Efficient Using Durian Shell Hydrolysate as Low-Cost Substrate for Bacterial Cellulose Production by <Emphasis Type="Italic">Gluconacetobacter xylinus</Emphasis>

Durian is one important tropical fruit with high nutritional value, but its shell is usually useless and considered as waste. To explore the efficient and high-value utilization of this agricultural and food waste, in this study, durian shell was simply hydrolyzed by dilute sulfuric acid, and the durian shell hydrolysate after detoxification was used for bacterial cellulose (BC) production by Gluconacetobacter xylinus for the first time. BC was synthesized in static culture for 10 days and the highest BC yield (2.67 g/L) was obtained at the 8th day. The typical carbon sources in the substrate including glucose, xylose, formic acid, acetic acid, etc. can be utilized by G. xylinus. The highest chemical oxygen demand (COD) removal (16.40%) was obtained at the 8th day. The highest BC yield on COD consumption and the highest BC yield on sugar consumption were 93.51% and 22.98% (w/w), respectively, suggesting this is one efficient bioconversion for BC production. Durian shell hydrolysate showed small influence on the BC structure by comparison with the structure of BC generated in traditional Hestrin–Schramm medium detected by FE-SEM, FTIR, and XRD. Overall, this technology can both solve the issue of waste durian shell and produce valuable bio-polymer (BC).     

Comparative Metabolic Flux Analysis of Lysine-Producing Corynebacterium glutamicum Cultured on Glucose or Fructose

A comprehensive approach to ¹³C tracer studies, labeling measurements by gas chromatography-mass spectrometry, metabolite balancing, and isotopomer modeling, was applied for comparative metabolic network analysis of lysine-producing Corynebacterium glutamicum on glucose or fructose. Significantly reduced yields of lysine and biomass and enhanced formation of dihydroxyacetone, glycerol, and lactate in comparison to those for glucose resulted on fructose. Metabolic flux analysis revealed drastic differences in intracellular flux depending on the carbon source applied. On fructose, flux through the pentose phosphate pathway (PPP) was only 14.4% of the total substrate uptake flux and therefore markedly decreased compared to that for glucose (62.0%). This result is due mainly to (i) the predominance of phosphoenolpyruvate-dependent phosphotransferase systems for fructose uptake (PTS_Fructose) (92.3%), resulting in a major entry of fructose via fructose 1,6-bisphosphate, and (ii) the inactivity of fructose 1,6-bisphosphatase (0.0%). The uptake of fructose during flux via PTS_Mannose was only 7.7%. In glucose-grown cells, the flux through pyruvate dehydrogenase (70.9%) was much less than that in fructose-grown cells (95.2%). Accordingly, flux through the tricarboxylic acid cycle was decreased on glucose. Normalized to that for glucose uptake, the supply of NADPH during flux was only 112.4% on fructose compared to 176.9% on glucose, which might explain the substantially lower lysine yield of C. glutamicum on fructose. Balancing NADPH levels even revealed an apparent deficiency of NADPH on fructose, which is probably overcome by in vivo activity of malic enzyme. Based on these results, potential targets could be identified for optimization of lysine production by C. glutamicum on fructose, involving (i) modification of flux through the two PTS for fructose uptake, (ii) amplification of fructose 1,6-bisphosphatase to increase flux through the PPP, and (iii) knockout of a not-yet-annotated gene encoding dihydroxyacetone phosphatase or kinase activity to suppress overflow metabolism. Statistical evaluation revealed high precision of the estimates of flux, so the observed differences for metabolic flux are clearly substrate specific.     

