On transition bias in mitochondrial genes of pocket gophers

The relative contribution of mutation and purifying selection to transition bias has not been quantitatively assessed in mitochondrial protein genes. The observed transition/transversion (s/v) ratio is (μ _s P _s)/(μ _v P _v), where μ _s and μ _v denote mutation rate of transitions and transversions, respectively, andP _s andP _v denote fixation probabilities of transitions and transversions, respectively. Because selection against synonymous transitions can be assumed to be roughly equal to that against synonymous transversions,P _s/P_v ≈ 1 at fourfold degenerate sites, so that thes/v ratio at fourfold degenerate sites is approximately μ _s /μ _v , which is a measure of mutational contribution to transition bias. Similarly, thes/v ratio at nondegenerate sites is also an estimate of μ _s /μ _v if we assume that selection against nonsynonymous transitions is roughly equal to that against nonsynonymous transversions. In two mitochondrial genes, cytochrome oxidase subunit I (COI) and cytochromeb (cyt-b) in pocket gophers, thes/v ratio is about two at nondegenerate and fourfold degenerate sites for both the COI and the cyt-b genes. This implies that mutation contribution to transition bias is relatively small. In contrast, thes/v ratio is much greater at twofold degenerate sites, being 48 for COI and 40 for cyt-b. Given that the μ _s /μ _v ratio is about 2, theP _s/P_v ratio at twofold degenerate sites must be on the order of 20 or greater. This suggests a great effect of purifying selection on transition bias in mitochondrial protein genes because transitions are synonymous and transversions are nonsynonymous at twofold degenerate sites in mammalian mitochondrial genes. We also found that nonsynonymous mutations at twofold degenerate sites are more neutral than nonsynonymous mutations at nondegenerate sites, and that the COI gene is subject to stronger purifying selection than is the cyt-b gene. A model is presented to integrate the effect of purifying selection, codon bias, DNA repair and GC content ons/v ratio of protein-coding genes. Correspondence to: X. Xia     

Influence of ultraviolet-B (UV-B) radiation on photosynthetic and growth characteristics in field-grown cassava (Manihot esculentum Crantz)

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5kJ m^?2 d^?1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (F_v/F_m) were determined for fully expanded leaves after 64–76 d of UV-B exposure. AH plants were harvested after 95 d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of F_v to F_m ratios indicated a significant decline in F_v/F_m ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800μmol m^?2 s^?1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO₂ and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation. The increase in shoot/root ratio was due primarily to a significant decrease in root weight (–32%) with UV-B exposure. Because root production determines the harvest-able portion of cassava, UV-B radiation may still influence the yield of an important tropical agronomic species, even though photosynthesis and total dry biomass may not be directly affected.     

Species abundance distributions in neutral models with immigration or mutation and general lifetimes

We consider a general, neutral, dynamical model of biodiversity. Individuals have i.i.d. lifetime durations, which are not necessarily exponentially distributed, and each individual gives birth independently at constant rate λ. Thus, the population size is a homogeneous, binary Crump–Mode–Jagers process (which is not necessarily a Markov process). We assume that types are clonally inherited. We consider two classes of speciation models in this setting. In the immigration model, new individuals of an entirely new species singly enter the population at constant rate μ (e.g., from the mainland into the island). In the mutation model, each individual independently experiences point mutations in its germ line, at constant rate θ. We are interested in the species abundance distribution, i.e., in the numbers, denoted I _n (k) in the immigration model and A _n (k) in the mutation model, of species represented by k individuals, k = 1, 2, . . . , n, when there are n individuals in the total population. In the immigration model, we prove that the numbers (I _t (k); k ≥ 1) of species represented by k individuals at time t, are independent Poisson variables with parameters as in Fisher’s log-series. When conditioning on the total size of the population to equal n, this results in species abundance distributions given by Ewens’ sampling formula. In particular, I _n (k) converges as n → ∞ to a Poisson r.v. with mean γ/k, where γ : = μ/λ. In the mutation model, as n → ∞, we obtain the almost sure convergence of n ⁻¹ A _n (k) to a nonrandom explicit constant. In the case of a critical, linear birth–death process, this constant is given by Fisher’s log-series, namely n ⁻¹ A _n (k) converges to α ^k /k, where α : = λ/(λ + θ). In both models, the abundances of the most abundant species are briefly discussed.     

