Catabolic enzyme levels in bacteria grown on binary and ternary substrate mixtures in continuous culture

Bacteria grow on multicomponent substrates in most natural and engineered environments. To advance our ability to model bacterial growth on such substrates, axenic cultures were grown in chemostats at a low specific growth rate and a constant total energy flux on binary and ternary substrate mixtures and were assayed for key catabolic enzymes for each substrate. The substrates were benzoate, salicylate, and glucose, and the enzymes were catechol 1,2-dioxygenase, gentisate 1,2-dioxygenase, and glucose-6-phosphate dehydrogenase, respectively. The binary mixtures were salicylate with benzoate and salicylate with glucose. Measurements were also made of oxygen uptake rate by whole cells in response to each substrate. The effects of the substrate mixture on the oxygen uptake rate paralleled the effects on the measured enzymes. Catechol 1,2-dioxygenase exhibited a threshold response before synthesis occurred. Below the threshold flux of benzoate through the chemostat, either basal enzyme levels or nonspecific enzymes kept reactor concentrations too low for enzyme synthesis. Above the threshold, enzyme levels were linearly related to the fraction of the total energy flux through the chemostat due to benzoate. Gentisate 1,2-dioxygenase exhibited a linear response to the salicylate flux when mixed with benzoate, but a threshold response when mixed with glucose. Glucose-6-phosphate dehydrogenase activity increased in direct proportion to the glucose flux through the chemostat over the entire range studied. The results from two ternary mixtures were consistent with those from the binary mixtures.     

刺芹侧耳菌渣对肺形侧耳（秀珍菇）生长和营养成分的影响

以常规棉籽壳培养料为对照,用刺芹侧耳Pleurotus eryngii工厂化生产的废料（简称菌渣）代替部分棉籽壳进行格氏侧耳Pleurotus geesterani栽培试验,并测定格氏侧耳的营养成分。结果表明,供试培养料中菌丝长势均良好,随着菌渣替代比例的增加,菌丝满袋时间、原基出现时间和转潮期缩短,生长周期变短;菌渣代替比例增加,产量呈降低趋势,其中替代比例33%（1:2）和55%（1.2:1）时培养料的生物学效率与对照无显著差异,替代比例55%和78%（3.5:1）时培养料对第四潮和第五潮菇产量影响明显,显著低于对照;供试菌渣培养料栽培格氏侧耳的粗蛋白和总氨基酸含量略高于或显著高于对照,说明添加适量的菌渣栽培格氏侧耳可提高子实体的营养成分。     

Metabolic efficiency and turnover of soil microbial communities in biodegradation tests.

Biodegradability screening tests of soil commonly measure 14CO2 evolution from radiolabeled test compounds, and glucose has often served as a positive control. When constant amounts of radiolabel were added to soil in combination with increasing amounts of unlabeled substrates, glucose and some related hexoses behaved in an anomalous manner. In contrast to that of formate, benzoate, n-hexadecane, or bis(2-ethylhexyl) phthalate, dilution of glucose radiocarbon with unlabeled glucose increased rather than decreased the rate and extent of 14CO2 evolution. [14C]glucose incorporation into biomass and Vmax values were consistent with the interpretation that application of relatively high concentrations of glucose to soil shifts the balance of the soil microbial community from the autochthonous (humus-degrading) to the zymogeneous (opportunistic) segment. The higher growth and turnover rates that define zymogeneous microorganisms, combined with a lower level of carbon incorporation into their biomass, result in the evolution of disproportionate percentages of 14CO2. When used as positive controls, glucose and related hexoses may raise the expectations for percent 14CO2 evolution to levels that are not realistic for other biodegradable compounds.     

Substrate and energy costs of the production of exocellular enzymes by Bacillus licheniformis

