Kinetics of the liquid-phase oxidation of acid ferrous sulfate by the bacterium Thiobacillus ferrooxidens

The kinetics of the batch-wise liquid-phase oxidation of ferrous sulfate by the organism Thiobacillus ferrooxidans has been studied over a range of temperatures from 20°C to 31°C and in the presence of an abundant supply of oxygen, carbon dioxide, and other nutrients. The rate of oxidation was found to be accurately described by the equation where t = time hr, S = concentration of ferrous ions g Fe⁺⁺/1., μ_m = maximum specific growth rate of bacteria, hr^?1. Y = mass of bacteria produced per gram of iron oxidized g/g, K = saturation constant, g Fe⁺⁺/l., and X = concentration of bacteria g/1. The value for the maximum specific growth rate, μ_m, was found to vary from 0.12 hr^?1 at 20°C to 0.20 hr^?1 at 31°C, while the value for the saturation constant K varied randomly between 1 and 2 g/1. A method has also been described which permitted evaluation of the relevant rate constants μ_m and K without direct knowledge of the bacterial population. This method was found to yield values of μ_m and K which agreed with values determined accurately by a statistical regression analysis of the experimental data.     

Growth characteristics of a thermotolerant strain of lodderomyces elongisporus grown on sucrose

A thermotolerant yeast species of Lodderomyces elongisporus EH 60 was isolated and physiologically characterized. This yeast possesses a high specific growth rate with μ_max = 0.61 h^?1. The dependence of the specific growth rate and cell yield on temperature, dilution rate, sucrose concentration and pH-value is investigated. Sucrose concentrations greater than 10 g/l inhibit the growth velocity. The specific growth rate μ can be calculated by a simple combination equation in the form: . The total optimum values for a sucrose-based continuous growth process with regard to the optimum cell yeild are: Y_S = 0.50 g DM/g S. T_opt. = 38,6 °C and D_opt. = 0,35 h^?1. The function Y_S = f(D, T) is represented by a total model.     

Thermal denaturation of Na- and Li-DNA in salt-free solutions

Thermal denaturation of Na- and Li-DNA from chicken erythrocytes was studied by means of scanning microcalorimetry in salt-free solutions at DNA concentrations (C_p) from 4.5 · 10^?2 to 1 · 10^?3 moles of nucleotides/liter (M). Linear dependencies of DNA melting temperature (T_m) vs lgC_p were obtained: ((1)) ((2)) for Na- and Li-DNA, respectively. Microcalorimetry data were compared with the results of spectrophotometric studies at 260 nm of DNA thermal denaturation in Me-DNA + MeCl solutions at C_p ? (6–8) · 10^?5 M and C_s = 0–40 mM (Me is Na or Li, C_s is salt concentration). It was found that Eqs. (1) and (2) are valid in DNA salt-free solutions over the C_p range 6 · 10^?5?4.5 · 10^?2M. Protonation of DNA bases due to the absorption of CO₂ from air in Na-DNA + NaCl solutions affects DNA melting parameters at C_s < 4 mM. Linear dependence of T_m on lga₊ is found in Na-DNA + NaCl at C_s > 0.4 mMin the absence of contact of solutions with CO₂ from air (a₊ is cation activity). A dependence of [dT_m/dlga₊] on Li⁺ activity was observed in Li-DNA + LiCl solutions at C_s < 10 mM: [dT_m/dlga₊] increases from 17°–18° at C_s > 10 mM to 28°–30° at C_s ? 0.2–0.4 mM. Spectrophotometric measurements at 282 nm show that this effect was caused by protonation of bases in fragments of denatured DNA in neutral solutions. The Poisson–Boltzmann (PB) equation was solved for salt-free DNA at the melting point. The linear dependence of T_m vs lgC_p was interpreted in terms of Manning's condensation theory. PB and Manning's theories fit the experimental data if charge density parameter (ξ) of denatured DNA is in the range 1.8–2.1 (assuming for native DNA ξ = 4.2). Specificity of Li ions in interactions with DNA is discussed. © 1994 John Wiley & Sons, Inc.     

