Physiology of yeasts in relation to biomass yields

The stoichiometric limit to the biomass yield (maximal assimilation of the carbon source) is determined by the amount of CO2 lost in anabolism and the amount of carbon source required for generation of NADPH. This stoichiometric limit may be reached when yeasts utilize formate as an additional energy source. Factors affecting the biomass yield on single substrates are discussed under the following headings: Energy requirement for biomass formation (YATP). YATP depends strongly on the nature of the carbon source. Cell composition. The macroscopic composition of the biomass, and in particular the protein content, has a considerable effect on the ATP requirement for biomass formation. Hence, determination of for instance the protein content of biomass is relevant in studies on bioenergetics. Transport of the carbon source. Active (i.e. energy-requiring) transport, which occurs for a number of sugars and polyols, may contribute significantly to the calculated theoretical ATP requirement for biomass formation. P/O-ratio. The efficiency of mitochondrial energy generation has a strong effect on the cell yield. The P/O-ratio is determined to a major extent by the number of proton-translocating sites in the mitochondrial respiratory chain. Maintenance and environmental factors. Factors such as osmotic stress, heavy metals, oxygen and carbon dioxide pressures, temperature and pH affect the yield of yeasts. Various mechanisms may be involved, often affecting the maintenance energy requirement. Metabolites such as ethanol and weak acids. Ethanol increases the permeability of the plasma membrane, whereas weak acids can act as proton conductors. Energy content of the growth substrate. It has often been attempted in the literature to predict the biomass yield by correlating the energy content of the carbon source (represented by the degree of reduction) to the biomass yield or the percentage assimilation of the carbon source. An analysis of biomass yields of Candida utilis on a large number of carbon sources indicates that the biomass yield is mainly determined by the biochemical pathways leading to biomass formation, rather than by the energy content of the substrate.     

Misconceptions About the Relation Between Plant Growth and Respiration

Abstract: The relation between plant growth rate and respiration rate is readily derived from the overall chemical reaction for aerobic metabolism. The derived relation can be used to show that separation of respiration into growth (g) and maintenance (m) components is not a useful concept. g and m cannot be unambiguously measured or defined in terms of biochemical processes. Moreover, because growth yield calculations from biochemical pathway analysis, from biomass molecular composition, from biomass heat of combustion, and from biomass elemental composition have not included all of the energy costs for biosynthesis, they are not accurate measures of the carbon cost for plant growth. Improper definitions of growth-respiration relations are impeding the use of physiological properties for prediction of plant growth as a function of environmental variables.     

Utilization of mass–energy balance regularities in the analysis of continuous-culture data

Material and energy balances for continuous-culture processes are described based on the facts that the heat of reaction per electron transferred to oxygen for a wide variety of organic molecules, the number of available electrons per carbon atom in biomass, and the weight fraction carbon in biomass are relatively constant. Energy requirements for growth and maintenance are investigated and related to the biomass energetic yield. The consistency of experimental data is examined using material and energy balances and the regularities identified above. When extracellular products are absent, the consistency of yield models containing separate terms for growth and maintenance may be investigated using organic substrate consumption, biomass production, oxygen consumption (or heat evolution), and carbon dioxide evolution rate data for a series of dilution rates. The consistency of continuous-culture data in the published literature is examined.     

Time‐dependent climate impact of a bioenergy system – methodology development and application to Swedish conditions

