Improvement of xylitol production by controlling oxygen supply in Candida parapsilosis

Enhanced xylitol production using non-detoxified xylose rich pre-hydrolysate from sugarcane bagasse by newly isolated Pichia fermentans

BACKGROUND

Integrated management of hemicellulosic fraction and its economical transformation to value-added products is the key driver towards sustainable lignocellulosic biorefineries. In this aspect, microbial cell factories are harnessed for the sustainable production of commercially viable biochemicals by valorising C5 and C6 sugars generated from agro-industrial waste. However, in the terrestrial ecosystem, microbial systems can efficiently consume glucose. On the contrary, pentose sugars are less preferred carbon source as most of the microbes lack metabolic pathway for their utilization. The effective utilization of both pentose and hexose sugars is key for economical biorefinery.

RESULTS

Bioprospecting the food waste and selective enrichment on xylose-rich medium led to screening and isolation of yeast which was phylogenetically identified as Pichia fermentans. The newly isolated xylose assimilating yeast was explored for xylitol production. The wild type strain robustly grew on xylose and produced xylitol with > 40% conversion yield. Chemical mutagenesis of isolated yeast with ethyl methanesulphonate (EMS) yielded seven mutants. The mutant obtained after 15 min EMS exposure, exhibited best xylose bioconversion efficiency. This mutant under shake flask conditions produced maximum xylitol titer and yield of 34.0 g/L and 0.68 g/g, respectively. However, under the same conditions, the control wild type strain accumulated 27.0 g/L xylitol with a conversion yield of 0.45 g/g. Improved performance of the mutant was attributed to 34.6% activity enhancement in xylose reductase with simultaneous reduction of xylitol dehydrogenase activity by 22.9%. Later, the culture medium was optimized using statistical design and validated at shake flask and bioreactor level. Bioreactor studies affirmed the competence of the mutant for xylitol accumulation. The xylitol titer and yield obtained with pure xylose were 98.9 g/L and 0.67 g/g, respectively. In comparison, xylitol produced using non-detoxified xylose rich pre-hydrolysate from sugarcane bagasse was 79.0 g/L with an overall yield of 0.54 g/g.

CONCLUSION

This study demonstrates the potential of newly isolated P. fermentans in successfully valorising the hemicellulosic fraction for the sustainable xylitol production.

Batch bioethanol production via the biological and chemical saccharification of some Egyptian marine macroalgae

AIMS

Marine seaweeds (macroalgae) cause an eutrophication problem and affects the touristic activities. The success of the production of the third-generation bioethanol from marine macroalgae depends mainly on the development of an ecofriendly and eco-feasible pretreatment (i.e. hydrolysis) technique, a highly effective saccharification step and finally an efficient bioethanol fermentation step. Therefore, this study aimed to investigate the potentiality of different marine macroalgal strains, collected from Egyptian coasts, for bioethanol production via different saccharification processes.

METHODS AND RESULTS

Different marine macroalgal strains, red Jania rubens, green Ulva lactuca and brown Sargassum latifolium, have been collected from Egyptian Mediterranean and Red Sea shores. Different hydrolysis processes were evaluated to maximize the extraction of fermentable sugars; thermochemical hydrolysis with diluted acids (HCl and H₂ SO₄ ) and base (NaOH), hydrothermal hydrolysis followed by saccharification with different fungal strains and finally, thermochemical hydrolysis with diluted HCl, followed by fungal saccharification. The hydrothermal hydrolysis of S. latifolium followed by biological saccharification using Trichoderma asperellum RM1 produced maximum total sugars of 510 mg g^-1 macroalgal biomass. The integration of the hydrothermal and fungal hydrolyses of the macroalgal biomass with a separate batch fermentation of the produced sugars using two Saccharomyces cerevisiae strains, produced approximately 0·29 g bioethanol g^-1 total reducing sugars. A simulated regression modelling for the batch bioethanol fermentation was also performed.

CONCLUSIONS

This study supported the possibility of using seaweeds as a renewable source of bioethanol throughout a suggested integration of macroalgal biomass hydrothermal and fungal hydrolyses with a separate batch bioethanol fermentation process of the produced sugars.