Light/Dark Cycle and Temperature-Their Impact on Phosphate-Limited Growth of Oscillatoria redekei VAN GOOR in Semicontinuous Culture

Phosphate-limited growth of Oscillatoria redekei in semicontinuous culture has been studied under conditions of continuous illumination at 20 °C as well as in a 12/12 hours light-dark cycle at temperatures between 5 °C and 20 °C. The subsistence quota (q₀) amounted to 0.052 μmol mm⁻³ under all conditions, when the phosphate quota was expressed on the basis of cell volume. The interaction between temperature and phosphate quota and its impact on growth rate are described by the following model: Parameter values are t_opt=24.5 °C, t_min=0.95 °C, μ_max =0.873 d⁻¹. The maximum phosphate quota was found to depend on temperature and to increase along with declining temperature.     

Production of propionic acid by microbiological way. Part 1. Influence of the initial sugars and product concentrations

The best yields and productivities of 0.38 g · g^?1 and 0.35 g · l^?1 h^?1, respectively, for the propionic acid production in a batchsystem using sugar-cane final molasses as carbon source were obtained when an initial TRS concentration of 50 g · l^?1 was used. It was obvious that this process is severely inhibited by the acids produced and the most drastic effect (μ = 0) was at a TVA concentration near to 250 mmol · l^?1, independently of the initial TRS concentration employed. A generalizated equation of noncompetitive inhibition was adjusted: and kinetic inhibition constants for each initial TRS concentration studied were estimated.     

Polynucleotides. XI. Thermodynamics of (A) N -2(U) infinity from the dependence of T m N on oligomer length

The variation in the helix-coil transition temperature, T_m^N, with oligomer length, N, for the system ((I)) has been examined. The results for N = 4-13, measured in 0.2M Na+, have been analyzed in terms of the expression of Blake (1972): ((II)) where c_m is the free oligomer concentration at T_m^N, and V_r^f is the thermodynamic free volume available to a helical base-triplet residue. The correlation coefficient for the fit to expression (II) of data obtained over a 50° temperature range is 0.997 when ΔH_r = ?12.6 kcal/mole of base-triplets (independent of oligomer length (N ? 4) or temperature), the value previously obtained from both calorimetry of (A)_∞·2(U)_∞ and (A)₄ concentration dependence of T_m. It is found that V_r^f = 8.0 × 10^?4 1/mole (± 30%) or 1.33 ų per helical base-triplet, and is constant with temperature. A maximum value for V_r^f of 21.0 × 10^?4 1/M (± 1.3%), equivalent to 3.54 ų per helical basetriplet is obtained by the same treatment of the helix-coil transition data for the three-stranded helix formed by adenosine (N = 1) and 2(U)_∞ obtained by Davies and Davidson (1971).     

Sensitivity analysis of reaction-diffusion constraints in muscle energetics

Theoretical and experimental studies of aerobic metabolism on a wide range of skeletal muscle fibers have shown that while all fibers normally function within the reaction control regime, some fibers operate near the transition region where reaction control switches to diffusion control. Thus, the transition region between reaction and diffusion control may define the limits of muscle function, and analysis of factors that affect this transition is therefore needed. In order to assess the role of all important model parameters, a sensitivity analysis (SA) was performed to define the parameter space where muscle fibers transition from reaction to diffusion control. SA, performed on a previously developed reaction–diffusion model, shows that the maximum rate for the ATPase reaction (V_max,ATPase), boundary oxygen concentration in the capillary supply (O), the mitochondrial volume fraction (ε_mito), and the diffusion coefficient of oxygen ( ) are the most sensitive parameters affecting this transition to diffusion control. It is demonstrated that fibers are not limited by diffusion for slow reactions (V_max,ATPase < 25 mM/min), high oxygen supply for the capillaries (O ≥ 35 µM), and large amounts of mitochondria (ε_mito ≥ 0.1). These conditions are applicable to muscle cells spanning a very broad range of animals. Within the diffusion‐controlled region, the overall metabolic rate and ATP concentrations have much higher sensitivity to the diffusion coefficient of oxygen than to the diffusion coefficients of the other metabolites (ATP, ADP, P_i). Biotechnol. Bioeng. 2012; 109:559–571. © 2011 Wiley Periodicals, Inc.     

