Oxidative capacity determines the growth rate with Acetobacter methanolicus

The cultivation of Acetobacter methanolicus on various substrates revealed that the respective maximum growth rates are obtained at an almost identical oxidative capacity of about 16 mmoles of oxygen g (biomass)^?1·h^?1 under conditions of energy generations by complete substrate oxidation. This is considered to be an indication that the energy production rate determined by the capacity of the respiratory chain limits the growth rate in this strain. However, with glucose and glycerol, for example, a further increase in the growth rate is observed accompanied by the generation of products (gluconic acid or dihydroxyacetone, respectively). The incomplete oxidation should play the role of an additional energy generation. The potential for this rate increase is looked for in a higher energy gain derived from reduction equivalents (PQQH₂) in this periplasmic oxidation step in relation to the cytoplasmic reduction equivalents.     

Growth of Hansenula polymorpha in a methanol-limited chemostat

Hansenula polymorpha has been grown in a methanol-limited continuous culture at a variety of dilution rates. Cell suspensions of the yeast grown at a dilution rate of 0.16 h^-1 showed a maximal capacity to oxidize excess methanol (QO ₂ ^max ) which was 1.6 times higher than the rate required to sustain the growth rate (Q _O2). When the dilution rate was decreased to 0.03 h^-1, QO ₂ ^max of the cells increased to a value of more than 20 times that of Q _O2. The enzymatic basis for this tremendous overcapacity for the oxidation of excess methanol at low growth rates was found to be the methanol oxidase content of the cells. The level of this enzyme increased from 7% to approximately 20% of the soluble protein when the growth rate was decreased from 0.16 to 0.03 h^-1. These results were explained on the basis of the poor affinity of methanol oxidase for its substrates. Methanol oxidase purified from Hansenula polymorpha showed an apparent K _mfor methanol of 1.3 mM in air saturated reaction mixtures and the apparent K _mof the enzyme for oxygen was 0.4 mM at a methanol concentration of 100 mM.The involvement of an oxygen dependent methanol oxidase in the dissimilation of methanol in Hansenula polymorpha was also reflected in the growth yield of the organism. The maximal yield of the yeast was found to be low (0.38 g cells/g methanol). This was not due to a very high maintenance energy requirement which was estimated to be 17 mg methanol/g cells x h.     

Respiration of individual honeybee larvae in relation to age and ambient temperature

The CO₂ production of individual larvae of Apis mellifera carnica, which were incubated within their cells at a natural air humidity of 60–80%, was determined by an open-flow gas analyzer in relation to larval age and ambient temperature. In larvae incubated at 34 °C the amount of CO₂ produced appeared to fall only moderately from 3.89±1.57 µl mg^–1 h^–1 in 0.5-day-old larvae to 2.98±0.57 µl mg^–1 h^–1 in 3.5-day-old larvae. The decline was steeper up to an age of 5.5 days (0.95±1.15 µl mg^–1 h^–1). Our measurements show that the respiration and energy turnover of larvae younger than about 80 h is considerably lower (up to 35%) than expected from extrapolations of data determined in older larvae. The temperature dependency of CO₂ production was determined in 3.5-day-old larvae, which were incubated at temperatures varying from 18 to 38 °C in steps of 4 °C. The larvae generated 0.48±0.03 µl mg^–1 h^–1 CO₂ at 18 °C, and 3.97±0.50 µl mg^–1 h^–1 CO₂ at 38 °C. The temperature-dependent respiration rate was fitted to a logistic curve. We found that the inflection point of this curve (32.5 °C) is below the normal brood nest temperature (33–36 °C). The average Q₁₀ was 3.13, which is higher than in freshly emerged resting honeybees but similar to adult bees. This strong temperature dependency enables the bees to speed up brood development by achieving high temperatures. On the other hand, the results suggest that the strong temperature dependency forces the bees to maintain thermal homeostasis of the brood nest to avoid delayed brood development during periods of low temperature.Abbreviations m body mass - R rate of development or respiration - T_I inflexion point of a logistic (sigmoid) curve - T_L lethal temperature - T_O temperature of optimum (maximum) developmentCommunicated by G. Heldmaier     