Substrate and energy costs of the production of exocellular enzymes from glucose and citrate by B. Iicheniformis S1684 as well as molar growth yields corrected for these costs of product formation were calculated using data from chemostat experiments. The calculations showed that 1.46-1.73 mol glucose and 2.31-2.77 mol citrate are needed for formation and excretion of 1 mol protein. Consequently, the values of the maximal product yield from substrate (Y(psm') g/mol) are 80 < Y(psm) < 95 when product is formed from glucose and 50 < Y(psm) < 60 when product is formed from citrate. The higher substrate costs for product formation from citrate are due to a higher level of CO(2) production during protein formation and a higher substrate requirement for the energy supply of product formation and excretion than when product is formed from glucose. The theoretical ATP requirement for protein synthesis could be determined reasonably well, but the energy costs of protein excretion could not be determined exactly. The energy costs of protein formation are higher than those of biomass formation or protein excretion. Molar growth yields corrected for the substrate costs of product formation were high, indicating a high efficiency of growth.Growth and production parameters were determined as well from experimental data of recycling fermentor experiments using a parameter optimization procedure based on a mathematical model describing biomass growth as a linear function of the substrate consumption rate and the rate of product formation as a linear function of biomass growth rate. The fitting procedure yielded two growth and production domains during glucose limitation. In the first domain the values for the maximal growth yield and maintenance coefficient were in agreement with those found in chemostat experiments at corresponding values of Y(spm). Domain 2 could be described best with linear growth and product formation. In domain 2 the rate of product formation decreased and more substrate became available for biomass formation. As a consequence the specific growth rate increased in the shift from domain 1 to 2. Domain 2 behavior most probably is caused by the rel-status of B. Iicheniformis S1684.     

A theoretical evaluation of growth yields of yeasts

Growth yields of Saccharomyces cerevisiae and Candida utilis in carbon-limited chemostat cultures were evaluated. The yields on ethanol and acetate were much lower in S. cerevisiae, in line with earlier reports that site I phosphorylation is absent in this yeast. However, during aerobic growth on glucose both organisms had the same cell yield. This can be attributed to two factors: --S. cerevisiae had a lower protein content than C. utilis; --uptake of glucose by C. utilis requires energy whereas in S. cerevisiae it occurs via facilitated diffusion. Theoretical calculations showed that, as a result of these two factors, the ATP requirement for biomass formation in C. utilis is 35% higher than in S. cerevisiae (theoretical YATP values of 20.8 and 28.1, respectively). The experimental YATP for anaerobic growth of S. cerevisiae on glucose was 16 g biomass.mol ATP-1. In vivo P/O-ratios can be calculated for aerobic growth on ethanol and acetate, provided that the gap between the theoretical and experimental ATP requirements as observed for growth on glucose is taken into account. This was done in two ways: --via the assumption that the gap is independent of the growth substrate (i.e. a fixed amount of ATP bridges the difference between the theoretical and experimental values). --alternatively, on the assumption that the difference is a fraction of the total ATP expenditure, that is dependent on the substrate. Calculations of P/O-ratios for growth of both yeasts on glucose, ethanol, and acetate made clear that only by assuming a fixed difference between theoretical and experimental ATP requirements, the P/O-ratios are more or less independent of the growth substrate. These P/O-ratios are approximately 30% lower than the calculated mechanistic values.     

Intensification of the exopolysaccharide synthesis by Acinetobacter sp. on an ethanol-glucose mixture: aspects related to biochemistry and bioenergetics

The possibility of intensifying the synthesis of microbial exopolysaccharides (EPS) by a strain of Acinetobacter sp. grown on a mixture of two substrates nonequivalent in terms of bioenergetics (ethanol + glucose) was shown. Based on theoretical calculations of the energy requirements for biomass and EPS synthesis from the energy-deficient substrate (glucose), the supplementary concentration of the energy-excessive substrate (ethanol) was determined that prevents the loss of glucose carbon that occurs when glucose is oxidized to CO2 to obtain energy for the processes of constructive metabolism. This made it possible to increase the efficiency of conversion of the substrate carbon to EPS. The introduction of ethanol into glucose-containing medium at a molar ratio of 3.1:1 allowed the amount of the EPS synthesized to be increased 1.8- to 1.9-fold; their yield relative to biomass increased 1.4- to 1.7-fold, and the yield of EPS relative to the substrate consumed increased 1.5- to 2-fold as compared to growth of the producer on single substrates. These results form the basis for the development of new technologies for obtaining secondary metabolites of practical value with the use of mixed growth substrates.     

