An elevation of the molar growth yield of Zymomonas mobilis during aerobic exponential growth

Elevated values of molar growth yield (Y_x/s = 14–26 g mol^–1) were obtained during exponential growth (μ > 0.4 h^–1) of Zymomonas mobilis ATCC 29191 by using reduced concentrations of glucose (6.25–100 mM) and increased oxygen supply (E _h > 300 mV) in the growth medium, as compared to the Y_x/s of anaerobic exponential growth (8–10 g mol^–1). Aerobically grown cells showed an increased maximum growth rate (μ_max), and a reduced specific glucose consumption rate (q_s), and specific ethanol formation rate (q_p), thus demonstrating a more pronounced energy-coupling growth under oxic conditions. These results can be neither explained by the concept of a solely operating Entner-Doudoroff pathway as an ATP source in aerobically growing cultures of Z. mobilis nor considered to be consistent with existing data on the lack of the Pasteur effect in this bacterium. Therefore, the results rather give evidence for the essential contribution of aerobic ATP generation under the reported conditions. Received: 24 September 1996 / Accepted: 9 December 1996     

The effect of the dissolved oxygen concentration and anabolic limitations on the behaviour of Rhizobium ORS571 in chemostat cultures

Chemostat cultures of Rhizobium ORS571 limited by the supply of oxygen or an anabolic substrate contained poly--hydroxybutyrate (PHB). Low amounts of PHB (about 10%) were present in ammonia- or nitrate-limited cultures; higher amounts were found in Mg⁺⁺-limited cultures (about 20%) and in oxygen-limited nitrogen-fixing cultures (37%). A method is described to calculate Y_ATP values (g PHB-free biomass · mol^-1 ATP) from the Y_succ values (g dry wt·mol^-1 succinate) measured. Y_succ and Y_ATP values in cultures limited by the supply of an anabolic substrate and in the oxygen-limited ammonia-assimilating culture were much lower than the values found in the PHB-free succinate-limited cultures. This shows that uncoupling of growth and energy production occurred. Therefore, H₂/N₂ ratio (mol hydrogen formed per mol nitrogen fixed) in nitrogen-fixing cultures could not be calculated from the comparison of the Y_ATP value found in the nitrogen-fixing culture and the value found in the corresponding ammonia-assimilating culture.Although the optimal dissolved oxygen concentration (d.o.c.) for nitrogen-fixing cultures of Rhizobium ORS571 is 5 or 10 M, nitrogen-fixing cultures could be obtained up to a d.o.c. of 40 M. Not only nitrogenase but also hydrogenase was active at this d.o.c. However, accumulation of PHB (10%) may indicate that cultures grown at unfavourable oxygen concentrations (15–40 M O₂) were N-limited rather than energy-limited, which may be the result of partial inactivation or repression of nitrogenase at a higher d.o.c.     

Oxygen effect on batch cultures of Leuconostoc mesenteroides: relationship between oxygen uptake,growth and end-products

Growth and lactose metabolism of a Leuconostoc mesenteroides strain were studied in batch cultures at pH 6.5 and 30° C in 101 modified MRS medium sparged with different gases: nitrogen, air and pure oxygen. In all cases, growth occurred, but in aerobiosis there was oxygen consumption, leading to an improvement of growth yield Y _x/s and specific growth rate compared to anaerobiosis. Whatever the extent of aerobic growth, oxygen uptake and biomass production increased with the oxygen transfer rate so that the oxygen growth yield, Y _x/o2, remained at a constant value of 11 g dry weight of biomass/mol oxygen consumed. Pure oxygen had a positive effect on Leuconostoc growth. Oxygen transfer was limiting under air, but pure oxygen provided bacteria with sufficient dissolved oxygen and leuconostocs were able to consume large amounts of oxygen. Acetate production increased progressively with oxygen consumption so that the total molar concentration of acetate plus ethanol remained constant. Maximal Y _x/s was obtained with a 120 l/h flow rate of pure oxygen: the switch from ethanol to acetate was almost complete. In this case, a 46.8 g/mol Y _x/s and a 0.69 h^–1 maximal growth rate could be reached.     