Diffusion of DNA at very low concentrations

The diffusion behavior of DNA samples of molecular weights between 1 × 10⁶ and 25 × 10⁶ Daltons was investigated under standard conditions at mean concentrations c? between 0.0009 and 0.017 g/dl. Special techniques described previously were used and supplemented. The sensitivity required was accomplished by multiple passage through the sample cells (effective path length of 10–45 cm) and application of the Gouy interference method. The maximum DNA refraction index difference has been determined more precisely from Gouy interference fringes by applying a systematic variation procedure and a linear-plot criterion. Convection was prevented by a temperature constancy better than 0.002°C/day, vibrationless operation, and by application of a slight density gradient of heavy water, which also improved the boundary-forming procedure. The corresponding optical HDO gradient was compensated. The concentration dependence of the DNA diffusion coefficient average D_A was found to be positive and very small at extremely low concentrations, that is, below c? = 0.008 g/dl, for the sample of highest molecular weight investigated. With beginning penetration of different DNA molecules, D_A increases markedly. The diffusion constant averages of our polydisperse samples will be corrected for monodisperse subfractions in a following paper. The resulting molecular weights M from diffusion and sedimentation constants (D⁰, s⁰) together with data from literature are the basis of new s⁰–M, D⁰ ? M, and [η]–M relations for monodisperse DNA samples.     

Thermodynamics of oligo (A) N -2poly(U) from the dependence of the temperature of the helix-coil transition on oligomer concentration

On the basis of elementary two-state, ideal solution thermocynamics, a modified expression for the melting of oligo. polynucleotide helices is derived which is applicable to variations in T_m^N and/or oligomer concentration, C_m with oligomer length, N: ((I)) ΔH_r is the enthalpy per helix residue, i.e., per base-pair or base-triplet, V_r^f is the thermodynamic “available” or “reaction” volume, in liters/mole of helical residues; and n is the number of polynucleotide strands, e.g., n = 2 for oligo (A)_N·2 poly(U)∞. Some earlier treatments have engendered confusion in the interpretation of the “reaction volume,” but with the derivation herein, the entropic origin and physical significance of V_r^f is unequivocal. The following approximation was arrived at for the reduction expected in the configurational entropy, ΔS_r^{conf, ∞}, for (A)∞·2(U)∞, when the poly(A), strand is substituted for by an equivalent strand of contiguous oligo(A)_N,′s: ((II)) This adjustment of ΔS_r^{conf, ∞} represents the source of the coefficient to 1/T_m^∞ in expression (I). The expectation that ΔS_r^{conf, N} < ΔS_r^{conf, ∞} is due to the effect of releasing normal internucleotide configurational restrictions every Nth residue in one-third of the strands of the (A)_N·2(U)∞ helix. Although the reduction in ΔS_r^{conf, ∞} (II) may seem small (i.e., only 5.5% for the tetramer), its effect on the magnitude of V_r^f in expression (I) is exponential. Thus, without these considerations the quantitative applicability of earlier expressions is questionable. By examining the variation in T_m^N with c_m for a single N, all assumptions, required for evaluating V_r^f or the entropic effects of discontinuities in the (A)_N strand are avoided in the determination of a reliable enthalpy. We have therefore examined the system ((III)) and obtained a ΔH_r = 12.58 ± 0.08 kcal per mole (A)·2(U) base-triplets between 5 and 2.5°C. That this value for ΔH_r is in such excellent agreement with all calorimetric values reported for (A)∞·2(U)∞ suggests that the enthalpy for reaction(III) is not significantly affected by disconnections in the backbone of (A)₄·2(U)∞. From (I), V_r^f = 6.0 × 10^?4 1/mole or 1 Å ³per helical residue. ΔH_r°, corrected for residual single-strand stacking in (A)₄, is in excellent agreement with that found earlier for (A)₁·2(U)∞. A residual heat capacity of 90 kcal(±20) per mole (A)·2(U) base-triplets per °C is deduced from the decrease of ΔH_r° with temperature.     