Evaluation of growth yield of Spirulina (Arthrospira) sp. in photoautotrophic, heterotrophic and mixotrophic cultures

In microbial cultures, both cellular growth rate and yield (defined as the degree of substrate conversion into the biomass) are important. Although effect of culture conditions on growth kinetics has been well documented for various microbial strains, there is almost no literature concerning the effect of environmental conditions on growth equilibrium, expressed as biomass yield coefficients from substrate. The present paper discusses the effect of culture conditions: irradiance (physical substrate) and glucose concentration (chemical substrate) on biomass yield coefficients from two chemical substrates: glucose and nitrate-nitrogen in photoautotrophic, heterotrophic and mixotrophic culture of blue-green alga Spirulina (Arthrospira) sp. The efficiency of substrates incorporation into the biomass can be precisely determined only if the elemental composition of the biomass is known. The experimental results showed that culture conditions had a substantial influence on biomass yield coefficients (biomass yield from glucose and nitrate-nitrogen). It was found that, the increase of irradiance favoured increase of biomass yield coefficient from both, glucose and nitrate-nitrogen. However, in the case of yield from nitrogen in mixotrophic culture, the effect was opposite. The effect of glucose concentration was different: the higher the initial glucose concentration, the lower the biomass yield coefficients from chemical substrates.     

C<Subscript>1</Subscript> compounds as auxiliary substrate for engineered <Emphasis Type="Italic">Pseudomonas putida</Emphasis> S12

The solvent-tolerant bacterium Pseudomonas putida S12 was engineered to efficiently utilize the C₁ compounds methanol and formaldehyde as auxiliary substrate. The hps and phi genes of Bacillus brevis, encoding two key steps of the ribulose monophosphate (RuMP) pathway, were introduced to construct a pathway for the metabolism of the toxic methanol oxidation intermediate formaldehyde. This approach resulted in a remarkably increased biomass yield on the primary substrate glucose when cultured in C-limited chemostats fed with a mixture of glucose and formaldehyde. With increasing relative formaldehyde feed concentrations, the biomass yield increased from 35% (C-mol biomass/C-mol glucose) without formaldehyde to 91% at 60% relative formaldehyde concentration. The RuMP-pathway expressing strain was also capable of growing to higher relative formaldehyde concentrations than the control strain. The presence of an endogenous methanol oxidizing enzyme activity in P. putida S12 allowed the replacement of formaldehyde with the less toxic methanol, resulting in an 84% (C-mol/C-mol) biomass yield. Thus, by introducing two enzymes of the RuMP pathway, co-utilization of the cheap and renewable substrate methanol was achieved, making an important contribution to the efficient use of P. putida S12 as a bioconversion platform host.     

Xylose anaerobic conversion by open-mixed cultures

Xylose is, after glucose, the dominant sugar in agricultural wastes. In anaerobic environments, carbohydrates are converted into volatile fatty acids and alcohols. These can be used as building blocks in biotechnological or chemical processes, e.g., to produce bioplastics. In this study, xylose fermentation by mixed microbial cultures was investigated and compared with glucose under the same conditions. The product spectrum obtained with both substrates was comparable. It was observed that, in the case of xylose, a higher fraction of the carbon was converted into catabolic products (butyrate, acetate, and ethanol) and the biomass yield was approximately 20% lower than on glucose, 0.16 versus 0.21 Cmol X/Cmol S. This lower yield is likely related to the need of an extra ATP during xylose uptake. When submitted to a pulse of glucose, the population cultivated on xylose could instantaneously convert the glucose. No substrate preference was observed when glucose and xylose were fed simultaneously to the continuously operated bioreactor.     

Biochemical limits to microbial growth yields: An analysis of mixed substrate utilization

A theoretical analysis has been made of carbon conversion efficiency during heterotrophic microbial growth. The expectation was that the maximal growth yield occurs when all the substrate is assimilated and the net flow of carbon through dissimilation is zero. This, however, is not identical to a 100% carbon conversion, since assimilatory pathways lead to a net production of CO(2). It can be shown that the amount of CO(2) produced by way of assimilatory processes is dependent upon the nature of the carbon source, but independent of its degree of reduction and varies between 12 and 29% of the substrate carbon. An analysis of published yield data reveals that nearly complete assimilation can occur during growth on substrates with a high energy content. This holds for substrates with a heat of combustion of ca. 550 kJ/mol C, or a degree of reduction higher than 5 (e.g. ethane, ethanol, and methanol). Complete assimilation can also be achieved on substrates with a lower energy content, provided that an auxiliary energy source is present that cannot be used as a carbon source. This is evident from the cell yields reported for Candida utilis grown on glucose plus formate and for Thiobacillus versutus grown on acetate plus thiosulfate. This evaluation of the carbon conversion efficiency during assimilation also made it possible to compare the energy content of the auxiliary energy substrate added with the quantity of the carbon source it had replaced. It will be shown that utilization of the auxiliary energy source may lead to extreme changes in the efficiency of dissimilatory processes.     