SIGNIFICANCE AND IMPACT OF THE STUDY

The usage of marine macroalgae (i.e. seaweeds) as feedstock for bioethanol; an alternative and/or complimentary to petro-fuel, would act as triple fact solution; bioremediation process for ecosystem, renewable energy source and economy savings.

Efficient detoxification of corn cob hydrolysate with ion-exchange resins for enhanced xylitol production by Candida tropicalis MTCC 6192

The present study demonstrates utilization of secondary agricultural wastes for xylitol production. The highest xylan-to-xylose (70%) conversion was achieved using dilute nitric acid as catalyst followed by resin treatment. Results show that resin treatment efficiently removed nitrate salt (70%), phenolic content and 5-HMF (70%). Highest xylitol yield (85%) was achieved during fermentation using Candida tropicalis MTCC 6192 from the neutralized hemicellulosic hydrolysate medium. Good recovery (>15%) was achieved from corncob with 85% xylose to xylitol conversion during fermentation. This two-step process for transformation of agri-waste to xylitol is much simpler and it could possibly be considered for up scaling after process optimization parameters.

RETRACTED ARTICLE: Comparative analysis of the Corynebacterium glutamicum transcriptome in response to changes in dissolved oxygen levels

The dissolved oxygen (DO) level of a culture of Corynebacterium glutamicum (C. glutamicum) in a bioreactor has a significant impact on the cellular redox potential and the distribution of energy and metabolites. In this study, to gain a deeper understanding of the effects of DO on the metabolism of C. glutamicum, we sought to systematically explore the influence of different DO concentrations on genetic regulation and metabolism through transcriptomic analysis. The results revealed that after 20 h of fermentation, oxygen limitation enhanced the glucose metabolism, pyruvate metabolism and carbon overflow, and restricted NAD+ availability. A high oxygen supply enhanced the TCA cycle and reduced glyoxylate metabolism. Several key genes involved in response of C. glutamicum to different oxygen concentrations were examined, which provided suggestions for target site modifications in developing optimized oxygen supply strategies. These data provided new insights into the relationship between oxygen supply and metabolism of C. glutamicum.

Utilization of xylose by engineered strains of Ashbya gossypii for the production of microbial oils

BACKGROUND

Ashbya gossypii is a filamentous fungus that is currently exploited for the industrial production of riboflavin. The utilization of A. gossypii as a microbial biocatalyst is further supported by its ability to grow in low-cost feedstocks, inexpensive downstream processing and the availability of an ease to use molecular toolbox for genetic and genomic modifications. Consequently, A. gossypii has been also introduced as an ideal biotechnological chassis for the production of inosine, folic acid, and microbial oils. However, A. gossypii cannot use xylose, the most common pentose in hydrolysates of plant biomass.

RESULTS

In this work, we aimed at designing A. gossypii strains able to utilize xylose as the carbon source for the production of biolipids. An endogenous xylose utilization pathway was identified and overexpressed, resulting in an A. gossypii xylose-metabolizing strain showing prominent conversion rates of xylose to xylitol (up to 97% after 48 h). In addition, metabolic flux channeling from xylulose-5-phosphate to acetyl-CoA, using aheterologous phosphoketolase pathway, increased the lipid content in the xylose-metabolizing strain a 54% over the parental strain growing in glucose-based media. This increase raised to 69% when lipid accumulation was further boosted by blocking the beta-oxidation pathway.

CONCLUSIONS

Ashbya gossypii has been engineered for the utilization of xylose. We present here a proof-of-concept study for the production of microbial oils from xylose in A. gossypii, thus introducing a novel biocatalyst with very promising properties in developing consolidated bioprocessing to produce fine chemicals and biofuels from xylose-rich hydrolysates of plant biomass.