Characterization flavanone 3β-hydroxylase expressed from <Emphasis Type="Italic">Populus euphratica</Emphasis> in <Emphasis Type="Italic">Escherichia coli</Emphasis> and its application in dihydroflavonol production

Flavanone 3β-hydroxylase plays very important role in the biosynthesis of flavonoids. A putative flavanone 3β-hydroxylase gene (Pef3h) from Populus euphratica was cloned and over-expressed in Escherichia coli. Induction performed with 0.1 mM IPTG at 20°C led to localization of PeF3H in the soluble fraction. Recombinant enzyme was purified by Ni-NTA affinity. The optimal activity of PeF3H was revealed at pH 7.6 and 35°C. The purified enzyme was stable over pH range of 7.6–8.8 and had a half-life of 1 h at 50°C. The activity of PeF3H was significantly enhanced in the presence of Fe²⁺ and Fe³⁺. The K _M and V _max for the enzyme using naringenin as substrate were 0.23 mM and 0.069 μmoles mg–1min-1, respectively. The K _m and V _max for eriodictyol were 0.18 mM and 0.013 μmoles mg^–1min^–1, respectively. The optimal conditions for naringenin bioconversion in dihydrokaempferol were obtained: OD₆₀₀ of 3.5 for cell concentration, 0.1 mM IPTG, 5 mM α-ketoglutaric acid and 20°C. Under the optimal conditions, naringenin (0.2 g/L) was transformed into 0.18 g/L dihydrokaempferol within 24 h by the recombinant E. coli with a corresponding molar conversion of 88%. Thus, this study provides a promising flavanone 3β-hydroxylase that may be used in biosynthetic applications.     

A kinetic study of the alcoholic fermentation of grape juice

The alcoholic fermentation of grape juice by a wine yeast was studied batchwise at pH 3.6 and 4.05 to develop kinetic equations relating cell concentration, N, to product concentration, P. In the exponential growth phase where A, B, and C are constants, and μ is the specific growth rate. In the stationary phase, where the cell population is constant, was found to apply. This equation, which incorporates a stoichiometric constant, P_m, predicted correctly the operation of a continuous fermentor at pH 3.6 and at 4.05. To study more fully the effect of alcohol concentration on yeast growth, a continuous fermentor was used in which the grape juice feed was supplemented with pure alcohol. At pH 3.6 the specific growth rate varied as, There was no growth inhibition below an alcohol concentration of 2.6 g./100 cc., but inhibition was complete above 6.85 g./100 cc. This is a modified form of the relation suggested by Hinshelwood.¹ The data suggest that growth in batch culture was limited not only by alcohol but also by some other factor, probably a nutritional deficiency.     

Production of Candida utilis protein from peat extracts

Sphagnum peat extracts or hydrolysates have been obtained and used as a culture medium for the production of Candida utilis biomass as single cell proteins. Acid hydrolysis of ground peat (4–60 mesh) in an autoclave operated under a set of conditions for acid strength (0.3-1.5 (v/v) H₂SO₄), holding time (1–4 hr), temperature (100–165°C), and weight ratio of dry peat to solution (3.3–16.7 g dry peat/100 g solution) yielded carbohydrate-rich extracts of different concentrations (1–34g/liter). The best yield (mg total carbohydrate/g dry peat) was obtained for a holding time of I hr and a temperature of 152°C. Low peat concentratio (4.1 g dry peat/100 g solution)resulted in high yield(280mg total carbohydrate/gdry peat) with a corresponding low carbohydrate content in hydrolysate (13 g/liter), while a lower yield with a higher carbohydrate content (34 g/liter)in hydrolysate were found when increasing peat concentration (16.7 g dry peat/100 g solution). Shake-fladk experiments using peat hydrolysates as the culture medium together with NH₄OH (～4.8 g/liter) and K₂HPO₄(5 g/liter) as nitrogen and phosphate supplement, respectively, gave a maximum biomass concentration of 7.5 g/liter after 60 hr at 30°C and 200rpm. Batch cultivation in a fermentor under controlled conditions for aeration (4.2 liter/min), agitation (500rpm), temperature (30°C), and pH (5.0) produced a maximum biomass of 10 g/liter after 20 hr with a specific growth rate of 0.13 hr^?1. For the continuous cultivation, a maximal biomass productivity of 1.24 g/gliter-he was obtained at a dilution rate of 0.125 hr ^?1. Monod's equation's equation has been used for the estimation of the coefficients μ_Max, K_s, and Y. It was found that the yield coefficient Y is not constant during the progress of batch cultivation.     