Metabolism of H2-CO2, methanol, and glucose by Butyribacterium methylotrophicum.

The fermentative metabolism of Butyribacterium methylotrophicum grown on either H2-CO2, methanol, glucose, or CO is described. The following reaction stoichiometries were obtained: 1.00 H2 + 0.52 CO2 leads to 0.22 acetate + 0.06 cell C; 1 methanol + 0.18 CO2 + 0.01 acetate leads to 0.24 butyrate + 0.29 cell C; and 1.00 glucose leads to 0.31 CO2 + 1.59 acetate + 0.21 butyrate + 0.13 H2 + 1.58 cell C. Cell yields of 1.7 g (dry weight) per mol of H2, 8.2 g (dry weight) per mol of methanol, 42.7 g (dry weight) per mol of glucose, and 3.0 g (dry weight) per mol of CO were obtained from linear plots of cell synthesis and substrate consumption. Doubling times of 9.0, 9.0, and 3 to 4 h were observed during batch growth on H2-CO2, methanol, and glucose, respectively. Indicative of a growth factor limitation, glucose fermentation in defined medium displayed a lower cell synthesis efficiency than when yeast extract (0.05%) was present. B. methylotrophicum fermentation displayed atypically high substrate/cell carbon synthesis conversion ratios for an anaerobe, as greater than 24% of the carbon was assimilated into cells during growth on methanol or glucose. The data indicate that B. methylotrophicum conserves carbon-bound electrons during growth on single-carbon or multicarbon substrates.     

Intracellular carbon fluxes in riboflavin-producing Bacillus subtilis during growth on two-carbon substrate mixtures

Metabolic responses to cofeeding of different carbon substrates in carbon-limited chemostat cultures were investigated with riboflavin-producing Bacillus subtilis. Relative to the carbon content (or energy content) of the substrates, the biomass yield was lower in all cofeeding experiments than with glucose alone. The riboflavin yield, in contrast, was significantly increased in the acetoin- and gluconate-cofed cultures. In these two scenarios, unusually high intracellular ATP-to-ADP ratios correlated with improved riboflavin yields. Nuclear magnetic resonance spectra recorded with amino acids obtained from biosynthetically directed fractional (13)C labeling experiments were used in an isotope isomer balancing framework to estimate intracellular carbon fluxes. The glycolysis-to-pentose phosphate (PP) pathway split ratio was almost invariant at about 80% in all experiments, a result that was particularly surprising for the cosubstrate gluconate, which feeds directly into the PP pathway. The in vivo activities of the tricarboxylic acid cycle, in contrast, varied more than twofold. The malic enzyme was active with acetate, gluconate, or acetoin cofeeding but not with citrate cofeeding or with glucose alone. The in vivo activity of the gluconeogenic phosphoenolpyruvate carboxykinase was found to be relatively high in all experiments, with the sole exception of the gluconate-cofed culture.     

不同微生物菌剂对基质酶活性和番茄产量及品质的影响

以鸡粪配方基质(腐熟鸡粪:腐熟玉米秸秆:河沙体积为3:4:3)和牛粪配方基质(腐熟牛粪:腐熟玉米秸秆:河沙体积为3:4:3)为试材,研究了基质中分别添加地福来、酵素菌、EM菌、枯草芽孢杆菌和农用微生物菌剂对基质酶活性和番茄产量及品质的影响.结果表明: 两种配方基质添加微生物菌剂40和60 d时根际基质的脲酶、蔗糖酶和碱性磷酸酶活性均显著提高,番茄植株生长量、果实产量及维生素C含量显著高于对照.鸡粪和牛粪配方基质均以添加地福来效果最好,番茄单株产量分别较各自对照增加14.7%和40.0%,果实维生素C含量分别提高22.2%和39.7%.在不添加微生物菌剂的情况下,鸡粪配方基质栽培的番茄单株产量和果实维生素C含量高于牛粪配方基质;分别添加地福来后,两种基质栽培的番茄单株产量和果实维生素C含量无显著差异.     