Co-fermentation of carbon sources by Enterobacter aerogenes ATCC 29007 to enhance the production of bioethanol

We investigated the enhancement of bioethanol production in Enterobacter aerogenes ATCC 29007 by co-fermentation of carbon sources such as glycerol, glucose, galactose, sucrose, fructose, xylose, starch, mannitol and citric acid. Biofuel production increases with increasing growth rate of microorganisms; that is why we investigated the optimal growth rate of E. aerogenes ATCC 29007, using mixtures of different carbon sources with glycerol. E. aerogenes ATCC 29007 was incubated in media containing each carbon source and glycerol; growth rate and bioethanol production improved in all cases compared to those in medium containing glycerol alone. The growth rate and bioethanol production were highest with mannitol. Fermentation was carried out at 37 °C for 18 h, pH 7, using 50 mL defined production medium in 100 mL serum bottles at 200 rpm. Bioethanol production under optimized conditions in medium containing 16 g/L mannitol and 20 g/L glycerol increased sixfold (32.10 g/L) than that containing glycerol alone (5.23 g/L) as the carbon source in anaerobic conditions. Similarly, bioethanol production using free cells in continuous co-fermentation also improved (27.28 g/L) when 90.37 % of 16 g/L mannitol and 67.15 % of 20 g/L glycerol were used. Although naturally existing or engineered microorganisms can ferment mixed sugars sequentially, the preferential utilization of glucose to non-glucose sugars often results in lower overall yield and productivity of ethanol. Here, we present new findings in E. aerogenes ATCC 29007 that can be used to improve bioethanol production by simultaneous co-fermentation of glycerol and mannitol.     

Efficient bioconversion from acid hydrolysate of waste oleaginous yeast biomass after microbial oil extraction to bacterial cellulose by Komagataeibacter xylinus

Biomass acid hydrolysate of oleaginous yeast Trichosporon cutaneum after microbial oil extraction was applied as substrate for bacterial cellulose (BC) production by Komagataeibacter xylinus (also named as Gluconacetobacter xylinus previously) for the first time. BC was synthesized in static culture for 10 days, and the maximum BC yield (2.9?g/L) was got at the 4th day of fermentation. Most carbon sources in the substrate (glucose, mannose, formic acid, acetic acid) can be utilized by K. xylinus. The highest chemical oxygen demand (COD) removal (40.7?±?3.0%) was obtained at the 6th day of fermentation, and then the COD increased possibly due to the degradation of BC. The highest BC yield on COD consumption was 38.7?±?4.0% (w/w), suggesting that this is one efficient bioconversion for BC production. The BC structure was affected little by the substrate by comparison with that generated in classical HS medium using field-emission scanning electron microscope (FE-SEM), Fourier transform infrared, and X-ray diffraction. Overall, this technology can both solve the issue of waste oleaginous yeast biomass and produce valuable biopolymer (BC).     

Comparative metabolic flux analysis of lysine-producing Corynebacterium glutamicum cultured on glucose or fructose

A comprehensive approach to (13)C tracer studies, labeling measurements by gas chromatography-mass spectrometry, metabolite balancing, and isotopomer modeling, was applied for comparative metabolic network analysis of lysine-producing Corynebacterium glutamicum on glucose or fructose. Significantly reduced yields of lysine and biomass and enhanced formation of dihydroxyacetone, glycerol, and lactate in comparison to those for glucose resulted on fructose. Metabolic flux analysis revealed drastic differences in intracellular flux depending on the carbon source applied. On fructose, flux through the pentose phosphate pathway (PPP) was only 14.4% of the total substrate uptake flux and therefore markedly decreased compared to that for glucose (62.0%). This result is due mainly to (i) the predominance of phosphoenolpyruvate-dependent phosphotransferase systems for fructose uptake (PTS(Fructose)) (92.3%), resulting in a major entry of fructose via fructose 1,6-bisphosphate, and (ii) the inactivity of fructose 1,6-bisphosphatase (0.0%). The uptake of fructose during flux via PTS(Mannose) was only 7.7%. In glucose-grown cells, the flux through pyruvate dehydrogenase (70.9%) was much less than that in fructose-grown cells (95.2%). Accordingly, flux through the tricarboxylic acid cycle was decreased on glucose. Normalized to that for glucose uptake, the supply of NADPH during flux was only 112.4% on fructose compared to 176.9% on glucose, which might explain the substantially lower lysine yield of C. glutamicum on fructose. Balancing NADPH levels even revealed an apparent deficiency of NADPH on fructose, which is probably overcome by in vivo activity of malic enzyme. Based on these results, potential targets could be identified for optimization of lysine production by C. glutamicum on fructose, involving (i) modification of flux through the two PTS for fructose uptake, (ii) amplification of fructose 1,6-bisphosphatase to increase flux through the PPP, and (iii) knockout of a not-yet-annotated gene encoding dihydroxyacetone phosphatase or kinase activity to suppress overflow metabolism. Statistical evaluation revealed high precision of the estimates of flux, so the observed differences for metabolic flux are clearly substrate specific.     

