source of carbohydrate for the growth of either Bacteroides succinogenes or Clostridium thermocellum, suggesting that inhibitors were released during the steam treatment.

Fermentation equations for propionic-acid bacteria and production of assorted oxychemicals from various sugars

Fermentation (stoichiometric) equations are derived for anaerobic fermentations of propionic-acid bacteria (of both the Propionibacterium and acrylate pathways) and for production of various oxychemicals (butanol, acetone, isopropanol, butanediol, butyrate, acetate, propionate, succinate, lactate, and acrylate) from pentoses, hexoses, and cellobiose. The derivations of the equations are based on the fermentation biochemistries of the various bacterial classes. The validity of the equations is tested using fermentation data from the literature. The equations are shown to be valuable, among other uses, for calculating maximal yields and selectivities of the various fermentation products, as "gateway sensors" for monitoring of the fermentations, and for calculating the extents of the various intracellular reactions of the fermentation biochemistry.

Development of an industrial medium for economical 2,3-butanediol production through co-fermentation of glucose and xylose by Klebsiella oxytoca

An industrial medium containing urea as a sole nitrogen source, low levels of corn steep liquor and mineral salts as nutrition factors to retain high 2,3-butanediol production through co-fermentation of glucose and xylose (2:1, wt/wt) by Klebsiella oxytoca was developed. Urea and corn steep liquor were identified as the most significant factors by the two-level Plackett-Burman design. Steepest ascent experiments were applied to approach the optimal region of the two factors and a central composite design was employed to determine their optimal levels. Under the optimal medium, the yield of 2,3-butanediol plus acetoin relative to glucose and xylose was up to 0.428 g/g, which was 85.6% of theoretical value. The cheap nitrogen source and nutrition factors combining the co-fermentation process using lignocellulose derived glucose and xylose as the carbon source in the developed medium would be a potential solution to improve the economics of microbial 2,3-butanediol production.

In situ hydrogen and nitrous oxide as indicators of concomitant fermentation and denitrification in the alimentary canal of the earthworm Lumbricus terrestris

The earthworm gut is a unique microzone in aerated soils that has been proposed to selectively stimulate ingested soil microorganisms by its in situ conditions, which include anoxia, high water content, a near-neutral pH, and high concentrations of organic compounds. The central objective of this study was to resolve potential links between in situ conditions and anaerobic microbial activities during the gut passage of Lumbricus terrestris. Both H(2) and N(2)O were emitted by living earthworms, and in situ microsensor analyses revealed both H(2) and N(2)O in the O(2)-free gut center. The highest H(2) concentrations occurred in foregut and midgut regions, whereas the highest N(2)O concentrations occurred in crop/gizzard and hindgut regions. Thus, H(2)-producing fermentations were more localized in the foregut and midgut, whereas denitrification was more localized in the crop/gizzard and hindgut. Moisture content, total carbon, and total nitrogen were highest in the foregut and decreased from the anterior to posterior end of the gut. Nitrite, ammonium, and iron(II) concentrations were highest in the crop/gizzard and decreased from the anterior to posterior end of the alimentary canal. Concentrations of soluble organic compounds were indicative of distinct fermentation processes along the alimentary canal, with maximal concentrations of organic acids (e.g., acetate and butyrate) occurring in the midgut. These findings suggest that earthworms (i) contribute to the terrestrial cycling of carbon and nitrogen via anaerobic microbial activities in the alimentary canal and (ii) constitute a mobile source of reductant (i.e., emitted H(2)) for microbiota in aerated soils.