The area of dedicated energy crops is expected to increase in Sweden. This will result in direct land use changes, which may affect the carbon stocks in soil and biomass, as well as yield levels and the use of inputs. Carbon dioxide (CO₂) fluxes of biomass are often not considered when calculating the climate impact in life cycle assessments (LCA) assuming that the CO₂ released at combustion has recently been captured by the biomass in question. With the extended time lag between capture and release of CO₂ inherent in many perennial bioenergy systems, the relation between carbon neutrality and climate neutrality may be questioned. In this paper, previously published methodologies and models are combined in a methodological framework that can assist LCA practitioners in interpreting the time‐dependent climate impact of a bioenergy system. The treatment of carbon differs from conventional LCA practice in that no distinction is made between fossil and biogenic carbon. A time‐dependent indicator is used to enable a representation of the climate impact that is not dependent on the choice of a specific characterization time horizon or time of evaluation and that does not use characterization factors, such as global warming potential and global temperature potential. The indicator used to aid in the interpretation phase of this paper is global mean surface temperature change (ΔT_s(n)). A theoretical system producing willow for district heating was used to study land use change effects depending on previous land use and variations in the standing biomass carbon stocks. When replacing annual crops with willow this system presented a cooling contribution to ΔT_s(n). However, the first years after establishing the willow plantation it presented a warming contribution to ΔT_s(n). This behavior was due mainly to soil organic carbon (SOC) variation. A rapid initial increase in standing biomass counteracted the initial SOC loss.     

Improved ε‐poly‐l‐lysine productivity partly resulting from rapid cell growth in cultures using a glucose–glycerol mixed carbon source

Productivity enhancements with mixed carbon sources are usually accompanied by simultaneous improvement of cell growth. However, whether the enhanced cell growth in mixed carbon sources influences the biochemical productivity of ε‐poly‐l ‐lysine (ε‐PL) still remains unclear. In this study, we investigated the effect of growth rate on the ε‐PL productivity in a glucose–glycerol mixed carbon source. Based on the typical ε‐PL fed batch fermentation, chemostat culture and relevant physiological analyses were carried out. The ε‐PL productivity was positively correlated to the growth rate ranging from 0.02 to 0.06 h^?1. The primary metabolism activity was enhanced at higher growth rate, providing sufficient precursor l ‐lysine and energy for ε‐PL production. Meanwhile, these two key elements were equally important for biomass production, which could be quickly produced when the cells were fast growing. In addition, rapid growth also strengthened the antioxidant capacity of cells to defend potential oxidative stress. The positive correlation between the growth rate and ε‐PL productivity indicated that the improvement of ε‐PL productivity in the mixed carbon source was partly attributed to the simultaneously enhanced cell growth. Information obtained may provide references for further studies on other secondary biochemicals’ production using mixed carbon sources.     

Growth and biomass production with enhanced β‐glucan and dietary fibre contents of Ganoderma australe ATHUM 4345 in a batch‐stirred tank bioreactor

In this study we maximized biomass production by the basidiomycete Ganoderma australe ATHUM 4345, a species of pharmaceutical interest as it is a valuable source of nutraceuticals, including dietary fibers and glucans. We used the Biolog FF MicroPlate to screen 95 different carbon sources for growth monitoring. The pattern of substrate catabolism forms a substrate assimilation fingerprint, which is useful in selecting components for media optimization of maximum biomass production. Response surface methodology, based on the central composite design was applied to explore the optimum concentrations of carbon and nitrogen sources of culture medium in shake flask cultures. When the improved culture medium was tested in a 20‐L stirred tank bioreactor, using 13.7 g/L glucose and 30.0 g/L yeast extract, high biomass yields (10.1±0.4 g/L) and productivity of 0.09 g L^?1 h^?1 were obtained. The yield coefficients for total glucan and dietary fibers on biomass formed were 94.82±6 and 341.15±12.3 mg/g mycelium dry weight, respectively.     

Simultaneous utilization of methanol and glucose by Hansenula polymorpha (Torulopsis sp.) MH 26, a chemostatic investigation on the distribution of 14C-methanol

Simultaneous utilization of methanol and glucose by Hansenula polymorpha (Torulopsis sp.) MH 26 results in an increase in growth yield of up to 18% in dependence of the mixing proportion. The distribution of carbon from ¹⁴C-methanol into biomass and carbon dioxide was investigated. The distribution pattern was strongly influenced by the mixing proportion showing that methanol plays an increasing role as an energy donor as the glucose content in the mixture increased. Due to these data and verified by theoretical considerations the effect on growth yield was discussed to be caused by an interconnection in the conversion of the individual substrates. This is attributed to glucose delivering the acceptor for C₁-assimilation and resulting in a more efficient utilization of glucose carbon on the one as well as the energy content of methanol on the other hand.     