Anaerobic acetate oxidation to CO2 by Desulfotomaculum acetoxidans

Desulfotomaculum acetoxidans oxidizes acetate to CO₂ with sulfate. This organism metabolizes acetate via a pathway in which C₁ units rather than tri- and dicarboxylic acids are intermediates. We report here that cell extracts of D. acetoxidans catalyzed an exchange between CO₂ and the carboxyl group of acetate at a rate of 90 nmol · min^-1 · mg^-1 protein which is sufficient to account for the in vivo acetate oxidation rate of 250 nmol · min^-1 · mg^-1 protein. The reaction was strictly dependent on both ATP and coenzyme A. The extracts contain high activities of acetate kinase (6.3 U · mg^-1 protein) and phosphotransacetylase (60 U · mg^-1 protein). These findings indicate that acetyl-CoA rather than acetyl-phosphate or acetate is the substrate of the carbon-carbon cleavage activity. Exchange was only observed in the presence of strong reducing agents such as Ti³⁺. Interestingly, the cell extracts also catalyzed the reduction of CO₂ to CO with Ti³⁺ as electron donor (120 nmol · min^-1 · mg^-1 protein). Carbon monoxide dehydrogenase and other oxidoreductases involved in acetate oxidation were found to be partially associated with the membrane fraction suggesting a membrane localization of these enzymes.Abbreviations MOPS Morpholinopropane sulfonic acid - Tricine N-tris(hydroxymethyl)-methylglycine - DTT d,l-1,4-Dithiothreitol - DMN 2,3-Dimethyl-1,4-naphthoquinone - MV_OX Methyl viologen, oxidized - APS Adenosinephosphosulfate - SRB Sulfate reducing bacteria - U mol product formed per min     

Modulation of photosynthesis and respiration in guard and mesophyll cell protoplasts by oxygen concentration

Stomatal movement is an energetic oxygen-requiring process. In the present study, the effect of oxygen concentration on mitochondrial respiratory activity and red-light-dependent photosynthetic oxygen evolution by Vicia faba and Brassica napus guard cell protoplasts was examined. Comparative measurements were made with mesophyll cell protoplasts isolated from the same species. At air saturated levels of dissolved oxygen in the protoplast suspension media, respiration rates by mesophyll protoplasts ranged from 6 to 10μmoles O₂ mg^?1 chl h^?1, while guard cell protoplasts respired at rates of 200–300 μmoles O₂ mg chl^?1 h^?1, depending on the species. Lowering the oxygen concentration below 50–60 mmol m^?3 resulted in a decrease in guard cell respiration rates, while rates by mesophyll cell protoplasts were reduced only at much lower concentrations of dissolved oxygen. Rates of photosynthesis in mesophyll cell protoplasts isolated from both species showed only a minor reduction in activity at low oxygen concentrations. In contrast, photosynthesis by guard cell protoplasts isolated from V. faba and B. napus decreased concomitantly with respiration. Oligomycin, an inhibitor of oxidative phos-phorylation, reduced photosynthesis in mesophyll cell protoplasts by 27–46% and in guard cell protoplasts by 51–58%. The reduction in both guard cell photosynthesis and respiration following exposure to low oxygen concentrations suggest close metabolic coupling between the two activities, possibly mediated by the availability of substrate for respiration associated with photosynthetic electron transport activity and subsequent export of redox equivalents.     

Induction by glucose of an antimycin-insensitive,azide-sensitive respiration in the yeast Kluyveromyces lactis

Increasing the glucose concentration from 0.1 to 10% in exponentially growing cultures of Kluyveromyces lactis CBS 2359 does not repress the antimycin-sensitive respiration (Q_{O 2} of 80 l O₂·h^-1·mg^-1 dry weight) but raises the antimycin-insensitive respiration from 3 to 12 l O₂·h^-1·mg^-1 dry weight. Antimycin A inhibits the growth of K. lactis on a variety of substrates with the exception of glucose at concentrations equal to or higher than 1% where substantial antimycin-insensitive respiratory rates are induced. It can be concluded that a minimal antimycin-insensitive Q_{O 2} is necessary for cellular growth when the normal respiratory pathway is not functional.The antimycin-insensitive respiration elicited by growth in high glucose concentrations is poorly inhibited by hydroxamate and is inhibited by 50% by 90 m azide or 1mm cyanide. These concentrations are much higher than those necessary to inhibit cytochrome c oxidase which is not involved in the antimycin-insensitive respiration as was demonstrated by spectral measurements. A pigment absorbing at 555 nm is specifically reduced after addition of glucose to antimycin-inhibited cells. The same pigment is reoxidized by further addition of high concentrations of sodium azide indicating its participation in the antimycin-insensitive, azide-sensitive respiration.     