Solar UV-B radiation, associated with ozone depletion, inhibits the Antarctic terrestrial microalga, Stichococcus bacillaris

This study shows that increased UV-B arising from stratospheric ozone depletion over Antarctica reduced cell viability and the maximum quantum yield of photochemistry (F _v/F _m) in a unicellular terrestrial microalga, Stichococcus bacillaris. In the UV waveband, rates of F _v/F _m decline increased with decreasing wavelength. Photosynthetically active radiation (PAR) also reduced F _v/F _m, though less than UV radiation. Further experiments under different ozone column thicknesses showed a significantly greater decline in cell viability and F _v/F _m under ozone depletion compared with non-depleted conditions. The inhibitory effects of ambient solar radiation suggest that S. bacillaris is unlikely to inhabit soil surfaces, but colonises shaded areas beneath soil surface particles. During periods of ozone depletion, increases in the ratio of UV-B:PAR may reduce the thickness of the sub-surface zone where light conditions are suitable for colonisation by this alga.     

A Simple Construction Procedure for Eesolvable Incomplete Block Designs for any Number of Treatments

A simple, straightforward procedure, which requires no special tables or generators, is presented for constructing resolvable incomplete block designs for v=pk, v=p²k, …, treatments, for k≥p, in incomplete blocks of size k. Also, it is shown, how to obtain incomplete block designs for any v in blocks of size k and k+1. The procedure allows construction of balanced incomplete block designs for p = k a prime number. For p = n not a prime number, incomplete block designs can be obtained by the procedure, but are not balanced. However, for p_s being the smallest prime factor of n, p_s + 1 for v = n², p_s²+ p_s + 1 for v = n³, …, arrangements can be obtained for which the occurrence of any treatment pair in the blocks is either zero or one. This is called a zero-one concurrence design. Procedures are described for obtaining additional zero-one concurrence arrangements. It is shown that the efficiency of these designs is maximum. Both intra-block and inter-block analyses are described.     

Assessment of the adiabatic transformability hypothesis in a ball-bouncing task

The adiabatic transformability hypothesis for non-conservative, non-rate-limited biological systems put forward by Kugler and Turvey [Kugler PN, Turvey MT (1987) Information, natural law, and the self-assembly of rhythmic movements, Erlbaum, Hillsdale, NJ] is evaluated in a ball-bouncing task as a function of skill level. We hypothesized that, when a basketball player increases or decreases the frequency of his limb motion in a vertical ball-bouncing task, the transformation should be characterized as adiabatic. Confirmation of this hypothesis would lend support to the body of knowledge that suggests that physical-biological laws guide the behavior of people engaged in motor tasks, including their acquisition of skill. We videotaped and analyzed four participants – two intermediates and two experts – bouncing a ball from various heights, and measured the energy and kinematic relations of the ball and the participants' body segments. The task presents a challenge to certain predictions of the adiabatic hypothesis in evaluating changes in movement velocity (v) or frequency (f), and energy as adiabatic transformations. Among these are the constant relation between kinetic energy (E _k) per cycle and v, constancy of energy dissipated per cycle (E _t) over changes in v and E _k, the ratio of E _k to E _t per cycle (the “Q” values), and the relation of amplitude to v. From our observations, which are examined in regard to the insights of Kugler and Turvey about the relation of Ehrenfest's adiabatic theorem to biological systems, we confirm the basic adiabatic character of the task in analyzing both the ball alone and the relations of various body segments. In the segmental frame of reference, we found evidence of differences in energy-kinematic relations in the Q values between skill levels. Received: 28 May 1998 / Accepted in revised form: 12 November 1999     

Influence of pH and micellar systems on the sensitized photo‐oxidation of bovine serum albumin