Enhancement of adenosine production by Bacillus subtilis CGMCC 4484 through metabolic flux analysis and simplified feeding strategies

The objective of this research was to understand how the initial glucose concentration influences adenosine (AR) production and metabolic flux shift on the cultivation of Bacillus subtilis CGMCC 4484. Experiments confirmed that initial glucose concentration affects cell growth, AR production and metabolites, significantly. The flux distribution at the key nodes of glucose-6-phosphate (G6P), pyruvate (PYR) and acetyl coenzyme-A (AcCoA) could be affected by changing the glucose concentration. Based on kinetic analysis of specific rates, the low-glucose concentration was better for both cell growth and AR production during the first 12 h. However, the high-glucose concentration was more favorable for AR formation after 18 h. Furthermore, different simplified feeding strategies were designed to achieve higher AR accumulation. The final AR concentration of 15.60 g L^?1 was achieved when an optimized constant-feeding strategy was used, which was 21.02 % higher than batch fermentation. This was the first time to investigate the regulation of the glucose metabolism of AR-producing B. subtilis.     

Growth of the yeast Kluyveromyces marxianus CBS 6556 on different sugar combinations as sole carbon and energy source

The yeast Kluyveromyces marxianus has been pointed out as a promising microorganism for a variety of industrial bioprocesses. Although genetic tools have been developed for this yeast and different potential applications have been investigated, quantitative physiological studies have rarely been reported. Here, we report and discuss the growth, substrate consumption, metabolite formation, and respiratory parameters of K. marxianus CBS 6556 during aerobic batch bioreactor cultivations, using a defined medium with different sugars as sole carbon and energy source, at 30 and 37 °C. Cultivations were carried out both on single sugars and on binary sugar mixtures. Carbon balances closed within 95 to 101 % in all experiments. Biomass and CO₂ were the main products of cell metabolism, whereas by-products were always present in very low proportion (<3 % of the carbon consumed), as long as full aerobiosis was guaranteed. On all sugars tested as sole carbon and energy source (glucose, fructose, sucrose, lactose, and galactose), the maximum specific growth rate remained between 0.39 and 0.49 h^?1, except for galactose at 37 °C, which only supported growth at 0.31 h^?1. Different growth behaviors were observed on the binary sugar mixtures investigated (glucose and lactose, glucose and galactose, lactose and galactose, glucose and fructose, galactose and fructose, fructose and lactose), and the observations were in agreement with previously published data on the sugar transport systems in K. marxianus. We conclude that K. marxianus CBS 6556 does not present any special nutritional requirements; grows well in the range of 30 to 37 °C on different sugars; is capable of growing on sugar mixtures in a shorter period of time than Saccharomyces cerevisiae, which is interesting from an industrial point of view; and deviates tiny amounts of carbon towards metabolite formation, as long as full aerobiosis is maintained.     