Formation of the exopolysaccharide ethapolan by <Emphasis Type="Italic">Acinetobacter</Emphasis> sp. IMV B-7005 on a fumarate-glucose mixture

Our studies enabled us to intensify the synthesis of the microbial exopolysaccharide (EPS) ethapolan produced by Acinetobacter sp. IMV B-7005 grown on a mixture of fumarate (an energy-excessive substrate) and glucose (an energy-deficient substrate). Supplementing glucose-containing medium with sodium (potassium) fumarate at a molar ratio of 4: 1 resulted in a 1.3–2.2-fold increase of the EPS amount synthesized and in a 1.3–2-fold increase of the EPS yield relative to the biomass compared to cultivation on monosubstrates. The conversion of the carbon of both substrates to EPS was the highest if the carbon/nitrogen ratio in the cultivation medium was 70.5 and inoculum grown on glucose monosubstrate was used.     

Intracellular Carbon Fluxes in Riboflavin-Producing Bacillussubtilis during Growth on Two-Carbon Substrate Mixtures

Metabolic responses to cofeeding of different carbon substrates in carbon-limited chemostat cultures were investigated with riboflavin-producing Bacillus subtilis. Relative to the carbon content (or energy content) of the substrates, the biomass yield was lower in all cofeeding experiments than with glucose alone. The riboflavin yield, in contrast, was significantly increased in the acetoin- and gluconate-cofed cultures. In these two scenarios, unusually high intracellular ATP-to-ADP ratios correlated with improved riboflavin yields. Nuclear magnetic resonance spectra recorded with amino acids obtained from biosynthetically directed fractional ¹³C labeling experiments were used in an isotope isomer balancing framework to estimate intracellular carbon fluxes. The glycolysis-to-pentose phosphate (PP) pathway split ratio was almost invariant at about 80% in all experiments, a result that was particularly surprising for the cosubstrate gluconate, which feeds directly into the PP pathway. The in vivo activities of the tricarboxylic acid cycle, in contrast, varied more than twofold. The malic enzyme was active with acetate, gluconate, or acetoin cofeeding but not with citrate cofeeding or with glucose alone. The in vivo activity of the gluconeogenic phosphoenolpyruvate carboxykinase was found to be relatively high in all experiments, with the sole exception of the gluconate-cofed culture.     

水杨酸钠对胰岛素抵抗大鼠胰岛素受体底物-1的影响

目的探讨抗炎药水杨酸钠对胰岛素抵抗大鼠胰岛素敏感性的影响及其作用机制。方法分别给大鼠静脉输注脂肪乳+肝素,脂肪乳+肝素+水杨酸钠和生理盐水7 h,并在输注的最后2 h,行清醒状态高胰岛素-正血糖钳夹试验,测定血浆葡萄糖、游离脂肪酸(FFA)、胰岛素和C-肽水平,检测肝脏、肌肉中胰岛素受体底物-1(IRS-1)及307位丝氨酸磷酸化的IRS-1表达。结果输注脂肪乳大鼠葡萄糖输注率(GIR)是输注生理盐水大鼠的45%,水杨酸钠可使GIR提高1.3倍(P0.01)。脂肪乳输注组大鼠肝脏及肌肉中307位丝氨酸磷酸化的IRS-1分别为生理盐水输注组大鼠的3倍和3.8倍(P0.001),输注水杨酸钠,肝脏、肌肉307位丝氨酸磷酸化的IRS-1下降45%、20%(P0.05)。结论 FFA增高引起肝脏及肌肉中307位丝氨酸磷酸化的IRS-1水平增高,可能是导致胰岛素抵抗发生的机制之一,应用水杨酸钠,大鼠肝脏及肌肉组织中IRS-1丝氨酸磷酸化水平下降,胰岛素抵抗改善。抗炎药物水杨酸钠可能通过抑制FFA引起的IRS-1丝氨酸磷酸化,而发挥改善胰岛素抵抗的作用。     

Improvement of Y-values of Hansenula polymorpha growth on methanol by simultaneous utilization of glucose

The simultaneous utilization of methanol and glucose by Hansenula polymorpha MH20 was investigated in chemostat (C-limited) cultivation. The mixed-substrate utilization results in biomass yields which are greater up to 20 to 25% as expected assuming an additive growth on both substrates. This is referred to as an auxiliary-substrate effect. Additionally, methanol can be utilized at higher growth rates in the presence of glucose compared to those obtained on this substrate alone. The extend of the auxiliary-substrate effect and the optimum ratio of substrates to reach this effect depend on dilution rate. The greatest stimulation in yield is obtained at D approximately 0.1 h-1, after raising the dilution rate this effect diminishes. At a rate of 0.1 h-1 the optimum mixed-substrate ratio of methanol: glucose is 7:1 (g). By increasing the growth rate the ratio changes toward glucose and reached a value of 1:1 (g) at D = 0.3 h-1. This change in the optimum ratio correlates with diminution in yield coefficient of methanol accompanying an increase in growth rate greater than 0.15 h-1. Energy balances of the utilization of the single substrates are used for interpretation of these results. From this it is evident that methanol does not play the role of an energy-rich substrate in the metabolism of yeast. Rather glucose is the energy-providing substrate in this combination.     