Photosensitized oxidation of bovine serum albumin (BSA), by using perinaphtenone as a sensitizer, has been studied at pH 7.4 and 11. The selected sensitizer does not present ground‐state complexation with BSA and ensures that the mechanism is mediated by O₂(¹△_g). Strong dependence between BSA–O₂(¹△_g) photo‐oxidation and the pH of the medium has been found. The relative oxygen uptake rate (v_? △ O2 ) and the total quenching rate constant (k_t) values are higher at pH 11 than pH 7.4. The enhancement in the alkaline condition is due to conformational changes in the protein and the reactivity of tyrosinate anion with O₂(¹△_g). Even when the tendency with the pH in the presence of sodium dodecyl sulfate (SDS) micelles is similar to that observed in homogeneous media, an increment on the k_t value is detected. This effect may be attributable to the strong interaction of BSA–SDS, which leads to the protein unfolding and could leave more exposed photo‐oxidizable amino acids. A protective effect against the O₂(¹△_g)‐mediated photo‐oxidation was observed in reverse micelles (RMs) of sodium bis(2‐ethylhexyl)sulfosuccinate (AOT) by comparing the k_t values obtained at W = 10 with respect to the one obtain in homogeneous media. The latter could be mainly explained by the modification in the solvent polarity. Also, another important observation was found, the internal pH inside RMs of AOT sensed through tyrosine absorption was independent of the one used for the formation of the water pool. Hence, the k_t values observed at both pH, are quite similar.     

3-Methylglutaconyl-CoA hydratase from Acinetobacter sp

Acinetobacter strain IVS-B aerobically grows on isovalerate as sole carbon and energy source. Isovalerate is metabolised via isovaleryl-CoA, an intermediate of the oxidative (S)-leucine degradation pathway. A 3-methylglutaconyl-CoA hydratase (EC 4.2.1.18) was purified 65-fold to apparent homogeneity from cell-free extracts of isovalerate-grown cells of Acinetobacter strain IVS-B. The enzyme was found to be a homotetramer (115.2 kDa) composed of four identical subunits of 28.8 kDa not containing any cofactors. The enzyme was shown to catalyse the hydration of (E)-glutaconyl-CoA (k _cat=18 s⁻¹, K _m=40 μM) and the dehydration of (S)-3-hydroxyglutaryl-CoA (k _cat=13 s⁻¹, K _m=52 μM), albeit with somewhat lower catalytic efficiencies as compared to the 3-methyl derivatives, 3-methylglutaconyl-CoA (k _cat=138 s⁻¹, K _m=14 μM) and (S)-3-hydroxy-3-methylglutaryl-CoA (k _cat=60 s⁻¹, K _m=36 μM). Thus, the mechanistically simple syn-addition of water to the (E)-isomer of 3-methylglutaconyl-CoA of the leucine degradative pathway leading to the common intermediate (S)-3-hydroxy-3-methylglutaryl-CoA was assigned as the major physiological role to this enzyme. The amino acid sequence of 3-methylglutaconyl-CoA hydratase from Acinetobacter sp. was found to be related to over 100 prokaryotic enoyl-CoA hydratases (up to 50% identity), possibly all being 3-methylglutaconyl-CoA hydratases.An erratum to this article can be found at     

Comparative evaluation of quantum theory of nerve excitation

Deliberate evaluation of the quantum theory of nerve excitation is made by comparing it with Hill's theory in fitting the experimental data on threshold-frequency relation, optimum frequency (v₀) for nerve excitation and strength-duration relation. Decrease of v₀ and increase of all the time constants (Hill's λ andk, Wei'sT ₂ and spike durationw) with decreasing temperature are interpreted on the basis of the dipole relaxation timeT ₂ but inexplicable from Hill's theory or any other existing theory. The closeness ofk,T ₂ andw values is explained. A variety of experimental results obtained by others is discussed. Finally, a comparison is made between the Hodgkin-Huxley equations and the quantum theory. Most of the facts (electrical and non-electrical) tend to support the thesis that nerve excitation is a macroscopic expression of quantum transitions of dipoles between energy states.     