The oxygen requirements of yeasts for the fermentation of d-xylose and d-glucose to ethanol

Summary The effect of oxygen availability on d-xylose and D-glucose metabolism by Pichia stipitis, Candida shehatae and Pachysolen tannophilus was investigated. Oxygen was not required for fermentation of d-xylose or d-glucose, but stimulated the ethanol production rate from both sugars. Under oxygen-limited conditions, the highest ethanol yield coefficient (Y_e/s) of 0.47 was obtained on d-xylose with. P. stipitis, while under similar conditions C. shehatae fermented d-xylose most rapidly with a specific productivity (q_pmax) of 0.32 h^-1. Both of these yeasts fermented d-xylose better and produced less xylitol than. P. tannophilus. Synthesis of polyols such as xylitol, arabitol, glycerol and ribitol reduced the ethanol yield in some instances and was related to the yeast strain, carbon source and oxygen availability. In general, these yeasts fermented d-glucose more rapidly than d-xylose. By contrast Saccharomyces cerevisiae fermented d-glucose at least three-fold faster under similar conditions.Nomenclature q_pmax maximum specific rate of ethanol production (g ethanol per g dry biomass per hour) - Y_e/s ethanol yield (g ethanol per g substrate utilized) - Y_p/s polyol yield (g polyol per g substrate utilized) - Y_x/s biomass yield (g dry biomass per g substrate utilized) - _max maximum specific growth rate (per hour)     

The influence of dissolved oxygen concentration on phosphofructokinase and the glucose metabolism ofEscherichia coli K-12

The glucose metabolism and the response of phosphofructokinase activity to oxygen were investigated using glucose-limited chemostat cultures ofE. coli K-12. With a dilution rate of 0.2 hr^–1 and a glucose input concentration of 0.83 g/litre, 10 steady states were obtained ranging from 320 to 0 mm HgO₂. Dissolved oxygen reached zero level at a pO₂ of 25.8 mm Hg. The specific phosphofructokinase activity was constant above 28 mm Hg O₂ and increased linearly at lower pO₂ levels until it reached highest activity at 0 mm Hg O₂. Cell dry weight also started to decrease linearly from 28 to 5.9 mm Hg O₂, and fell sharply thereafter. Acid production rate did not start before pO₂ reached 25.6 mm Hg, increased progressively with an additional sharp increase below 5.9 mm Hg O₂. The main endproducts formed were acetic acid and ethanol with lactic acid appearing below 5.9 mm Hg O₂. The results suggest an effect of oxygen on phosphofructokinase synthesis rather than an ATP inhibition of the enzyme.This work was supported by a grant from the Australian Research Grant Commission.     

Methane Metabolism of the Obligate Methane-utilizing Bacterium,Methylomonas flagellata,in Methane-limited and Oxygen-limited Chemostat Culture

The effect of a limited supply of methane and oxygen on growth of Methylomonas flagellata was analyzed in chemostat culture. A significant decrease in Y_CH4 was observed under oxygen-limitation. Accumulation of formaldehyde is supposed to cause a decrease of Y_CH4. A low level of formaldehyde oxidizing activity in cells grown under oxygen-limited steady-states suggests that formaldehyde dehydrogenase is repressed under oxygen deficiency. In spite of the suppression of respiration, methane oxidation progressed independently under oxygen-limitation. These facts support the idea that methanol oxidation couples with methane oxidation (a mono-oxygenase system) in this microorganism.     

Studies on the kinetic parameters for alcoholic fermentation by flocculent Saccharomyces cerevisiae strains and non-hydrogen sulfide-producing strains

Summary The growing demand for high quality products and the immense export potential that cacha?a represents, demonstrated especially during the past few years, have clearly indicated the necessity of establishing well-defined standards of quality, as well as effective means of controlling the process of production of this beverage. The objective of this study was the selection of S. cerevisiae yeast strains and the investigation of their influence on the kinetic parameters of fermentation. Ninety strains of S. cerevisiae isolated from distilleries of the state of Minas Gerais were evaluated with respect to the following parameters: flocculation capacity, production of H₂S and kinetic parameters of fermentation. The UFMGA 905 strain was used as a reference because it presented desirable characteristics for the production of cacha?a. Five strains presented high specific sedimentation velocities (SSV), indicating a high flocculation capacity, and two did not produce H₂S. The strains presented significant statistical differences for fermentation parameters: yield of ethanol; efficiency of substrate conversion to ethanol; ratio of substrate conversion to ethanol (Y _p/s), to cells (Y _x/s), to organic acids (Y _ac/s), and to glycerol (Y _g/s); and productivity. In general, the strains presented a good fermentative potential, with ethanol yields varying from 74.7 to 82.1% and an efficiency of 76.1–84.4%. All strains presented high productivities (4.6–6.6 g l⁻¹ h⁻¹), indicating that this parameter can be used in the selection of strains for the production of cacha?a.     