Products, requirements and efficiency of biosynthesis: a quantitative approach

The question of how many grams of an organism can grow heterotrophically from only 1·0 g of glucose and adequate minerals has been put forward many times. Only a few attempts have been made to answer this question theoretically and these attempts were rather rough. In this paper, it is demonstrated that the yield of a growth process may be accurately computed by considering the relevant biochemistry of conversion reactions and the cytological implications of biosynthesis and growth. Oxygen consumption and carbon dioxide production by these processes are also computed. The weight of the biomass synthesized from 1·0 g of substrate and the quantities of gases exchanged are independent of temperature.These results are obtained by adding the individual equations describing the formation of each compound synthesized by the organism from the substrate supplied. The sum represents an equation which accounts for all substrate molecules required for biosynthesis of the carbon skeletons of an end-product, whose chemical composition is given. It is then calculated how much energy is required for the non-synthetic processes which form a part of biosynthesis, such as intra- and intercellular transport of molecules and maintenance of RNA and enzymes. The additional amount of substrate required to provide this energy by combustion is easily calculated. Adding this substrate to the amount used for skeleton synthesis gives an overall equation which quantifies the substrate and oxygen demand as well as carbon dioxide evolution during biosynthesis of 1·0 g biomass. For example, it requires 1·34 g of glucose with adequate ammonia and minerals to synthesize 1·0 g maize plant biomass in darkness; during this process 0·14 g oxygen are consumed and 0·24 g carbon dioxide are produced. It has been described elsewhere that similar results were obtained experimentally with growing plants.Such results depend considerably upon the chemical composition of the biomass being synthesized and upon the state (oxidized or reduced) of the nitrogen source. Other parameters, such as the number of ATP molecules required for protein synthesis, the possibility for utilization of alternative pathways for synthesis or energy production, the presence or absence of compartmentation of synthetic processes and variations in the P/O ratio between two and three, under many conditions affect results of the computation less than 10%.Since maintenance of cellular structures is not considered, the approach concerns the gross yield of biosynthesis. It predicts therefore the dry matter yield of heterotrophic cells from a given quantity of substrate at high relative growth rates.     

Microbial Production of 4,4′-Dihydroxybiphenyl: Biphenyl Hydroxylation by Fungi

Of 15 species of fungi examined for their ability to hydroxylate biphenyl, 10 produced 4-hydroxybiphenyl. Seven of the 10 also produced 4,4′-dihydroxybiphenyl. The most efficient strains, Absidia pseudocylindrospora NRRL 2770 and Absidia sp. NRRL 1341, were more closely examined to determine their growth characteristics and the kinetics of biphenyl hydroxylation in batch fermentation. In the absence of biphenyl, A. pseudocylindrospora 2770 and Absidia sp. 1341 grew about as rapidly and efficiently in a defined glucose minimal medium as in a complex medium. Substrate yield coefficients for glucose in both media were 0.4 to 0.5 g of biomass per g of glucose, and the specific growth rate was about 0.17 h⁻¹ (doubling time, about 4 h). In this unoptimized system, 10 to 15 g of biomass per liter (dry weight) could be produced, using a defined salt solution and glucose as sole carbon and energy source. In the presence of biphenyl, growth was inhibited, more so for strain 1341 than for strain 2770. However, the specific activity for biphenyl hydroxylation (milligrams of biphenol per gram of biomass) was about 3.5 times greater for strain 1341. Furthermore, biphenyl hydroxylation appeared to require the presence of an oxidizable carbon and energy source (and perhaps growth) to proceed and, at least for strain 1341, hydroxylation seemed to be more efficient in the complex medium.     