Metabolic Fluxes in Corynebacterium glutamicum during Lysine Production with Sucrose as Carbon Source

Metabolic fluxes in the central metabolism were determined for lysine-producing Corynebacterium glutamicum ATCC 21526 with sucrose as a carbon source, providing an insight into molasses-based industrial production processes with this organism. For this purpose, ¹³C metabolic flux analysis with parallel studies on [1-¹³C^Fru]sucrose, [1-¹³C^Glc]sucrose, and [¹³C₆^Fru]sucrose was carried out. C. glutamicum directed 27.4% of sucrose toward extracellular lysine. The strain exhibited a relatively high flux of 55.7% (normalized to an uptake flux of hexose units of 100%) through the pentose phosphate pathway (PPP). The glucose monomer of sucrose was completely channeled into the PPP. After transient efflux, the fructose residue was mainly taken up by the fructose-specific phosphotransferase system (PTS) and entered glycolysis at the level of fructose-1,6-bisphosphate. Glucose-6-phosphate isomerase operated in the gluconeogenetic direction from fructose-6-phosphate to glucose-6-phosphate and supplied additional carbon (7.2%) from the fructose part of the substrate toward the PPP. This involved supply of fructose-6-phosphate from the fructose part of sucrose either by PTS^Man or by fructose-1,6-bisphosphatase. C. glutamicum further exhibited a high tricarboxylic acid (TCA) cycle flux of 78.2%. Isocitrate dehydrogenase therefore significantly contributed to the total NADPH supply of 190%. The demands for lysine (110%) and anabolism (32%) were lower than the supply, resulting in an apparent NADPH excess. The high TCA cycle flux and the significant secretion of dihydroxyacetone and glycerol display interesting targets to be approached by genetic engineers for optimization of the strain investigated.     

Production of biomass, carotenoid and other lipid metabolites by several red yeast strains cultivated on waste glycerol from biofuel production – a comparative screening study

This study was focused on a comparison of growth and production properties of seven red yeast strains of the genus Rhodotorula, Sporobolomyces and Cystofilobasidium cultivated on glycerol substrate. Production of enriched yeast biomas and specific yeast metabolites (carotenoids, ergosterol, lipids) was evaluated on medium with glucose, pure technical glycerol and/or waste glycerol from biofuel production (40 g/L) and mixture of glycerol and glucose (1:3, 1:1, 3:1; C/N ratio 57 in all cultivations). All tested strains were able to utilize glycerol as the only carbon source. Production of biomass on waste glycerol was in most strains higher than in control as well as in medium with pure technical glycerol and reached 15.97–21.76 g/L. Production of carotenoids and ergosterol was better in glucose medium than in medium with glycerol only. Nevertheless, using glycerol medium with addition of glucose, higher yields of total carotenoids, beta-carotene and ergosterol were obtained than in control. The highest yields of total pigments were reached by Sporobolomyces roseus (3.60 mg/g cell dry weight (CDW); glycerol:glucose 1:3), Sporobolomyces salmonicolor (2.85 mg/g CDW; glycerol:glucose 1:3) and Rhodotorula glutinis (2.80 mg/g CDW; glycerol:glucose 3:1) In glucose medium, most tested strains except Cystofilobasidium capitatum (22.6 %) produced neutral lipids in the range of 11–15 %. Production of triacylglycerols in all strains was in 10–30 % better in glycerol medium, in which Rhodotorula aurantiaca and Sporobolomyces shibatanus also reached intracellular triacylglycerol concentrations up to 20 % of biomass. This study has shown that oleaginous red yeasts could have great potential for converting crude glycerol to valuable lipids and carotenoids in respect of efficient bioresources utilization.     