P2Y purinoceptors induce changes in intracellular calcium in acinar cells of rat lacrimal glands

Adenosine 5′-triphosphate (ATP) is an extracellular signal that regulates various cellular functions. Cellular secretory activities are enhanced by ATP as well as by cholinergic and adrenergic stimuli. The present study aimed to determine which purinoceptors play a role in ATP-induced changes in the intracellular concentration of calcium ions ([Ca²⁺]_i) and in the fine structure of acinar cells of rat lacrimal glands. ATP induced exocytotic structures, vacuolation and an increase in [Ca²⁺]_i in acinar cells. The removal of extracellular Ca²⁺ or the use of Ca²⁺ channel blockers partially inhibited the ATP-induced [Ca²⁺]_i increase. U73122 (an antagonist of PLC) and heparin (an antagonist of IP₃ receptors) did not completely inhibit the ATP-induced [Ca²⁺]_i increase. P1 purinoceptor agonists did not induce any changes in [Ca²⁺]_i, whereas suramin (an antagonist of P2 receptors) completely inhibited ATP-induced changes in [Ca²⁺]_i. A P2Y receptor agonist, 2-MeSATP, induced a strong increase in [Ca²⁺]_i, although UTP (a P2Y_2,4,6 receptor agonist) had no effect, and reactive blue 2 (a P2Y receptor antagonist) resulted in partial inhibition. The potency order of ATP analogs (2-MeSATP > ATP ⋙ UTP) suggested that P2Y₁ played a significant role in the cellular response to ATP. BzATP (a P2X₇ receptor agonist) induced a small increase in [Ca²⁺]_i, but α,β-meATP (a P2X_1,3 receptor agonist) had no effect. RT-PCR indicated that P2X_2,3,4,5,6,7 and P2Y_1,2,4,12,14 are expressed in acinar cells. In conclusion, the response of acinar cells to ATP is mediated by P2Y (especially P2Y₁) as well as by P2X purinoceptors.     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence responses of <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> seedlings subjected to progressive saline stress in the Tarim desert highway ecological shelterbelt

In order to assess the long-term impacts of saline groundwater irrigation to Haloxylon ammodendron, one of the main shrubs in the Tarim desert highway ecological shelterbelt, we irrigated the H. ammodendron seedlings with progressive saline groundwater (3–30 g L⁻¹, simulation environment in the Tarim desert highway ecological shelterbelt) and investigated the diurnal variations of chlorophyll a (Chl a) fluorescence parameters, such as maximal quantum yield of photosystem II (PSII) photochemistry (F_v/F_m), quantum yield of photochemical energy conversion in PSII (Y_II), the apparent rate of electron transport at the PSII level (ETR), photochemical quenching coefficient (q_P), non-photochemical quenching (NPQ), quantum yield of nonregulated non-photochemical energy loss in PSII (Y_NO) and quantum yield of regulated non-photochemical energy loss in PSII (Y_II), at approximately 2-h intervals. F_v/F_m with 5 g L⁻¹ (S2) was lower than that with 2 g L⁻¹ (S1) but a little higher than 20 g L⁻¹ (S5), respectively. Under the low light [photosyntheticallyactive radiation (PAR) ≤ 250 μmol m⁻² s⁻¹, at 08:00, 10:00 and 20:00 h of the local time], S1 kept the lowest Y_II and the highest Y_NPQ; while under the high light (PAR ≥ 1500 μmol m⁻² s⁻¹), the Y_II performed S1>S2>S5, and the reverse Y_NPQ; under mild light (250 μmol m^t-2 s⁻¹ ≤ PAR ≤ 1500 μmol m⁻² s⁻¹), S1 remained the highest Y_II, no matter the light and the salinity, the similar Y_NO almost occurred basically. The results showed that the sand-binding plant H. ammodendron could regulate its energy-utilizing strategies. The S2 might be the most suitable salinity of the irrigation water for H. ammodendron in the Tarim desert highway ecological shelterbelt in the northwest of China.     