Equilibrium and dynamic investigations of organic acids adsorption onto ion-exchange resins

The aim of the study was to determine properties of selected ion-exchange resins for citric and lactic acids recovery, to define sorption isotherms for these acids at different temperatures (in the range of 20–60°C) and to determine diffusion coefficients inside sorbent particles. A mathematical model of the ion-exchange process in the chromatographic column and its experimental verification is also presented. During investigations 18 types of ion-exchange resins were tested. It was found that weakly basic resins were more suitable for the recovery process than strongly basic ones. The best resin for the separation of citric acid was Amberlite IRA-67 and for lactic acid Amberlite IRA-92. As a result of transient-state sorption experiments diffusion coefficients of the citric acid inside the sorbent particle at different temperatures were obtained. It was found that D_p increased with the temperature by two times in the range of 20–60°C, and its value at 60°C was 7.2×10^–10 m²/s. The proposed mathematical model was applied to identify bed operation parameters in the column for the needs of the simulated moving bed chromatography method.List of symbols b Equilibrium constant in Langmuir equation, [dm³/g] - c Acid concentration in the liquid phase inside the particle pores, [g/dm³] - C Acid concentration in the liquid, [g/dm³] - D_L Axial dispersion coefficient, [m²/s] - D_p Intraparticle diffusion coefficient, [m²/s] - k_f Liquid film mass transfer coefficient, [m/s] - L Ion-exchanger bed height, [m] - q Acid concentration in the adsorbent phase, [g/dm³] - R_p Particle radius, [m] - U Volumetric flow rate of the feeding solution, [dm³/s] - V Volume of the solution, [dm³] - W Weight of the wet resin particles, [g] - The ion-exchanger bed porosity, [-] - _p Particle porosity, [-] - Linear liquid velocity, [m/s] - Apparent density of the wet resin, [g/dm³]     

Inhibitory effects of ethanol in alcoholic fermentation

The ethanol tolerance behaviour of the strain Saccharomyces cerevisiae Sc 5 regarding the growth is characterized by a threshold ethanol inhibitory concentration (P₁' = 42.5 g/l) and a linear relationship between the specific growth rate and the ethanol concentration within the limits P₁' < P < P′. The maximum ethanol concentration for growth amounts to P′ = 84 g/l. A general model for the inhibition of growth and alcohol production, respectively, caused by ethanol, is deduced from experimental and bibliographical data: If the inhibitory effects are linear, the exponents b, b' become 1.     

Developmental changes in oxygen consumption and hypoxia tolerance in the heat and hypoxia-adapted tabasco line of the Nile tilapia Oreochromis niloticus,with a survey of the metabolic literature for the genus Oreochromis