Alkaline conditions stimulate the production of 1,3-propanediol in Lactobacillus panis PM1 through shifting metabolic pathways

A novel Lactobacillus panis PM1 isolate was found to be capable of converting glycerol to 1,3-propanediol (1,3-PDO), an increasingly valuable commodity chemical. In this study the effects of various process parameters, including glucose and glycerol concentrations, inoculum size, temperature, aeration, pH, and carbon source were examined to determine the optimal conditions for the production of 1,3-PDO using a culture method simulating late log to early stationary phases. Inoculum size did not influence the production of 1,3-PDO, and temperature variance showed similar 1,3-PDO production between 25 and 37 °C under the examined conditions. Glycerol concentration and pH played a primary role in the final concentration of 1,3-PDO. The highest production occurred at 150–250 mM glycerol when 50 mM glucose was available. Alkaline initial conditions (pH 9–10) stimulated the production of 1,3-PDO which concurrently occurred with increased acetic acid production. Under these conditions, 213.6 mM of 1,3-PDO were produced from 300 mM glycerol (conversion efficiency was 71 %). These observations indicated that the production of 1,3-PDO was associated with the shift of the metabolic end-product ethanol to acetic acid, and that this shift resulted in an excess concentration of NADH available for the processing of glycerol to 1,3-PDO.     

Metabolic fluxes in Schizosaccharomyces pombe grown on glucose and mixtures of glycerol and acetate

Growth on glycerol has already been a topic of research for several yeast species, and recent publications deal with the regulatory mechanisms of glycerol assimilation by the fission yeast Schizosaccharomyces pombe. We investigated glycerol metabolism of S. pombe from a physiological point of view, characterizing growth and metabolism on a mixture of glycerol and acetate and comparing it to growth on glucose under respirative growth conditions in chemostat experiments. On glycerol/acetate mixtures, the cells grew with a maximum specific growth rate of 0.11 h^?1 where 46 % of the carbon was channeled into biomass and the key fermentation product ethanol was not detectable. ¹³C-assisted metabolic flux analysis resolved substrate distributions through central carbon metabolism, proving that glycerol is used as a precursor for glycolysis, gluconeogenesis, and the pentose phosphate pathway, while acetate enters the tricarboxylic acid cycle via acetyl-CoA. Considering compartmentalization between cytosol and mitochondria in the metabolic model, we found compartmentalization of biosynthesis for the amino acids aspartate and leucine. Balancing of redox cofactors revealed an abundant production of cytosolic NADPH that must be finally regenerated via the respiratory chain shown by the simulated and measured CO₂ production and oxygen consumption rates which were in good agreement.     

Increased product formation induced by a directed secondary substrate limitation in a batch Hansenula polymorpha culture

By the use of directed limitations of secondary substrates, the metabolic flux should be deflected from biomass production to product formation. In order to study the impact of directed limitations caused by various secondary substrates on the growth and product formation of the methylotrophic yeast Hansenula polymorpha, the cultivation systems respiration activity monitoring system (RAMOS) and BioLector were used in parallel. While the RAMOS device allows the online monitoring of the oxygen transfer rate in shake flasks, the BioLector enables in microtiter plates the monitoring of scattered light and the fluorescence intensity of the green fluorescent protein (GFP). Secondary substrate limitations of phosphate, potassium, and magnesium were analyzed in batch fermentations. The sole carbon source was either 10 g/L glucose or 10 g/L glycerol. The expression of the GFP gene is controlled by the FMD promoter (formate dehydrogenase). In batch cultures with glucose as carbon source, a directed limitation of phosphate increased the GFP production 1.87-fold, compared to phosphate unlimited conditions. Under potassium-limited conditions with glycerol as sole carbon source, the GFP production was 1.41-fold higher compared to unlimited conditions. A limitation of the substrate magnesium resulted in a 1.22-fold increase GFP formation in the case of glycerol as carbon source.     