The role of formate for the growth of Rhodococcus opacus UFZ B 408 on pyridine

R. opacus UFZ B 408 is able to use pyridine, a potentially growth-inhibiting substrate, as the sole source of carbon, energy and nitrogen. In a previous publication [1] we reported that with the simultaneous utilization of a second carbon and energy source in carbon-substrate-limited chemostat culture, stable steady states could be achieved at higher dilution rates than with growth on pyridine as the sole substrate. Owing to the higher growth yield during growth on such a substrate mixture, both the specific pyridine consumption rates and the residual pyridine concentrations were lower at similar dilution rates than with growth on pyridine alone. Therefore, the critical growth-inhibitory pyridine concentration was only achieved at a higher dilution rate. With the investigations presented here in carbon-substrate-limited continuous culture, the simultaneous utilization of pyridine and formate by R. opacus UFZ B 408 was studied. The yield coefficient during growth on pyridine as the sole substrate amounted to about 0.55 g dry mass/g pyridine. Theoretically, however, the carbon-metabolism-determined yield coefficient should have been about 0.915 g dry mass/g pyridine. Because of the difference between these two values the conclusion was drawn that pyridine is energetically deficient. That means that during growth on pyridine a part of the substrate was dissimilated to supply the energy required for the incorporation of the pyridine carbon into biomass. Formate cannot be used as a carbon source for growth by R. opacus UFZ B 408. However, with growth on pyridine, formate was oxidized simultaneously. During growth on pyridine/formate mixtures, the yield coefficient could be enhanced up to 0.7 g dry mass/g pyridine. That means that biologically usable energy, generated in the course of the formate oxidation, was used for the assimilation of pyridine carbon. The increase in the yield coefficient was related to the utilization ratio of formate to pyridine in a linear manner. However, the carbon-metabolism-determined yield coefficient of 0.915 g dry mass/g pyridine could not be achieved. That can be put down to the fact that R. opacus UFZ B 408 possesses only a limited capacity to oxidize externally supplied formate. Because of the limited formate oxidation capacity the probability is low that, with simultaneous utilization of formate, stable steady states could be achieved at substantially higher dilution rates than with growth on pyridine alone. Enzymatic studies revealed the induction of both NAD(P)⁺-linked glutaric dialdehyde dehydrogenase and isocitrate lyase during growth on pyridine. Therefore, the conclusion was drawn that pyridine is metabolized by R. opacus UFZ B 408 via the same pathway described for the utilization of pyridine by Nocardia Z1 [2]. This conclusion implies that the ability to oxidize formate represents a metabolic performance which seems not to be directly related to the pyridine metabolism of R. opacus UFZ B 408.     

Modeling of microbial substrate conversion,growth and product formation in a recycling fermentor

Paracoccus denitrificans and Bacillus licheniformis were grown in a carbon- and energy source-limited recycling fermentor with 100% biomass feedback. Experimental data for biomass accumulation and product formation as well as rates of carbon dioxide evolution and oxygen consumption were used in a parameter optimization procedure. This procedure was applied on a model which describes biomass growth as a linear function of the substrate consumption rate and the rate of product formation as a linear function of the biomass growth rate. The fitting procedure yielded two growth domains for P. denitrificans. In the first domain the values for the maximal growth yield and the maintenance coefficient were identical to those found in a series of chemostat experiments. The second domain could be described best with linear biomass increase, which is equal to a constant growth yield. Experimental data of a protease producing B. licheniformis also yielded two growth domains via the fitting procedure. Again, in the first domain, maximal growth yield and maintenance requirements were not significantly different from those derived from a series of chemostat experiments. Domain 2 behaviour was different from that observed with P. denitrificans. Product formation halts and more glucose becomes available for biomass formation, and consequently the specific growth rate increases in the shift from domain 1 to 2. It is concluded that for many industrial production processes, it is important to select organisms on the basis of a low maintenance coefficient and a high basic production of the desired product. It seems less important that the maximal production becomes optimized, which is the basis of most selection procedures.     