The genus Oreochromis is among the most popular of the tilapiine cichlid tribe for aquaculture. However, their temperature and hypoxia tolerance, if tested at all, is usually tested at temperatures of 20–25°C, rather than at the considerably higher temperatures of 30–35°C typical of tropical aquaculture. We hypothesized that both larvae and adults of the heat and hypoxia-adapted Tabasco-line of the Nile tilapia Oreochromis niloticus would be relatively hypoxia-tolerant. Oxygen consumption rate (), Q₁₀ and aquatic surface respiration (ASR) was measured using closed respirometry at 2 (c. 0.2 g), 30 (c. 2–5 g), 105 c. (10–15 g) and 240 (c. 250 g) days of development, at 25°C, 30°C and 35°C. at 30°C was inversely related to body mass: c. 90 μM O₂ g⁻¹/h in larvae down to c. 1 μM O₂ g⁻¹/h in young adults. Q₁₀ for was typical for fish over the range 25–35°C of 1.5–2.0. ASR was exhibited by 50% of the fish at pO₂ of 15–50 mmHg in a temperature-dependent fashion. However, the largest adults showed notable ASR only when pO₂ fell to below 10 mmHg. Remarkably, p_crit for was 12–17 mmHg at 25–30°C and still only 20–25 mmHg across development at 35°C. These values are among the lowest measured for teleost fishes. Noteworthy is that all fish maintain equilibrium, ventilated their gills and showed routine locomotor action for 10–20 min after ceased at near anoxia and when then returned to oxygenated waters, all fish survived, further indicating a remarkable hypoxic tolerance. Remarkably, data assembled for from >30 studies showed a > x2000 difference, which we attribute to calculation or conversion errors. Nonetheless, p_crit was very low for all Oreochromis sp. and lowest in the heat and hypoxia-adapted Tabasco line.     

Morphometric diffusing capacity and functional anatomy of the book lungs in the spider Tegenaria spp. (Agelenidae)

The presence of both book lungs and a tracheal system in many spiders raises the question of the functional significance of this double respiratory system. The present physiological and morphometric study of the house spider (Tegenaria spp.) reveals that the diffusing capacity (Dto₂) of the lungs alone suffices during rest and following exercise to meet measured rates of oxygen consumption (\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm V}\limits^{\rm.} $\end{document}o₂) at driving pressures (ΔPto ₂) similar to those calculated for vertebrate lungs. During moulting ΔPto ₂ may rise to more than double the vertebrate values, implying the possible insufficiency of book lungs during this critical life phase. Resting \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm V}\limits^{\rm .} $\end{document}o₂ is greatest (92 mm³/h · g) during the early morning and lowest (66 mm³/h · g) near midday: during moulting \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm V}\limits^{\rm .} $\end{document}o₂ rises to 278.7 mm³/h · g. In spiders recovering from exercise \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm V}\limits^{\rm .} $\end{document}o₂ is consistently greater than during rest: neither value is significantly reduced by blockage of the tracheal stigmas. Regression calculations of morphometric values for a hypothetical 100-mg Tegenaria yield a total lung volume of 0.578 mm³, a pulmonary surface area of 69.8 mm², and a surface-to-volume ratio of 120.89 mm²/mm³. In spite of the similar thickness of the chitinous and hypodermal components of the air-hemolymph barrier (each ca. 0.2 μm in nonmoulting animals), the low permeability of chitin for oxygen makes this layer the greater barrier to diffusion. For a 100-mg specimen Dto₂ is 3.5 mm³/h · torr: similar to that of a turtle (Pseudemys) on a gram-body weight basis.     

Thermodynamic functions of biopolymer hydration. II. Enthalpy–entropy compensation in hydrophilic hydration processes