Photosynthesis performance in sweet almond [<Emphasis Type="Italic">Prunus dulcis</Emphasis> (Mill) D. Webb] exposed to supplemental UV-B radiation

Due to anthropogenic influences, solar UV-B irradiance at the earth’s surface is increasing. To determine the effects of enhanced UV-B radiation on photosynthetic characteristics of Prunus dulcis, two-year-old seedlings of the species were submitted to four levels of UV-B stress, namely 0 (UV-B_c), 4.42 (UV-B₁), 7.32 (UV-B₂) and 9.36 (UV-B₃) kJ m⁻² d⁻¹. Effects of UV-B stress on a range of chlorophyll (Chl) fluorescence parameters (FPs), Chl contents and photosynthetic gas-exchange parameters were investigated. UV-B stress promoted an increase in minimal fluorescence of dark-adapted state (F₀) and F₀/F_m, and a decrease in variable fluorescence (F_v, F_v/F_m, F_v/F₀ and F₀/F_m) due to its adverse effects on photosystem II (PSII) activity. No significant change was observed for maximal fluorescence of dark-adapted state (F_m). Enhanced UV-B radiation caused a significant inhibition of net photosynthetic rate (P _N) at UV-B₂ and UV-B₃ levels and this was accompanied by a reduction in stomatal conductance (g _s) and transpiration rate (E). The contents of Chl a, b, and total Chl content (a+b) were also significantly reduced at increased UV-B stress. In general, adverse UV-B effects became significant at the highest tested radiation dose 9.36 kJ m⁻² d⁻¹. The most sensitive indicators for UV-B stress were F_v/F₀, Chl a content and P _N. Significant P<0.05 alteration in these parameters was found indicating the drastic effect of UV-B radiation on P. dulcis.     

Photochemical Properties of Photosystem 2 in Primary Leaves of Barley Seedlings Grown Under Various Blue or Red Irradiances

Photosystem 2 (PS2)-driven electron transfer was studied in primary leaves of barley (Hordeum vulgare L.) seedlings grown under various photon fluxes (0.3–170.0 mol m^–2 s^–1) of blue (BR) or red (RR) radiation using modulated chlorophyll fluorescence. The F_v/F_m ratio was 0.78–0.79 in leaves of all radiation variants, except in seedlings grown under BR or RR of 0.3 mol m^–2 s^–1. The extent of the photochemical phase of the polyphasic F_v rise induced by very strong white light was similar in leaves of all radiation treatments. Neither radiation quality nor photon flux under plant cultivation influenced the amount of non Q_B-transferring centres of PS2 except in leaves of seedlings grown under BR of 0.3 mol m^–2 s^–1, in which the amount of such centres increased threefold. Both BR and RR stimulated the development of photochemically competent PS2 at photon fluxes as low as 3 mol m^–2 s^–1. Three exponential components with highly different half times were distinguished in the kinetics of F_v dark decay. This indicates different pathways of electron transfer from Q_A ^–, the reduced primary acceptor of PS2, to other acceptors. Relative magnitudes of the individual decay components did not depend on the radiation quality or the photon flux during plant cultivation. Significant differences were found, however, between plants grown under BR or RR in the rate of the middle and fast components of F_v dark decay, which showed 1.5-times faster intersystem linear electron transport in BR-grown leaves.     

Nitrogen and Phosphorus Release from Decomposing Leaves under Sub‐Humid Tropical Conditions1