Enhancement of xylitol production in Candida tropicalis by co-expression of two genes involved in pentose phosphate pathway

The yeast Candida tropicalis produces xylitol, a natural, low-calorie sweetener whose metabolism does not require insulin, by catalytic activity of NADPH-dependent xylose reductase. The oxidative pentose phosphate pathway (PPP) is a major basis for NADPH biosynthesis in C. tropicalis. In order to increase xylitol production rate, xylitol dehydrogenase gene (XYL2)disrupted C. tropicalis strain BSXDH-3 was engineered to co-express zwf and gnd genes which, respectively encodes glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6-PGDH), under the control of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter. NADPH-dependent xylitol production was higher in the engineered strain, termed "PP", than in BSXDH-3. In fermentation experiments using glycerol as a co-substrate with xylose, strain PP showed volumetric xylitol productivity of 1.25 g l(-1) h(-1), 21% higher than the rate (1.04 g l(-1) h(-1)) in BSXDH-3. This is the first report of increased metabolic flux toward PPP in C. tropicalis for NADPH regeneration and enhanced xylitol production.

Purification and biochemical characterization of a moderately halotolerant NADPH dependent xylose reductase from Debaryomyces nepalensis NCYC 3413

A Xylose reductase (XR) from the halotolerant yeast, Debaryomyces nepalensis NCYC 3413 was purified to apparent homogeneity. The enzyme has a molecular mass of 74 kDa with monomeric subunit of 36.4 kDa (MALDI-TOF/MS) and pI of 6.0. The enzyme exhibited its maximum activity at pH 7.0 and 45 °C (21.2U/mg). In situ gel digestion and peptide mass fingerprinting analysis showed 12-22% sequence homology with XR from other yeasts. Inhibition of the enzyme by DEPC (diethylpyrocarbonate) confirmed the presence of histidine residue in its active site. The enzyme exhibited high preference for pentoses over hexoses with greater catalytic efficiency for arabinose than xylose. The enzyme also showed absolute specificity with NADPH over NADH. The enzyme retained 90% activity with 100 mM of NaCl or KCl and 40% activity with 1 M KCl which suggest that the enzyme is moderately halotolerant and can be utilized for commercial production of xylitol under conditions where salts are present.

Xylitol production is increased by expression of codon-optimized Neurospora crassa xylose reductase gene in Candida tropicalis

Xylose reductase (XR) is the first enzyme in d-xylose metabolism, catalyzing the reduction of d-xylose to xylitol. Formation of XR in the yeast Candida tropicalis is significantly repressed in cells grown on medium that contains glucose as carbon and energy source, because of the repressive effect of glucose. This is one reason why glucose is not a suitable co-substrate for cell growth in industrial xylitol production. XR from the ascomycete Neurospora crassa (NcXR) has high catalytic efficiency; however, NcXR is not expressed in C. tropicalis because of difference in codon usage between the two species. In this study, NcXR codons were changed to those preferred in C. tropicalis. This codon-optimized NcXR gene (termed NXRG) was placed under control of a constitutive glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter derived from C. tropicalis, and integrated into the genome of xylitol dehydrogenase gene (XYL2)-disrupted C. tropicalis. High expression level of NXRG was confirmed by determining XR activity in cells grown on glucose medium. The resulting recombinant strain, LNG2, showed high XR activity (2.86 U (mg of protein)⁻¹), whereas parent strain BSXDH-3 showed no activity. In xylitol fermentation using glucose as a co-substrate with xylose, LNG2 showed xylitol production rate 1.44 g L⁻¹ h⁻¹ and xylitol yield of 96% at 44 h, which were 73 and 62%, respectively, higher than corresponding values for BSXDH-3 (rate 0.83 g L⁻¹ h⁻¹; yield 59%).