Flow of 14C-methanol via assimilatory and dissimilatory sequences with yeast in presence of glucose

Experiments were performed to reveal the extent to which individual heterotrophic substrates of a mixture contribute to the overall carbon and energy metabolism. For this reason Hansenula polymorpha MH 20 was chemostatically (C-limited) cultivated at different growth rates on mixtures of methanol and glucose fed at proportions of 3:1 and 1:3 (in weight units), respectively. The distributions of ¹⁴C-carbon from methanol in biomass as well as carbon dioxide (and supernatant) fractions were determined. From these results it followed, firstly, that energy derived from methanol dissimilation was used in part for the incorporation of glucose carbon, resulting in carbon conversion efficiencies for this substrate equivalent to yield coefficients of 0.61–0.69 g/g. Secondly, the growth yield data revealed that the efficiency of methanol conversion had to be increased in order to account for the experimentally determined yield figures. This was further confirmed by theoretical treatment of the growth yield data which showed that these could only be obtained if P/O-quotients for methanol conversion similar to those for glucose, i.e. 2.0–2.5, were considered. The latter property was regarded as the main reason for the observed improvement of growth yield accompanying the simultaneous utilization of methanol and glucose in this yeast.Abbreviations ATP_M,a ATP required for incorporation of assimilated methanol at a given P/O-quotient - ATP_M,d ATP generated from dissimilated methanol at a given P/O-quotient - G and M glucose and methanol; respectively (the indices u, a, d and e mean utilized, assimilated, dissimilated and incorporated by excess energy, respectively) - PGA 3-phosphoglyceric acid - Y _G ^app apparent growth yield on glucose in presence of methanol - Y _G ^P/O theoretical growth yield on glucose at a given P/O-quotient     

Physiological diversity within the <Emphasis Type="Italic">kluyveromyces marxianus</Emphasis> species

The Kluyveromyces marxianus strains CBS 6556, CBS 397 and CBS 712^T were cultivated on a defined medium with either glucose, lactose or sucrose as the sole carbon source, at 30 and 37°C. The aim of this work was to evaluate the diversity within this species, in terms of the macroscopic physiology. The main properties evaluated were: intensity of the Crabtree effect, specific growth rate, biomass yield on substrate, metabolite excretion and protein secretion capacity, inferred by measuring extracellular inulinase activity. The strain Kluyveromyces lactis CBS 2359 was evaluated in parallel, since it is the best described Kluyveromyces yeast and thus can be used as a control for the experimental setup. K. marxianus CBS 6556 presented the highest specific growth rate (0.70 h⁻¹) and the highest specific inulinase activity (1.65 U mg⁻¹ dry cell weight) among all strains investigated, when grown at 37°C with sucrose as the sole carbon source. The lowest metabolite formation and highest biomass yield on substrate (0.59 g dry cell weight g sucrose⁻¹) was achieved by K. marxianus CBS 712^T at 37°C. Taken together, the results show a systematic comparison of carbon and energy metabolism among three of the best known K. marxianus strains, in parallel to K. lactis CBS 2359.     

Extractability of polyhydroxyalkanoate synthesized by <Emphasis Type="Italic">Bacillus flexus</Emphasis> cultivated in organic and inorganic nutrient media

Bacillus flexus was isolated from local soil sample and identified by molecular methods. In inorganic nutrient medium (IM) containing sucrose as carbon source, yield of biomass and polyhydroxyalkanoate (PHA) were 2 g/l and 1 g/l (50% of biomass), respectively. Substitution of inorganic nitrogen by peptone, yeast extract or beef extract resulted in biomass yields of 4.1, 3.9 and 1.6 g/l, respectively. Corresponding yields of PHA in biomass was 30%, 40% and 44%. Cells subjected to change in nutrient condition from organic to inorganic, lacked diaminopimelic acid in the cell wall and the concentration of amino acids also decreased. Under these conditions the extractability of the polymer from the cells by hot chloroform or mild alkali hydrolysis was 86–100% compared to those grown in yeast extract or peptone (32–56%). The results demonstrated that growth, PHA production and the composition of cell wall of B. flexus are influenced by the organic or inorganic nutrients present in the growth medium. Cells grown in inorganic medium lysed easily and this can be further exploited for easier recovery of the intracellular PHA.     