For many soils of the tropics, inputs of organic materials are essential to sustain soil fertility and crop production. Research in the quality of organic inputs, a key factor controlling rates of decomposition and nutrient release, continues to guide selection and use of organic materials as nutrient sources. The relationship between decomposition patterns and the quality parameters of the fresh leaves of six agroforestry species: Sesbania sesban, Croton megalocarpus, Calliandra calothyrsus, Tithonia diversifolia, Lantana camara, and Senna spectabilis, was investigated in a litterbag study over a period of 77 days in the highlands of western Kenya. The litterbags were buried 1 cm below the soil surface and covered with soil of ca 1 cm thickness. Percent leaf mass and total N and P that remained with time strongly correlated with total P and C/P ratio (R²= 0.60‐0.90) during the first 35 days of study; but afterwards, correlation was stronger with the initial soluble polyphenolics (Pp)/P ratio (R²= 0.69‐0.92) than with total P and C/P ratio. Loss of leaf mass and release of N and P followed the exponential function, y_t= y₀* e‐^kt, from which the specific decay rate constants (k) were calculated for loss of leaf mass (k_B) and release of N (k_N) and P (K_p). Among the plant species, the k values were lowest in Calliandra with k_B= 0.012/d, k_N= 0.017/d and k_p= 0.044/d. Lantana had the highest K values with k_g= 0.067/d and k_p= 0.119/d, but the highest k_N value of 0.109/d occurred in Tithonia. The k_B values for all organic materials were lower than their corresponding k_N and k_p values, suggesting that leaching of N and P from litters may have augmented the microbial mineralization of N and P. There was a strong correlation between the k_B, k_N, and k_p values and total P (r = 0.82‐0.96; P 0.01), but not total N, lignin (LIG), or Pp. Rates of N and P release followed the general trend: Tithonia > Senna > Lantana > Sesbania > Croton > Calliandra. The results indicated that, among the quality parameters studied, total P is the most important factor controlling rate of decomposition and N and P release from organic inputs in the area of study.     

Interparticle mass transfer study with a packed column of immobilized microbes

The kinetics of whole-cell lactase of Escherichia coli immobilized in spherical agar gels was determined under the influence of interparticle mass transfer in a fixed bed reactor. The dependence of the pseudokinetic parameters (K′_m and v′_max) fluid conditions was in accordance with the prediction derived from combinedeffects of kinetics and mass transfer. Within the limited conditions of theexperiments, it was observed that the apparent Michaelis constant could be represented by the following simple equation, K′_m/K_m = 1 + (?/k_La)(v_max/K_m). The mass transfer coeflicient (k_L) needed in the theoretical analysis was determined in correlation with particle size (0.12–0.45 cm) and liquid flow rate (0.30–12.0 cm/min) using the more stable enzyme, cell-bound invertase of Saccharomyces pastorianus, as the material to be entrapped in the gelatinous particles. The relationship was expressed in the following dimensionless equation, (1–?)Sh = 2 + 0.6Re^1/2Sc^1/3, except that marked deviation resulting in the reduction of k_L was noticed with a Reynolds number less than 0.8.     

Small scale vertical gradients of Arctic ice algal photophysiological properties

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (F _v/F _m), maximum relative electron transport rate (rETR_max), photosynthetic efficiency (α), and the photoadaptive index (E _k). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0–5 cm from ice–water interface) expressed low F _v/F _m (0.331–0.426) and low α (0.098–0.130 (μmol photons m⁻²s⁻¹)⁻¹) in December. F _v/F _m and α increased in March and May (0.468–0.588 and 0.141–0.438 (μmol photons m⁻²s⁻¹)⁻¹, respectively) indicating increased photosynthetic activity. In addition, increases in rETR_max (3.3–16.4 a.u.) and E _k (20–88 μmol photons m⁻² s⁻¹) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion, photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).     

Photoadaptation of an ice algal community in thin sea ice,Saroma-Ko Lagoon,Hokkaido, Japan

We investigated the shade adaptation of a seasonally well-developed ice algal community in thin sea ice at Saroma-Ko Lagoon, Hokkaido, Japan on 3–4 March 2006 and 4–5 March 2007, by examining photosynthetic pigment concentrations, the chlorophyll a-specific light-absorption coefficient (a _ph *), and the light-saturation index (E _k ). The high proportions of photosynthetic pigments, including chlorophyll a, fucoxanthin, and chlorophyll c, and the low values of a _ph *₍₄₄₀₎ and a _ph *₍₆₇₅₎ suggested that the lagoon’s ice algal community was shade-adapted. The high ratio of E _k to total photosynthetically active radiation (PAR) in the ice algal habitat suggested that the degree of shade adaptation is weak. Scaling of E _k to total PAR could be extended to studies of the degree of photoadaptive succession of ice algal communities in the Northern Hemisphere. The degree of shade adaptation of ice algal communities in the Northern Hemisphere might be related to ice thickness, regardless of latitude.     