Fermentation characteristics as criteria for selection of cachaça yeast

The fermentation characteristics of 24 strains of Saccharomyces cerevisiae and one strain of Candida apicola, C. famata, C. guilliermondii, Hanseniospora occidentalis, Pichia subpelicullosa and Schizosaccharomyces pombe were evaluated for the production of cachaça. They were isolated from small cachaça distilleries (27), industrial cachaça distilleries (2) and one sugarcane alcohol distillery. The yeasts showed significant differences in ethanol yield, substrate conversion, efficiency, conversion factors of substrate into ethanol (Y _p/s), cells (Y _x/s), organic acids (Y _ac/s) and glycerol (Y _g/s), and maximum specific growth rate ( _max). In general the S. cerevisiae strains showed better fermentation potential, with yields between 83 and 91% and _max between 0.450 and 0.640 h^–1, several of them being comparable with the high performance yeast used in the industrial production of ethanol, which was adopted as a reference. The non-Saccharomyces strains showed high efficiency, very low ethanol yield and very high Y _ac/s and Y _g/s values, except Pichia subpelliculosa, which behaved very similarly to the S. cerevisiae strains. Hierarchical Cluster Analysis and Principal Component Analysis showed the fermentation yield (or substrate conversion) as being the variable which contributed most to the separation of the strains into different groups.     

Hydrogen oxidation and nitrogen fixation in rhizobia,with special attention focused on strain ORS 571

In this survey we describe the influence of hydrogen oxidation on the physiology ofRhizobium ORS 571. The presence of hydrogen is required for the synthesis of hydrogenase. Carbon substrates do not repress the synthesis of hydrogenase. The respiratory system contains cytrochromes of theb- andc-type. Cytochromea ₆₀₀ is present after growth at high oxygen tensions. The nature of the terminal oxidases functioning at low oxygen tensions has not been established yet. → H⁺/O values with endogenous substrates are between 6 and 7. The results show the presence of two phosphorylation sites: site 1 (ATP/2e=1.0) and site 2(ATP/2e=1.33). By measuring molar growth yields it has been demonstrated that carbon-limited, nitrogen-fixing cultures obtain additional ATP from hydrogen oxidation, and that site 2 of oxidative phosphorylation is passed during hydrogen oxidation. A method is described to calculate ATP/N₂ values (the total amount of ATP used by nitrogenase during the fixation of 1 mol N₂) and H₂/N₂ ratios (mol hydrogen formed per mol N₂ fixed) in aerobic organisms. ForRhizobium ORS 571 the ATP/N₂ value is about 40 and the H₂/N₂ ratio is between 5 and 7.5. Cells obtained from oxygen-limited nitrogen-fixing cultures contain 30–40% poly-β-hydroxybutyrate, which explains the high molar growth yields found. Hydrogen has not been detected in the effluent gas of these cultures, which may point to reoxidation of the hydrogen formed at nitrogen fixation. Calculations show that the effect of hydrogen reoxidation on the efficiency of nitrogen fixation (g N fixed × mol⁻¹ substrate converted) is not very large and that the actual H₂/N₂ ratio is of much more importance. After addition of hydrogen to succinate-limited, ammonia-assimilating cultures, an initial increase of the Y_succinate value (g dry wt × mol⁻¹ succinate) is followed by a gradual decrease. This is accompanied by a large decrease of the value, and an increased permeability of the cytoplasmic membrane to protons. The results may be explained by a transition of the culture from an energy-limited state to a carbon-limited state.     