Comparative transcriptome analysis of Bacillus subtilis responding to dissolved oxygen in adenosine fermentation

Dissolved oxygen (DO) is an important factor for adenosine fermentation. Our previous experiments have shown that low oxygen supply in the growth period was optimal for high adenosine yield. Herein, to better understand the link between oxygen supply and adenosine productivity in B. subtilis (ATCC21616), we sought to systematically explore the effect of DO on genetic regulation and metabolism through transcriptome analysis. The microarrays representing 4,106 genes were used to study temporal transcript profiles of B. subtilis fermentation in response to high oxygen supply (agitation 700 r/min) and low oxygen supply (agitation 450 r/min). The transcriptome data analysis revealed that low oxygen supply has three major effects on metabolism: enhance carbon metabolism (glucose metabolism, pyruvate metabolism and carbon overflow), inhibit degradation of nitrogen sources (glutamate family amino acids and xanthine) and purine synthesis. Inhibition of xanthine degradation was the reason that low oxygen supply enhanced adenosine production. These provide us with potential targets, which can be modified to achieve higher adenosine yield. Expression of genes involved in energy, cell type differentiation, protein synthesis was also influenced by oxygen supply. These results provided new insights into the relationship between oxygen supply and metabolism.

Integrated production of xylitol and ethanol using corncob

Xylitol production from corncob hemicellulose is a popular process in China. Microbial conversion of xylose to xylitol, as a biological process with many advantages, has drawn increasing attention. As a by-product from the manufacturing of xylitol, corncob cellulosic residues are produced in very large amounts and represent an environmental problem. As a result, considering the large amount of xylitol production in China, the conversion of corncob cellulosic residues has become a widespread issue having to be tackled. After the hemicellulose in corncob has been hydrolyzed for xylitol production, the corncob cellulosic residue is porous and can easily be hydrolyzed by cellulases into glucose and further converted to ethanol, another high-added-value chemical. Based on the latest technology advancements in xylitol, cellulase, and ethanol production, the integrated production of ethanol from corncob cellulosic residues appears as a promising way to improve the profit of the whole xylitol production process.

Enhancement of xylitol productivity and yield using a xylitol dehydrogenase gene-disrupted mutant of Candida tropicalis under fully aerobic conditions

The effects of glycerol and the oxygen transfer rate on the xylitol production rate by a xylitol dehydrogenase gene (XYL2)-disrupted mutant of Candida tropicalis were investigated. The mutant produced xylitol near the almost yield of 100% from D: -xylose using glycerol as a co-substrate for cell growth and NADPH regeneration: 50 g D: -xylose l(-1) was completely converted into xylitol when at least 20 g glycerol l(-1) was used as a co-substrate. The xylitol production rate increased with the O(2) transfer rate until saturation and it was not necessary to control the dissolved O(2) tension precisely. Under the optimum conditions, the volumetric productivity and xylitol yield were 3.2 g l(-1) h(-1) and 97% (w/w), respectively.

Production of xylitol from D-xylose by a xylitol dehydrogenase gene-disrupted mutant of Candida tropicalis

Xylitol dehydrogenase (XDH) is one of the key enzymes in d-xylose metabolism, catalyzing the oxidation of xylitol to d-xylulose. Two copies of the XYL2 gene encoding XDH in the diploid yeast Candida tropicalis were sequentially disrupted using the Ura-blasting method. The XYL2-disrupted mutant, BSXDH-3, did not grow on a minimal medium containing d-xylose as a sole carbon source. An enzyme assay experiment indicated that BSXDH-3 lost apparently all XDH activity. Xylitol production by BSXDH-3 was evaluated using a xylitol fermentation medium with glucose as a cosubstrate. As glucose was found to be an insufficient cosubstrate, various carbon sources were screened for efficient cofactor regeneration, and glycerol was found to be the best cosubstrate. BSXDH-3 produced xylitol with a volumetric productivity of 3.23 g liter(-1) h(-1), a specific productivity of 0.76 g g(-1) h(-1), and a xylitol yield of 98%. This is the first report of gene disruption of C. tropicalis for enhancing the efficiency of xylitol production.