Utilization of glycerol and crude glycerol for polysaccharide production by an endophytic fungus Chaetomium globosum CGMCC 6882

Abstract

Crude glycerol is becoming a financial and environmental liability due to its surplus production from biodiesel industry, and its utilization as a fermentation feedstock for value-added chemicals production has been widely studied. In present work, the capacity of an endophytic fungus, Chaetomium globosum CGMCC 6882, using glycerol and crude glycerol for polysaccharide production was investigated. Results showed that the polysaccharide titers from glucose and glycerol were 1.85 and 3.8?g/L, respectively. Moreover, spore morphology of C. globosum CGMCC 6882 was favorable for polysaccharide production. Meanwhile, impurities in crude glycerol have no effect on polysaccharide production by C. globosum CGMCC 6882. Finally, characteristic results of polysaccharides produced from glucose, glycerol, and crude glycerol have suggested that metabolic flux might be a determinant factor on polysaccharide structure. Taken together, this research provided an innovative approach of utilizing crude glycerol produced from the biodiesel production process.     

Comparative study on the production of poly(3-hydroxybutyrate) by thermophilic Chelatococcus daeguensis TAD1: a good candidate for large-scale production

In spite of numerous advantages on operating fermentation at elevated temperatures, very few thermophilic bacteria with polyhydroxyalkanoates (PHAs)-accumulating ability have yet been found in contrast to the tremendous mesophiles with the same ability. In this study, a thermophilic poly(3-hydroxybutyrate) (PHB)-accumulating bacteria (Chelatococcus daeguensis TAD1), isolated from the biofilm of a biotrickling filter used for NOx removal, was extensively investigated and compared to other PHB-accumulating bacteria. The results demonstrate that C. daeguensis TAD1 is a growth-associated PHB-accumulating bacterium without obvious nutrient limitation, which was capable of accumulating PHB up to 83.6 % of cell dry weight (CDW, w/w) within just 24 h at 45 °C from glucose. Surprisingly, the PHB production of C. daeguensis TAD1 exhibited strong tolerance to high heat stress as well as nitrogen loads compared to that of other PHB-accumulating bacterium, while the optimal PHB amount (3.44?±?0.3 g l^?1) occurred at 50 °C and C/N?=?30 (molar) with glucose as the sole carbon source. In addition, C. daeguensis TAD1 could effectively utilize various cheap substrates (starch or glycerol) for PHB production without pre-hydrolyzed, particularly the glycerol, exhibiting the highest product yield (Y _P/S, 0.26 g PHB per gram substrate used) as well as PHB content (80.4 % of CDW, w/w) compared to other carbon sources. Consequently, C. daeguensis TAD1 is a viable candidate for large-scale production of PHB via utilizing starch or glycerol as the raw materials.     

Rational design of a synthetic Entner–Doudoroff pathway for enhancing glucose transformation to isobutanol in <Emphasis Type="Italic">Escherichia coli</Emphasis>

Isobutanol as a more desirable biofuel has attracted much attention. In our previous work, an isobutanol-producing strain Escherichia coli LA09 had been obtained by rational redox status improvement under guidance of the genome-scale metabolic model. However, the low transformation from sugar to isobutanol is a limiting factor for isobutanol production by E. coli LA09. In this study, the intracellular metabolic profiles of the isobutanol-producing E. coli LA09 with different initial glucose concentrations were investigated and the metabolic reaction of fructose 6-phosphate to 1, 6-diphosphate fructose in glycolytic pathway was identified as the rate-limiting step of glucose transformation. Thus, redesigned carbon catabolism was implemented by altering flux of sugar metabolism. Here, the heterologous Entner–Doudoroff (ED) pathway from Zymomonas mobilis was constructed, and the adaptation of upper and lower parts of ED pathway was further improved with artificial promoters to alleviate the accumulation of toxic intermediate metabolite 2-keto-3-deoxy-6-phospho-gluconate (KDPG). Finally, the best isobutanol-producing E. coli ED02 with higher glucose transformation and isobutanol production was obtained. In the fermentation of strain E. coli ED02 with 45 g/L initial glucose, the isobutanol titer, yield and average producing rate were, respectively, increased by 56.8, 47.4 and 88.1% to 13.67 g/L, 0.50 C-mol/C-mol and 0.456 g/(L × h) in a shorter time of 30 h, compared with that of the starting strain E. coli LA09.     