Effect of aeration and carbon dioxide on cell morphology of Candida utilis

The effect of CO2 availability on cell size, shape, and aggregation in continuous cultures of Candida utilis was studied in minimal medium with glucose or ethanol as the sole carbon and energy source. Enrichment with CO2 was achieved (i) by using the substrate with more C atoms, (ii) by using pure oxygen and thus decreasing aeration intensity at the same dissolved-oxygen concentration, or (iii) by adding CO2 to the aeration gas. The cells were always of yeast shape, and no filaments were formed. In cultures with a biomass concentration above 6 g (dry weight) per liter, no cell aggregates were observed. In cultures with a lower biomass, the daughter cells failed to separate from the parent cells and formed aggregates with thickened walls. The average cell number per aggregate was found to be higher, and the average protoplast volume lower, under conditions of probable CO2 limitation. Simultaneously, the ratio of total dry weight to wet weight of protoplasts was considerably higher, indicating an increased share of wall or extracellular material. The possible effect of the observed morphological changes for maintaining a suitable concentration gradient of CO2 around the cell is discussed.     

Effect of aeration and carbon dioxide on cell morphology of Candida utilis.

The effect of CO2 availability on cell size, shape, and aggregation in continuous cultures of Candida utilis was studied in minimal medium with glucose or ethanol as the sole carbon and energy source. Enrichment with CO2 was achieved (i) by using the substrate with more C atoms, (ii) by using pure oxygen and thus decreasing aeration intensity at the same dissolved-oxygen concentration, or (iii) by adding CO2 to the aeration gas. The cells were always of yeast shape, and no filaments were formed. In cultures with a biomass concentration above 6 g (dry weight) per liter, no cell aggregates were observed. In cultures with a lower biomass, the daughter cells failed to separate from the parent cells and formed aggregates with thickened walls. The average cell number per aggregate was found to be higher, and the average protoplast volume lower, under conditions of probable CO2 limitation. Simultaneously, the ratio of total dry weight to wet weight of protoplasts was considerably higher, indicating an increased share of wall or extracellular material. The possible effect of the observed morphological changes for maintaining a suitable concentration gradient of CO2 around the cell is discussed.     

Continuous and biomass recycle fermentations of Clostridium acetobutylicum

The availability and demand of biosynthetic energy (ATP) is an important factor in the regulation of solvent production in steady state continuous cultures of Clostridium acetobutylicum. The effect of biomass recycle at a variety of dilution rates and recycle ratios under both glucose and non-glucose limited conditions on product yields and selectivities has been investigated. Under conditions of non-glucose limitation, when the ATP supply is not growth-limiting, a lower growth rate imposed by biomass recycle leads to a reduced demand for ATP and substantially higher acetone and butanol yields. When the culture is glucose limited, however, biomass recycle results in lower solvent yields and higher acid yields.List of Symbols A ₆₀₀ absorbance at 600 nm - ATP adenosine triphosphate - C _imol/dm³ concentration of componenti in the fermentor - C _i ⁰ mol/dm³ concentration of componenti in the feed - D h^–1 dilution rate - F dm³/h feed flow rate - FdH₂ ferredoxin, reduced form - NAD nicotinamide adenine dinucleotide, oxidized form - NADH nicotinamide adenine dinucleotide, reduced form - NfF mmol/g/h NADH produced from oxidation of FdH₂ per unit biomass per unit time - P dm³/h filtrate flow during biomass recycle operation - PCRP C-mole carbon per C-mole glucose utilized percent of (substrate) carbon recovered in products - R recycle ratio,P/F - SPR mmol/g/h specific production rate - X _imol product/100 mol glucose utilized product yield - Y _ATP g biomass/mol ATP biomass yield on ATP - Y _GLU g biomass/mol glucose biomass yield on glucose - Y _ig biomass/mol biomass yield on nutrienti - h^–1 specific growth rate