Increase of xylitol productivity by cell-recycle fermentation of Candida tropicalis using submerged membrane bioreactor

Candida tropicalis, an osmophilic strain isolated from honeycomb, produced xylitol at a maximal volumetric productivity of 3.5 g l(-1) h(-1) from an initial xylose concentration of 200 g l(-1). Even at a very high xylose concentration, e.g., 350 g l(-1), this strain produced xylitol at a moderate rate of 2.07 g l(-1) h(-1). In a fed-batch fermentation of xylose and glucose, 260 g l(-1) xylose was added, and the xylitol production was 234 g l(-1) for 48 h, corresponding to a rate of 4.88 g l(-1) h(-1). To increase xylitol productivity, cells were recycled in a submerged membrane bioreactor with suction pressure and air sparging. For each recycle round in cell-recycle fermentation, the average concentration of xylitol produced, fermentation time, volumetric productivity, and product yield were 180 g l(-1), 19.5 h, 8.5 g l(-1) h(-1), and 85%, respectively. When cell-recycle fermentation was started with the cell mass concentrated twofold after batch fermentation and performed for 10 recycle rounds, we achieved a very high productivity of 12 g l(-1) h(-1). The productivity and total amount of xylitol in cell-recycle fermentation were 3.4- and 11.0-fold higher than those in batch fermentation, respectively.

Production of highly concentrated xylitol by Candida magnoliae under a microaerobic condition maintained by simple fuzzy control

Microbial production of xylitol from xylose was investigated using Candida magnoliae. In particular, the effect of the oxygenation condition on the xylitol production yield was examined and the significance of maintaining a microaerobic condition was demonstrated. A simple system of fuzzy logic control (FLC) was devised to maintain the microaerobic condition in the xylitol production phase by regulating the proportion of air (air flow rate) supplied to the fermentor. The input variables to the fuzzy control system were the dissolved oxygen (DO) concentration in the culture broth and the CO2 concentration in the exit gas. A batch cultivation test using the FLC system confirmed the importance of maintaining a constant microaerobic condition throughout the xylitol production phase, and indicated it would be advantageous for this phase to be prolonged. An intermittent fed-batch culture was therefore carried out. The FLC system allowed a constant microaerobic condition to be maintained, resulting in minimal cell mass production and constant xylitol accumulation in the culture medium. As a consequence, a very high xylitol concentration of 356 g/dm3 could be attained. The xylitol yield in the fed-batch culture was 0.75, which corresponded to 82% of the theoretical yield.

Xylitol production using recombinant Saccharomyces cerevisiae containing multiple xylose reductase genes at chromosomal delta-sequences

Xylitol production from xylose was studied using recombinant Saccharomyces cerevisiae 2805 containing xylose reductase genes (XYL1) of Pichia stipitis at chromosomal delta-sequences. S. cerevisiae 2805-39-40, which contains about 40 copies of the XYL1 gene on the chromosome, was obtained by a sequential transformation using a dominant selection marker neor and an auxotrophic marker URA3. The multiple XYL1 genes were stably maintained on the chromosome even after 21 and 10 days in the non-selective sequential batch and chemostat cultures, respectively, whereas S. cerevisiae 2805:pVTXR, which harbors the episomal plasmid pVTXR having the XYL1 gene, showed mitotic plasmid instability and more than 95% of the cells lost the plasmid under the same culture conditions. In the first batch (3 days) of the sequential batch culture, volumetric xylitol productivity was 0.18 g l-1 h-1 for S. cerevisiae 2805-39-40, as compared to 0.21 g l-1 h-1 for S. cerevisiae 2805:pVTXR. However, the xylitol productivity of the latter started to decrease rapidly in the third batch and dropped to 0.04 g l-1 h-1 in the seventh batch, whereas the former maintained the stable xylitol productivity at 0.18 g l-1 h-1 through the entire sequential batch culture. The xylitol production level in the chemostat culture was about 8 g l-1 for S. cerevisiae 2805-39-40, as compared to 2.0 g l-1 for S. cerevisiae 2805:pVTXR after 10 days of cultures even though the xylitol production level of the latter was higher than that of the former for the first 5 days. The results of this experiment indicate that S. cerevisiae containing the multiple XYL1 genes on the chromosome is much more efficient for the xylitol production in the long-term non-selective culture than S. cerevisiae harboring the episomal plasmid containing the XYL1 gene.