Elongation Factor P is Dispensable in Escherichia coli and Pseudomonas aeruginosa

3-Hydroxypropionic acid (3-HP) is a commercially important platform chemical from which a panel of chemicals can be generated. Klebsiella pneumoniae has been regarded as a promising host strain in glycerol-based 3-HP production for its exceptional ability to metabolize glycerol. Since the glycerol dissimilation mechanism governs the carbon flux distribution from glycerol, inducible strong promoters were usually employed to enhance the glycerol consumption and 3-HP production. Here, we report an alternative strategy that the native promoter of dhaB gene was applied to enhance 3-HP production in K. pneumoniae. The key enzyme genes (ald4 and dhaB) for 3-HP biosynthesis were co-expressed under this promoter. Metabolic analysis revealed that the 3-HP formation was partially coupled with cell metabolism. To optimize the production of 3-HP, the effects of glucose as energy source assistant were investigated based on the analysis of fermentation process kinetics. The highest 3-HP yield (3.77 g/L in flask) was observed upon optimized conditions. Since there were no additional inducers needed, the strategy of employing native promoter seems more feasible to industrial application. More importantly, the employment of constitutive promoter demonstrated an effective approach for decoupling the natural correlation between respiratory metabolism and glycerol dissimilation in K. pneumoniae.     

Genome-scale reconstruction and analysis of the metabolic network in the hyperthermophilic archaeon sulfolobus solfataricus

We describe the reconstruction of a genome-scale metabolic model of the crenarchaeon Sulfolobus solfataricus, a hyperthermoacidophilic microorganism. It grows in terrestrial volcanic hot springs with growth occurring at pH 2-4 (optimum 3.5) and a temperature of 75-80°C (optimum 80°C). The genome of Sulfolobus solfataricus P2 contains 2,992,245 bp on a single circular chromosome and encodes 2,977 proteins and a number of RNAs. The network comprises 718 metabolic and 58 transport/exchange reactions and 705 unique metabolites, based on the annotated genome and available biochemical data. Using the model in conjunction with constraint-based methods, we simulated the metabolic fluxes induced by different environmental and genetic conditions. The predictions were compared to experimental measurements and phenotypes of S. solfataricus. Furthermore, the performance of the network for 35 different carbon sources known for S. solfataricus from the literature was simulated. Comparing the growth on different carbon sources revealed that glycerol is the carbon source with the highest biomass flux per imported carbon atom (75% higher than glucose). Experimental data was also used to fit the model to phenotypic observations. In addition to the commonly known heterotrophic growth of S. solfataricus, the crenarchaeon is also able to grow autotrophically using the hydroxypropionate-hydroxybutyrate cycle for bicarbonate fixation. We integrated this pathway into our model and compared bicarbonate fixation with growth on glucose as sole carbon source. Finally, we tested the robustness of the metabolism with respect to gene deletions using the method of Minimization of Metabolic Adjustment (MOMA), which predicted that 18% of all possible single gene deletions would be lethal for the organism.     

氧化还原电位对Actinobacillus succinogenes厌氧发酵生产丁二酸的影响

为提高琥珀酸放线菌Actinobacillus succinogenes CGMCC1593厌氧发酵产丁二酸的水平。研究了以葡萄糖为C源,发酵液中不同氧化还原电位（VORP）对A．succirtogenes CGMCC1593生长和代谢产物分布的影响。结果表明：菌体生长和丁二酸积累的较佳VORP分别为-220mV和-270mV;利用代谢流分析法,比较VORP在-220mV和-270mV时发酵对数生长期（8h）和稳定期（20h）的代谢通量分布,以及发酵过程中磷酸烯醇式丙酮酸（PEP）、丙酮酸（Pyr）节点,NADH通量分配的变化,由此得出在VORP为-270mV时,NADH总通量和丁二酸方向代谢通量增幅明显。在发酵过程中,通过降低VORP至-270mV,使丁二酸的产率从70％提高到85％。