Genome‐derived minimal metabolic models for Escherichia coli MG1655 with estimated in vivo respiratory ATP stoichiometry

Metabolic network models describing growth of Escherichia coli on glucose, glycerol and acetate were derived from a genome scale model of E. coli. One of the uncertainties in the metabolic networks is the exact stoichiometry of energy generating and consuming processes. Accurate estimation of biomass and product yields requires correct information on the ATP stoichiometry. The unknown ATP stoichiometry parameters of the constructed E. coli network were estimated from experimental data of eight different aerobic chemostat experiments carried out with E. coli MG1655, grown at different dilution rates (0.025, 0.05, 0.1, and 0.3 h^?1) and on different carbon substrates (glucose, glycerol, and acetate). Proper estimation of the ATP stoichiometry requires proper information on the biomass composition of the organism as well as accurate assessment of net conversion rates under well‐defined conditions. For this purpose a growth rate dependent biomass composition was derived, based on measurements and literature data. After incorporation of the growth rate dependent biomass composition in a metabolic network model, an effective P/O ratio of 1.49 ± 0.26 mol of ATP/mol of O, K_X (growth dependent maintenance) of 0.46 ± 0.27 mol of ATP/C‐mol of biomass and m_ATP (growth independent maintenance) of 0.075 ± 0.015 mol of ATP/C‐mol of biomass/h were estimated using a newly developed Comprehensive Data Reconciliation (CDR) method, assuming that the three energetic parameters were independent of the growth rate and the used substrate. The resulting metabolic network model only requires the specific rate of growth, µ, as an input in order to accurately predict all other fluxes and yields. Biotechnol. Bioeng. 2010;107: 369–381. © 2010 Wiley Periodicals, Inc.     

Data consistency,yield and maintenance coefficient for the growth of Candida lypoytica and Pseudomonas aeruginosa on n-hexadecane

Summary When more than the minimum number of variables are measured, and measurement error is taken into account, the results of parameter estimation depend on which of the measured variables are selected for this purpose. The reparameterization of Pirt's models for growth produces multiresponse models with common parameters. By using the covariate adjustment technique, a unit variate linear model with covariates is obtained. This allows a combined point and interval estimates of biomass energetic yield and maintenance coefficient to be obtained using standard multiple regression programmes. When this method was applied using form I and form II of the Pirt's models, good combined estimates were obtained and compared. Using data from the literature for Candida lipolytica produced reliable results. However, for Pseudomonas aeruginosa, which has been known to produce intermediate products, a modified Pirt's model is required for a good estimate of the biomass energetic yield.Nomenclature a Mole of ammonia per quantity of organic substrate containing 1 g atom carbon, g mole/g atom carbon - b Moles of oxygen per quantity of organic substrate containing 1 g atom carbon, g mole/g atom carbon - c Moles of water per quantity of organic substrate containing 1 g atom carbon, g mole/g atom carbon; no of covariates included in model - d Moles of carbon dioxide per quantity of organic substrate containing 1 g atom carbon, g mole/g atom carbon - e _i Error terms in Eqs. (6–8) - l Atomic ratio of oxygen to carbon in organic substrate, dimensionless - m Atomic ratio of hydrogen to carbon in organic substrate, dimensionless - m _e Rate of organic substrate consumption for maintenance, g equiv. of available electrons in biomass (h) or kcal/Kcal of biomass(h) - n Atomic ratio of oxygen to carbon in biomass, dimensionless - p Atomic ratio of hydrogen to carbon in biomass, dimensionless - Q _{CO 2} Rate of evolution of carbon dioxide, g moles/g dry wt (h) - Q _{O 2} Rate of oxygen consumption, g moles/g dry wt (h) - Q _s Rate of organic substrate consumption g/g dry wt (h) - q Atomic ratio of nitrogen to carbon in biomass, dimensionless - r Atom ratio of hydrogen to carbon in products, dimensionless; the number of parameters of interest - s Atomic ratio of oxygen to carbon in products, dimensionless - t Atomic ratio of nitrogen to carbon in products, dimensionless - r Mean of k responses in Eq. (10) - x _ki Kth response in the ith observation - y _c Biomass carbon yield (fraction of organic substrate carbon in biomass), dimensionless - z _i Covariate matrix - z Fraction of organic substrate carbon in products, dimensionless - a _i Parameters associated with covariates - _s Reductance degree of biomass, equivalents of available electrons per gram atom carbon - Reductance degree of organic substrate, equivalents of available electrons per gram atom carbon - Fraction of energy in organic substrate which is evolved as heat, dimensionless - Fraction of available electrons transferred to biomass; biomass energetic yield - True growth yield - Specific growth rate, h^-1 - _p Fraction of available electrons incorporated into products; product energetic yield - Correlation coefficient - Mass fraction carbon - ² Mean square error of model (10)     