Chemostat studies on the assimilation of hexadecane by the yeastCandida tropicalis

Summary The yeastCandida tropicalis was grown in a chemostat with hexadecane as the sole carbon source. The influence of the dilution rate, oxygen and ammonium limitation on s, x, Y_s, Q_{O 2}, Q_{CO 2}, and Q_s was investigated. When the extracellular hexadecane concentration exceeded 120 mg l^–1 (at dilution rates close to D_c, at pO₂ below 2.54 kPa and at ammonium limited growth) Y_s decreased and Q_{O 2}, Q_{CO 2}, and Q_s increased. It was concluded that uncoupling of the mitochondrial respiratory chain occurred under these conditions. The Q_{O 2} was determined by two different methods: first in situ, with a gasanalyzer directly connected to the bioreactor to analyze the outcoming gas, and second, with a sample from the bioreactor transferred to a Clark-type oxygen electrode. When cell growth was not oxygen limited in the chemostat (pO₂ above 2.54 kPa), no apparent difference between the in situ and the dynamically determined Q_{O 2} was observed. In contrast, when cell growth was oxygen limited in the chemostat, the Q_{O 2} measured in the Clark-type oxygen electrode was remarkably higher than the in situ Q_{O 2}. This indicates that the electron transport chains are limited bythe oxidases, when the cells lack oxygen.     

The Nature of Conformational Correlations in α- and β-Anomers of Nucleic Acid Components in Aqueous Solution by Nuclear Magnetic Resonance

Abstract

The solution distribution of combinations of the sugar ring puckering domains, C2′endo(S), C3′endo(N), and C4′-C5′ rotamers, +sc(g⁺), ap(t), -sc(g^?), in α and β-anomers in ribo- and deoxyribo- pyrimidine nucleic acid components can be determined from vicinal coupling constants (M. Remin, J. Biomol. Str. Dyn. 2, 211 (1984). A general correlation pattern with a conformational constant λ, reflecting an intrinsic physical property of the sugar - side chain ensemble, is developed and expressed in terms of four principles:

I) The +sc rotamer contributes to the C3′endo population to a higher extent (1 - Y_t) than to C2′endo,(l-Y_t-Y_g-/X_s).

II) The ap rotamer contributes to both C2′endo and C3′endo populations to the same extent (Y_t).

III) The—sc rotamer contributes only to the C2′endo population, (Y_g-/X_s).

IV) The molar fractions X_s, Y_t and Y_g- of conformations C2′endo, ap and—sc, respectively, are strongly correlated, λ = (Y_g-/X_s)/Y_t ≈ 0.5, and therefore Y_t is a basic variable parameter which determines all others in the correlation pattern.

In α-anomers, regardless of the type and conformation of the sugar ring and base, the molar fraction Y_t = 0.37 ± 0.02. This finding means that different α-anomers show one correlation pattern free of the influence of the base. In β-anomers, structure and conformation of the base are important factors which modulate (through Y_t) the correlation pattern, conserving its fundamental features. Y_t is considerably increased by a syn-oriented pyrimidine base, but decreases when the base is anti. The transition from anti to syn orientation of the base is followed by destabilization of (C2′endo, +sc) in favor of (C3′endo, ap). The principles of conformational correlations rationalize a variety of correlations observed in the past.     

Regulation of intracellular H+ balance in Zymomonas mobilis 113 during the shift from anaerobic to aerobic conditions

Summary The energetics, enzyme activities and end-product synthesis of Zymomonas mobilis 113 in continuous culture were studied after the shift from an anaerobic to an aerobic environment. Aeration diminished ethanol yield and lactic acid concentration, but increased glucose consumption rate and production of acetic acid. After the shift to aerobic conditions reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H]-oxidase activity was stimulated. Washed cell suspensions consumed oxygen with glucose, lactate and ethanol as substrates. The aerobic Z. mobilis 113 regulated their intracellular redox balance by production and reoxidation of the end products, coupled with the formation of NAD(P)H. An increase in transmembrane pH gradient (pH) and a decrease in intracellular ATP concentration were observed after the shift to aerobic conditions. At low medium redox potential (Eh) values the H⁺ balance was regulated in an energy-independent way via end-product excretion. Under aerobic conditions this was supplemented by ATP-dependent H⁺ excretion by the membrane H⁺-ATPase.Abbreviations D dilution rate (h^-1) - S ₀ initial glucose concentration (g/l) - Y _x/s growth yield (g/mol) - Y _p/s product yield (g/g) - q _s specific rate of substrate utilization (g/g per hour) - q _p specific rate of ethanol formation (g/g per hour) - qo ₂ specific rate of CO₂ production (mmol/g per hour) - specific growth rate (h^-1) - X dry biomass concentration (g/l) - Eh redox potential of culture medium (mV) - pH transmembrane pH gradient (pH units) - pH_in intracellular pH - SASE sum of activities of specific enmymes of Entner-Doudoroff pathway     