Mesoscopic simulation studies on micellar phases of Pluronic P103 solution

The microphase separation dynamics of the triblock copolymer surfactant P103 [(ethylene oxide)₁₇(propylene oxide)₆₀(ethylene oxide)₁₇] was investigated by a dynamic variant of mean-field density functional theory. Different self-assembled aggregates, spherical micelles, micellar clusters and disk-like micelles, are explored in the solution. The spherical micelle above critical micelle concentration (CMC) is a dense core consisting mainly of PPO and a hydrated PEO swollen corona, and is in good agreement with the experimental results concerning their structures. At a concentration of 10–15%, micellar clusters with a larger PPO core form as a result of coalescence among spherical micelles. At concentrations above 16% by volume, a series of disk-like micelles come into being. The order parameters show that spherical micelles are easily formed, while the micellar clusters or disk-like micelles need a longer time to reach steady equilibrium. The results show that mesoscopic simulation can augment experimental results on amphiphilic polymers, and provide some mesoscopic information at the mesoscale level. Figure Coalescence of Micelles with time evolution for 15% vol system. □ represents spherical micelle that coalesce. (a) 180 μs, (b) 190 μs, (c)225 μs, and (d) 250 μs     

Photosynthetic performance of regenerated protoplasts from disintegrated cells of <Emphasis Type="Italic">Bryopsis hypnoides</Emphasis> (Chlorophyta)

The photosynthetic performances of regenerated protoplasts of Bryopsis hypnoides, which were incubated in seawater for 1, 6, 12, and 24 h, were studied using chlorophyll (Chl) fluorescence and oxygen measurements. Results showed that for the regenerated protoplasts, the pigment content, the ratios of photosynthetic rate to respiration rate, the maximal photosystem II (PSII) quantum yield (F_v/F_m), and the effective PSII quantum yield (Φ_PSII) decreased gradually along with the regeneration progress, indicated that during 24 h of regeneration there was a remarkable reduction in PSII activity of those newly formed protoplasts. We assumed that during the cultivation progress the regenerated protoplasts had different photosynthetic vigor, with only some of them able to germinate and develop into mature thalli. The above results only reflected the photosynthetic features of the regenerated protoplasts at each time point as a whole, rather than the actual photosynthetic activity of individual aggregations. Further investigation suggested a relationship between the size of regenerated protoplasts and their viability. The results showed that the middle-sized group (diameter 20–60 μm) retained the largest number of protoplasts for 24 h of growth. The changes in F_v/F_m and Φ_PSII of the four groups of differently sized protoplasts (i.e. < 20, 20–60, 60–100, and > 100 μm) revealed that the protoplasts 20–60 μm in diameter had the highest potential activity of the photosynthetic light energy absorption and conversion for several hours.     

Evolution of enzyme catalytic power. Characteristics of optimal catalysis evaluated for the simplest plausible kinetic model