Biotransformation of 5‐Hydroxymethylfurfural to 2,5‐Furandicarboxylic Acid by a Syntrophic Consortium of Engineered Synechococcus elongatus and Pseudomonas putida

2,5‐furandicarboxylic acid (FDCA) is one of the top platform chemicals that can be produced from biomass feedstock. To make the cost of industrial FDCA production compatible with plastics made from fossils, the price of substrates and process complexity should be reduced. The aim of this research is to create a CO₂‐driven syntrophic consortium for the catalytic conversion of renewable biomass‐derived 5‐hydroxymethylfurfural (HMF) to FDCA. Sucrose produced from carbon fixation by the engineered Synechococcus elongatus serves as the sole carbon source for the engineered Pseudomonas putida to catalyze the reaction of HMF to FDCA. The yield of FDCA by the consortium reaches around 70% while the conversion of HMF is close to 100%. With further surface engineering to clump the two strains, the FDCA yield is elevated to almost 100% via the specific association between an Src homology 3 (SH3) domain and its ligand. The syntrophic consortium successfully demonstrates its green and cost‐effective characteristics for the conversion of CO₂ and biomass into platform chemicals.     

Computation of metabolic fluxes and efficiencies for biological carbon dioxide fixation

With rising energy prices and concern over the environmental impact of fossil fuel consumption, the push to develop biomass derived fuels has increased significantly. Although most global carbon fixation occurs via the Calvin Benson Bassham cycle, there are currently five other known pathways for carbon fixation; the goal of this study was to determine the thermodynamic efficiencies of all six carbon fixation pathways for the production of biomass using flux balance analysis. The three chemotrophic pathways, the reductive acetyl-CoA pathway, the 3-hydroxypropionate/4-hydroxybutyrate cycle and the dicarboxylate/4-hydroxybutyrate cycle, were found to be more efficient than photoautotrophic carbon fixation pathways. However, as hydrogen is not freely available, the energetic cost of hydrogen production from sunlight was calculated and included in the overall energy demand, which results in a 5 fold increase in the energy demand of chemoautotrophic carbon fixation. Therefore, when the cost of hydrogen production is included, photoautotrophic pathways are more efficient. However, the energetic cost for the production of 12 metabolic precursors was found to vary widely across the different carbon fixation pathways; therefore, different pathways may be more efficient at producing products from a single precursor than others. The results of this study have significant impact on the selection or design of autotrophic organisms for biofuel or biochemical production. Overall biomass production from solar energy is most efficient in organisms using the reductive TCA cycle, however, products derived from one metabolic precursor may be more efficiently produced using other carbon fixation pathways.     

Sugar fermentation by <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> to produce ethanol

As a first step in the research on ethanol production from lignocellulose residues, sugar fermentation by Fusarium oxysporum in oxygen-limited conditions is studied in this work. As a substrate, solutions of arabinose, glucose, xylose and glucose/xylose mixtures are employed. The main kinetic and yield parameters of the process are determined according to a time-dependent model. The microorganism growth is characterized by the maximum specific growth rate and biomass productivity, the substrate consumption is studied through the specific consumption rate and biomass yield, and the product formation via the specific production rate and product yields. In conclusion, F. oxysporum can convert glucose and xylose into ethanol with product yields of 0.38 and 0.25, respectively; when using a glucose/xylose mixture as carbon source, the sugars are utilized sequentially and a maximum value of 0.28 g/g ethanol yield is determined from a 50% glucose/50% xylose mixture. Although fermentation performance by F.␣oxysporum is somewhat lower than that of other fermenting microorganisms, its ability for simultaneous lignocellulose-residue saccharification and fermentation is considered as a potential advantage.