The P2Y2 Nucleotide Receptor Mediates the Proliferation and Migration of Human Hepatocellular Carcinoma Cells Induced by ATP

ATP is an abundant biochemical component of the tumor microenvironment and a physiologic ligand for the P2Y₂ nucleotide receptor (P2Y₂R). In this study, we investigated the effect of ATP on the cellular behavior of human hepatocellular carcinoma (HCC) cells and the role of P2Y₂R in ATP action and aimed to find a new therapeutic target against HCC. The experiments were performed in native isolated human HCC cells, normal hepatocytes, human HCC cell lines, and nude mice. We found that the mRNA and protein expression levels of P2Y₂R in native human HCC cells and the human HCC cell lines HepG2 and BEL-7404 were enhanced markedly compared with human normal hepatocytes and the normal hepatocyte line LO2, respectively. ATP induced intracellular Ca²⁺ increases in HCC cells and promoted the proliferation and migration of HCC cells and the growth of HCC in nude mice. The P2Y receptor antagonist suramin, P2Y₂R-specific shRNA, the store-operated calcium channel inhibitors 2-aminoethoxydiphenyl borate (2-APB) and 1-(β-3-(4-methoxy-phenyl) propoxyl-4-methoxyphenethyl)1H-imidazole-hydrochloride ({"type":"entrez-protein","attrs":{"text":"SKF96365","term_id":"1156357400","term_text":"SKF96365"}}SKF96365), and stromal interaction molecule (STIM1)-specific shRNA inhibited the action of ATP on HCC cells. In conclusion, P2Y₂R mediated the action of ATP on the cellular behavior of HCC cells through store-operated calcium channel-mediated Ca²⁺ signaling, and targeting P2Y₂R may be a promising therapeutic strategy against human HCC.     

The scale-up of plant cell culture: Engineering considerations

The enormous versatility of plants has continued to provide the impetus for the development of plant tissue culture as a commercial production strategy for secondary metabolites. Unfortunately problems with slow growth rates and low products yields, which are generally non-growth associated and intracellular, have made plant cell culture-based processes, with a few exceptions, economically unrealistic. Recent developments in reactor design and control, elicitor technology, molecular biology, and consumer demand for natural products, are fuelling a renaissance in plant cell culture as a production strategy. In this review we address the engineering consequences of the unique characteristics of plant cells on the scale-up of plant cell culture.Abbreviations a gas-liquid interfacial area per volume - C dissolved oxygen concentration - C^* liquid phase oxygen concentration in equilibrium with the partial pressure of oxygen in the bulk gas phase - K_L overall mass transfer coefficient - k_L liquid film mass transfer coefficient - m_O2 cell maintenance coefficient for oxygen - OTR oxygen transfer rate - OUR oxygen uptake rate - pO₂ partial pressure of oxygen - STR stirred-tank reactor - v.v.m. volume of gas fed per unit operating volume of reactor per minute - X biomass concentration - Y_x/O2 biomass yield coefficient for oxygen - specific growth rate     

Shake-flask culture of Laccaria laccata,an ectomycorrhizal basidiomycete

 Large-scale exploitation of the potential benefits of ectomycorrhizal fungi in improving plantation yields means that fermentation techniques for these fungi will be required. Starting with a base performance on a rich, complex medium, the effect of variations in some physicochemical culture parameters on biomass yield was studied. It was possible to reduce the amount of phosphate salts (to 1/9th) and other ingredients (to 1/3rd) in the medium. A shaking speed of either 100 rpm or 200 rpm in an orbital incubator was satisfactory and biomass yield responded to an increase in carbon substrate (glucose, from 10 g l^-1 and 20 g l^-1) though Y _x/s declined. An increase in inoculum size shortened culture time but decreased biomass yield. The upper limit of the incubation temperature was between 25°C and 30°C. Biomass yields were about 12 g l^-1 dry weight (Y _x/s=0.63) when 20 g l^-1 glucose was supplied, and about 7 g l^-1 (Y _x/s=0.74) when 10 g l^-1 glucose was supplied. Received: 9 October 1995/Accepted: 4 December 1995     