Ensiling agricultural residues for bioethanol production

The potential of using ensiling, with and without supplemental enzymes, as a cost-effective pretreatment for bioethanol production from agricultural residues was investigated. Ensiling did not significantly affect the lignin content of barley straw, cotton stalk, and triticale hay ensiled without enzyme, but slightly increased the lignin content in triticale straw, wheat straw, and triticale hay ensiled with enzyme. The holocellulose (cellulose plus hemicellulose) losses in the feedstocks, as a result of ensiling, ranged from 1.31 to 9.93%. The percent holocellulose loss in hays during ensiling was lower than in straws and stalks. Ensiling of barley, triticale, wheat straws, and cotton stalk significantly increased the conversion of holocellulose to sugars during subsequent hydrolysis with two enzyme combinations. Enzymatic hydrolysis of ensiled and untreated feedstocks by Celluclast 1.5 L-Novozyme 188 enzyme combination resulted in equal or higher saccharification than with Spezyme CP-xylanase combination. Enzyme loadings of 40 and 60 FPU/g reducing sugars gave similar sugar yields. The percent saccharification with Celluclast 1.5 L-Novozyme 188 at 40 FPU/g reducing sugars was 17.1 to 43.6%, 22.4 to 46.9%, and 23.2 to 32.2% for untreated feedstocks, feedstocks ensiled with, and without enzymes, respectively. Fermentation of the hydrolysates from ensiled feedstocks resulted in ethanol yields ranging from 0.21 to 0.28 g/g reducing sugars.

Potential of agricultural residues and hay for bioethanol production

Production of bioethanol from agricultural residues and hays (wheat, barley, and triticale straws, and barley, triticale, pearl millet, and sweet sorghum hays) through a series of chemical pretreatment, enzymatic hydrolysis, and fermentation processes was investigated in this study. Composition analysis suggested that the agricultural straws and hays studied contained approximately 28.62-38.58% glucan, 11.19-20.78% xylan, and 22.01-27.57% lignin, making them good candidates for bioethanol production. Chemical pretreatment with sulfuric acid or sodium hydroxide at concentrations of 0.5, 1.0, and 2.0% indicated that concentration and treatment agent play a significant role during pretreatment. After 2.0% sulfuric acid pretreatment at 121 degrees C/15 psi for 60 min, 78.10-81.27% of the xylan in untreated feedstocks was solubilized, while 75.09-84.52% of the lignin was reduced after 2.0% sodium hydroxide pretreatment under similar conditions. Enzymatic hydrolysis of chemically pretreated (2.0% NaOH or H2SO4) solids with Celluclast 1.5 L-Novozym 188 (cellobiase) enzyme combination resulted in equal or higher glucan and xylan conversion than with Spezyme(R) CP- xylanase combination. The glucan and xylan conversions during hydrolysis with Celluclast 1.5 L-cellobiase at 40 FPU/g glucan were 78.09 to 100.36% and 74.03 to 84.89%, respectively. Increasing the enzyme loading from 40 to 60 FPU/g glucan did not significantly increase sugar yield. The ethanol yield after fermentation of the hydrolyzate from different feedstocks with Saccharomyces cerevisiae ranged from 0.27 to 0.34 g/g glucose or 52.00-65.82% of the theoretical maximum ethanol yield.

Physiology of carbohydrate to solvent conversion by clostridia

The solvent-forming clostridia have attracted interest because of their ability to convert a range of carbohydrates to end-products such as acetone, butanol and ethanol. Polymeric substrates such as cellulose, hemicellulose and starch are degraded by extracellular enzymes. The majority of cellulolytic clostridia, typified by Clostridium thermocellum, produce a multi-enzyme cellulase complex in which the organization of components is critical for activity against the crystalline substrate. A variety of enzymes involved in degradation of hemicellulose and starch have been identified in different strains. The products of degradation, and other soluble substrates, are accumulated via membrane-bound transport systems which are generally poorly characterized. It is clear, however, that the phosphoenolpyruvate-dependent phosphotransferase system (PTS) plays a major role in solute uptake in several species. Accumulated substrates are converted by intracellular enzymes to end-products characteristic of the organism, with production of ATP to support growth. The metabolic pathways have been described, but understanding of mechanisms of regulation of metabolism is incomplete. Synthesis of extracellular enzymes and membrane-bound transport systems is commonly subject to catabolite repression in the presence of a readily metabolized source of carbon and energy. While many genes encoding cellulases, xylanases and amylases have been cloned and sequenced, little is known of control of their expression. Although the mechanism of catabolite repression in clostridia is not understood, some recent findings implicate a role for the PTS as in other low G-C Gram-positive bacteria. Emphasis has been placed on describing the mechanisms underlying the switch of C. acetobutylicum fermentations from acidogenic to solventogenic metabolism at the end of the growth phase. Factors involved include a lowered pH and accumulation of undissociated butyric acid, intracellular concentration of ATP and reduced pyridine nucleotides, nutrient limitation, and the interplay between pathways of carbon and electron flow. Genes encoding enzymes of solvent pathways have been cloned and sequenced, and their expression correlated with the pattern of end-product formation in fermentations. There is evidence that the initiation of solvent formation may be subject to control mechanisms similar to other stationary-phase phenomena, including sporulation. The application of recently developed techniques for genetic manipulation of the bacterium is improving understanding of the regulatory circuits, but a complete molecular description of the control of solvent formation remains elusive. Experimental manipulation of the pathways of electron flow in other species has been shown to influence the range and yield of fermentation end-products. Acid-forming clostridia can, under appropriate conditions, be induced to form atypical solvents as products. While the mechanisms of regulation of gene expression are not at all understood, the capacity to adapt in this way further illustrates the metabolic flexibility of clostridial strains.