The thermodynamic functions of biopolymer hydration were investigated by multitemperature vapor pressure studies. Desorption measurements were performed that allowed determination of reversible isotherms in the hydration range of 0.1 to 0.3–0.5 g H₂O/g dry polymer. These isotherms are accessible to thermodynamic interpretation and are relevant to the interaction of water with biopolymers in their solution conformation. The results obtained on a series of different biopolymers (lysozyme, α-chymotrypsin, apo-lactoferrin, and desoxyribonucleic acid), show the following common features of interest: (1) The differential excess enthalpies (ΔH^e ) and entropies (ΔS^e ) are negative, and exhibit pronounced anomalies in a well-defined low-humidity range (approx. 0.1 g H₂O/g dry polymer). These initial extrema are interpretable by structural changes, induced in the native biopolymer structures by water removal below a critical degree of hydration. (2) The ΔH^e and ΔS^e terms exhibit statistically significant linear enthalpy–entropy compensation effects in all biopolymer–water systems investigated. The compensation temperatures \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta = \overline {\Delta H} ^e /\overline {\Delta S} ^e $\end{document} are approximately identical for all biopolymers, ranging from 360 to 500 K. The compensation effects are attributable to phase transitions of water molecules between the bulk liquid and the inner-sphere hydration shell of native biopolymers. (3) The negative excess free energies (ΔG^e ) decrease monotonically with increasing water content and are close to zero at 0.3 to 0.5 g H₂O/g polymer. This result indicates that only transitions between the bulk liquid and the inner-sphere hydration shell are associated with significant net free energy effects. The outer-sphere hydration water is thermodynamically comparable to bulk water. The importance of the proportionality factor \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta $\end{document} in the control of the free energy term is discussed.     

Buoyant densities of natural and synthetic DNA's in CsCl and Cs2SO4 considered as sequence-dependent properties

The buoyant densities of natural and synthetic DNA's can be accurately interrelated if second-neighbor influences are taken into account. We derive the following expressions, based partly on the buoyant densities of six synthetic DNA's, for the buoyant densities ρ (g/cm³) of DNA's having random sequences. In CsCl, and in Cs₂SO₄, . In these equations, H_G is the mole fraction of G : C base pairs in the DNA and the buoyant densities are calculated relative to densities for E. coli DNA of 1.703 and 1.426 (g/cm³) in CsCl and Cs₂SO₄, respectively.     

Verrucarin A and roridin E produced on spinach by <Emphasis Type="Italic">Myrothecium verrucaria</Emphasis> under different temperatures and CO<Subscript>2</Subscript> levels

The behavior of Myrothecium verrucaria, artificially inoculated on spinach, was studied under seven different temperature conditions (from 5 to 35 °C) and under eight different combinations of temperature and CO₂ concentration (14–30 °C and 775–870 or 1550–1650 mg/m³). The isolate used for this study was growing well on spinach, and the mycotoxins verrucarin A and roridin E were produced under all tested temperature and CO₂ conditions. The maximum levels of verrucarin A (18.59 ng/g) and roridin E (49.62 ng/g) were found at a temperature of 26–30 °C and a CO₂ level of 1550–1650 mg/m³. Rises in temperature as well as in temperature and CO₂ concentrations had a significant effect by increasing Myrothecium leaf spots on spinach. The biosynthesis of verrucarin A was significantly increased at the highest temperature (35 °C), while roridin E was influenced by the CO₂ concentration. These results show that a positive correlation between climate condition and macrocyclic trichothecene production is possible. However, because of the ability of M. verrucaria to produce mycotoxins, an increase in temperature could induce the spread of M. verrucaria in temperate regions; this pathogen may gain importance in the future.     

A theoretical model of hydraulic conductivity recovery from embolism with comparison to experimental data on Acer saccharum

A theoretical model of bubble dissolution in xylem conduits of stems was designed using the finite differential method and iterative calculations via computer. The model was based on Fick's, Henry's and Charles' laws and the capillary equation. The model predicted the tempo of recovery from embolism in small diameter branches of woody plants with various xylem structures under different xylem water pressures. The model predicted the time required to recover conductivity in any position in the stem. Repeated iterative solution of the model for different situations yielded an empirical formula to calculate the time for complete recovery of conductivity in stems from a fully embolised initial state. The time, t_p, is given by: where α is a temperature coefficient; D is the coefficient of diffusion of air in wood at 25°C; r_cs is the ratio of the area of total conduit cross section to the stem cross section; Ψ_xp is the stem xylem pressure potential (Pa, where 0 Pa equals atmospheric pressure); τ is solution surface tension (0.072 N m^?1); and D_c and D_s are diameters of the conduits and the stem, respectively (m). The equation is valid only when Ψ_xp > –4τ/D_c. The model predicts no recovery of conductivity when Ψ_xp≤–4τ/D_c. The model agreed with experiments.