Water fleas (Daphnia magna) provide a separate ventilatory mechanism for their brood

The planktonic filter feeder Daphnia magna depends on a steady oxygen supply by convection. In the ventral carapace chamber, this convection is established by the feeding current which is generated by the movement of the thoracic limbs. The present study revealed that this movement can cause an additional flow of medium which passes through the brood chamber of the animal. To visualise this current, ink or fluorescent microspheres were released by a microcapillary near the posterior opening of the brood chamber. The tracks of these probes were monitored by video microscopy. Digital motion analysis was used for the determination of flow velocity and flow rate. Ambient medium entered the brood chamber at the abdomen of the animal and moved then to the anterior end of the brood chamber before entering the ventral carapace chamber. Two horizontal lamellae, which are attached at both sides of the trunk and project laterally to contact the carapace walls, almost completely separate the dorsal brood chamber from the ventral carapace chamber. Water can only pass these barriers through small depressions in these lamellae at the level of the 3rd and 4th appendages. Female daphnids with embryos at late developmental stages showed more rapid water currents in the brood chamber than those with younger embryos. Moreover, animals showed higher flow rates when exposed to hypoxic conditions. As the oxygen uptake rate of older embryos is approximately three times higher than that of younger embryos, the enhanced brood chamber current could improve the oxygen availability for both the mother and its brood under conditions of reduced oxygen availability.List of symbols:a specific oxygen consumption rate (nmol mm-³ h-¹), K Krogh constant for oxygen diffusion (nmol mm-¹ h-¹ kPa-¹, P_c critical oxygen partial pressure (kPa), P o₂ oxygen partial pressure (kPa), r radius (mm), s distance (mm), t time (ms)     

A generalized mathematical model for the growth kinetics of Saccharomyces cerevisiae with experimental determination of parameters

A general model of the kinetics of microbial growth has been developed involving the kinetics of incorporation of substrate into biomass and the maintenance energy requirements. Results obtained from batch cultures of the yeast Saccharomyces cerevisiae growing in synthetic media at pH 5.1 and 30°C permitted all biological parameters in the model to be calculated. Values obtained for these parameters were: maximum specific glucose uptake rate (μS_m), 2.08 g/g biomass/hr; apparent Michaelis constant for glucose (K_S), 0.1 g/liter (5.5 × 10^?4M) apparent Michaelis constant for oxygen (K_L), 1.4% O₂ (3.2 × 10^?6 M) quantitative index of the Pasteur effect (b), 4.9 × 10^?4%^?1 O₂ (207 M ^?1). Under conditions of strongly substrate-repressed respiration the values obtained for Y_ATP and P/O were constant over the course of the exponential phase of growth (Y_ATP = 10.4 g biomass/mole ATP; P/O = 3 moles ATP/atom 0). Mass balances for aerobic and anaerobic cultures confirmed the results obtained form the generalized model. Results presented suggested the operation of a mechanism for regulating energy-yielding metabolism which involved an equilibrium between the systems of oxidative phosphorylation and dephosphorylation and was dependent upon the level of catbolite repression.     

The steady-state rate of ATP synthesis in the perfused rat heart measured by 31P NMR saturation transfer

The steady-state rate of ATP synthesis in the isolated, Langendorff-perfused rat heart was determined using a ³¹P NMR saturation transfer method. At 37°C and a perfusion pressure of 70 cm H₂O the value is 2.8 ± 0.3 (n=5 ± S.E.M.) μmol.s^?1. (g. dry wt.)^?1. The activity of creatine phosphokinase measured in the same experiments was 14.6 ± 1.0 μ mol.s^?1 .(g. dry wt.)^?1. From the rate of ATP synthesis and the separately measured oxygen consumption we calculated an apparent mitochondrial ADP:O ratio of 3.5 ± 0.8 in the intact tissue.