1. Evolutionary changes in the structure of an enzyme that provide an increase in its K_m value are considered. Provided that K_m increases as a result of increases in the forward rate constants of the catalysis relative to the reverse rate constants, the enzyme catalyses the conversion of a fixed concentration of its substrate more rapidly when its structure provides that K_m>[S] than when K_m<[S]. 2. Catalytic efficiency of enzymes is discussed in terms of the simplest plausible model, the Haldane [(1930) Enzymes, Longmans, London] reversible three-step model: [Formula: see text] The rate equation for the forward reaction of this model (formation of P) may be written in the simple form: [Formula: see text] K_eq. is the equilibrium constant (=[P]_eq./[S]_eq.), and k_cat.=V/[E]_T, where [E]_T is the total enzyme concentration. 3. To assess the effectiveness of an enzyme, it is necessary only to determine the extent to which the constraints of a particular kinetic mechanism permit v₂ (v when K_m»[S]) to approach v_d (the diffusion-limited rate). 4. The value of the optimal rate of catalysis (v_opt., the maximal value of v₂) is dictated by the equilibrium constant for the reaction, K_eq.; v₂=v_d/a, where [Formula: see text] when k₊₁ is assumed equal to k₋₃, and v_opt.=v_d/a_min.. When K_eq.≥1, it is necessary that k₊₂»k₋₁ for a to take its minimum value, a_min.; when K_eq.«1, it is necessary only that k₊₂»K_eq.·k₋₁, i.e. a can equal a_min. even if k₊₂<k₋₁. When K_eq.»1, v_opt.=v_d; when K_eq.=1, v_opt.=v_d/2, and when K_eq.«1, v_opt.=K_eq.·v_d. 5. The analysis, together with predicted effects of evolutionary pressure, suggests that in practice the rates of the fastest enzyme-catalysed freely reversible reactions might be expected to be lower than the value of k₊₁[E]_T[S] by about an order of magnitude, particularly if K_eq.<1. 6. The existing literature suggests that, in general, appropriate values of K_m have evolved for the provision of high rates of catalysis but that many values of k_cat. are not large enough to provide optimal rates of catalysis unless the value of k₊₁ in vivo is lower than its value in free solution.     

EFFECT OF LIGHT HISTORY ON THE ULTRASTRUCTURE AND PHYSIOLOGY OF PROROCENTRUM MARIAE-LEBOURIAE (DINOPHYCEAE)1

Ultrastructural and physiological responses of Prorocentrum mariae-lebouriae (Parke & Ballantine) Faust are reported for cultures maintained at growth irradiances (I_g) ranging from 20.6 to 0.3 E m^?2.d^?1 and following downward shifts in light intensity. We tested the hypothesis that Prorocentrum grown under light regimes that elicit different responses in photosynthesis and pigmentation exhibit distinctive cell ultrastructures. Prorocentrum from high-light conditions had high saturation intensities for photosynthesis (I_k) and low levels of Chl a, Chl c and peridinin-cell^?1 These cultures were morphologically distinguished by a large starch volume fraction (V_v), small chloroplast V_v and fewer thylakoids lamella^?1. I_k values were lower and pigment concentrations higher in low-light treatments, and cells showed reduced starch V_v, large chloroplast V_v, and higher numbers of thylakoids · lamella ^?1. Cells grown under extremely low-light conditions appeared stressed as indicated by the absence of starch reserves and the presence of large vacuoles within the cytoplasm. Results for presence of large vacuoles within the cytoplasm. Results for quantiative electron microscopy, photosynthesis-irradiance (P-I) relations and cell pigmentation indicate that photoadaptation in P. mariae-lebouriae involves a strategy that encompasses changes in both the “size” and “number” of photosynthetic units.     

Oxygen transfer to slurries treated in a rotating drum operated at atmospheric pressure

The objective of this work was to determine (1) the effect of rotational speed (N) and lifters on the oxygen transfer coefficient (k _L) of a mineral solution and (2) the effect of solids concentration of a slurry soil-mineral solution on k _L, at a fixed value N (0.25 s⁻¹); in both cases the treatment was carried out in an aerated rotating drum reactor (RDR) operated at atmospheric pressure. First, the k _L for the mineral solution was in the range 6.38 × 10⁻⁴–7.69 × 10⁻⁴ m s⁻¹, which was of the same order of magnitude as those calculated for closed rotating drums supplied with air flow. In general, k _L of RDR implemented with lifters was superior or equal to that of RDR without lifters. For RDR implemented with lifters, k _L increased with N in the range 6.65 × 10⁻⁴–10.51 × 10⁻⁴ m s⁻¹, whereas k _L of RDR without lifters first increased with N up to N = 0.102 s⁻¹, and decreased beyond this point. Second, regarding soil slurry experiments, an abrupt fall of k _L (ca. 50%) at low values of the solid concentration (C _v) and an asymptotic pattern at high C _v were observed at N = 0.25 s⁻¹. These results suggest that mass transfer phenomena were commanded by the slurry properties and a semi-empirical equation of the form Sh = f(Re, Sc) seems to corroborate this finding.