Butanediol production from cellulose and hemicellulose by Klebsiella pneumoniae grown in sequential coculture with Trichoderma harzianum

The bioconversion of cellulose and hemicellulose substrates to 2,3-butanediol by a sequential coculture approach was investigated with the cellulolytic fungus Trichoderma harzianum E58 and the fermentative bacterium Klebsiella pneumoniae. Vogel medium optimal for the production of the cellulolytic and xylanolytic enzymes of the fungus was found to be inhibitory to butanediol fermentation. This inhibition appeared to be due to a synergistic effect of various ingredients, particularly the salts, present in the fungal medium. The removal or replacement of such ingredients from Vogel medium led to the relief of fermentation inhibition, but the treatments also resulted in a significant decrease in fungal enzyme production. Resting cells of K. pneumoniae could be used for butanediol production in the fungal medium, indicating that the inhibitory effect on solvent production under such conditions was due to the indirect result of growth inhibition of the bacterial cells. The resting-cell approach could be combined with a fed-batch system for the direct conversion of 8 to 10% (wt/vol) of Solka-Floc or aspenwood xylan to butanediol at over 30% of the theoretical conversion efficiencies.

The enzymatic hydrolysis and fermentation of pretreated wood substrates

Aspenwood chips were pretreated by steam explosion. The various wood fractions obtained were assayed for their ability to act as substrates for growth and cellulase production of different Trichoderma and Clostridium thermocellum species. Steam exploded aspenwood was as efficiently utilized as solka floc and correspondingly high cellulase activities were detected in the various culture filtrates. When T. harzianum E58 was grown on increasing concentrations of solka floc, highest cellulase and xylanase activities were detected at 1% substrate concentrations while high substrate concentrations (10-20%) inhibited growth and enzyme production. When the cellulosic substrates were supplemented with increasing amounts of glucose, cellulase and xylanase production were inhibited when the glucose concentration exceeded 0.1%. Highest xylanase activities were detected after growth of T. reesei C30 and T. harianum E58 on xylan and solka floc respectively. All of the steam exploded fractions were at least partially hydrolyzed by the T. harzianum E58 cellulase system. The extent of the pretreatment also influenced the ability of Zymomonas mobilis and Saccharomyces cerevisiae to ferment the liberated sugars to ethanol. About 85% of the theoretical yield of ethanol from cellulose could be obtained from the combined hydrolysis and fermentation of pretreated aspenwood.

Fed-batch approach to production of 2,3-butanediol by Klebsiella pneumoniae grown on high substrate concentrations

The bioconversion of sugars present in wood hemicellulose to 2,3-butanediol by Klebsiella pneumoniae grown on high sugar concentrations was investigated. When K. pneumoniae was grown under finite air conditions in the presence of added acetic acid, 50 g of D-glucose and D-xylose per liter could be converted to 25 and 27 g of butanediol per liter, respectively. The efficiency of bioconversion decreased with increasing sugar substrate concentrations (up to 200 g/liter). Butanediol production at low sugar substrate concentrations was less efficient when the organism was grown under aerobic conditions; however, final butanediol values were higher for cultures grown on an initial sugar concentration of 150 g/liter, particularly when the inoculum was first acclimatized to high sugar levels. When a double fed-batch approach (daily additions of sugars together with yeast extract) was used under aerobic conditions, up to 88 and 113 g of combined butanediol and acetyl methyl carbinol per liter could be obtained from the utilization of 190 g of D-xylose and 226 g of D-glucose per liter, respectively.