Endogenous respiration reflects the energy load imposed by transport of nonmetabolizable substrates and by induced de novo protein synthesis in Rhodotorula glutinis

Uptake of the nonmetabolizable sugars 6-deoxy-d-glucose, l-rhamnose and l-xylose, which are taken up by a common carrier, stimulated significantly cell respiration in Rhodotorula glutinis. The extra oxygen consumption for uptake (0.5–0.7 equivalents O₂/mol transported sugar) was proportional to the uptake rate and was independent of the K _tvalue of the transport system. Sugars that become metabolized after induction, d-arabinose and methyl--d-glucoside, caused a higher stimulation, 1.4 and 3.6 equivalents O₂/mol respectively, which was reduced to 0.6 equivalents O₂/mol when de novo protein synthesis was blocked by cycloheximide. The stimulation of respiration thus includes a fraction related purely to the energy demand for uptake and another one related to the induced de novo protein synthesis. The net uptake-induced respiration boost was similar with all sugars under study irrespective of their transport systems. The estimated energy demand was equivalent to about 2 ATP/sugar molecule. For comparison, the amino acid analogue -aminoisobutyric acid (AIB) was also investigated; the overall energy demand for its uptake corresponded to the equivalent of about 4 ATP/molecule.Abbreviation AIB -aminoisobutyric acid     

Search for intermediates of photosynthetic water oxidation

Photosystem II of cyanobacteria and plants incorporates the catalytic centre of water oxidation. Powered and clocked by quanta of light the centre accumulates four oxidising equivalents before oxygen is released. The first three oxidising equivalents are stored on the Mn₄Ca-cluster, raising its formal oxidation state from S₀ to S₃ and the third on Y_Z, producing S₃ Y_Z^ox. From there on water oxidation proceeds in what appears as a single reaction step (S₃ Y_Z^ox(H₂O)₂O₂ + 4H⁺ + S₀. Intermediate oxidation products of bound water had not been detected, until our recent report on the stabilisation of such an intermediate by high oxygen pressure (NATURE 430, 2004, 480–483). Based on the oxygen titration (half-point 2.3 bar) the standard free-energy profile of a reaction sequence with a single intermediate was calculated. It revealed a rather small difference (−3 kJ mol⁻¹) between the starting state [S₃Y_Z^OX and the product state S₀Y_Z + O₂ + 4H⁺ . Here we describe the tests for side effects of exposing core particles to high oxygen pressure. We found the reduction of P₆₈₀^{+ ·} in ns and the reduction/dismutation of quinones at the acceptor side of PSII both unaffected, and the inhibition of the oxygen evolving reaction by exposure to high O₂-pressure was fully reversible by decompression to atmospheric conditions.     

Water cleavage by solar radiation—an inspiring challenge of photosynthesis research

Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states S_i (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states S_i, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S₃ with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with Y_Z ^ox and simultaneous internal ligand to metal charge transfer.Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.Abbreviations b559 cytochrome b559 - BChl bacteriochlorophyll - Chl chlorophyll - CP47 Chl a containing a 47 kDa polypeptide - D1/D2 polypeptides of the PS II reaction center - ENDOR electron nuclear double resonance - EPR electron paramagnetic resonance - ESEEM electron spin echo envelope modulation - EXAFS extended X-ray absorption fine structure - FTIR Fourier transform infrared - NMR nuclear magnetic resonance - P680, P700 photoactive Chl a of PS II and PS I, respectively - PS II Photosystem II - Q_A special plastoquinone of PS II - S_i redox states of WOC - WOC water oxidizing complex - WOS water oxidizing site - UV/VIS ultraviolet/visible - Y_D, Y_Z redox active tyrosines of polypeptides D2 and D1, respectively     

Branched respiratory chain in aerobically grown Staphylococcus aureus —oxidation of ethanol by cells and protoplasts

Addition of ethanol and some other primary alcohols, except methanol, to cells and protoplasts (but not membrane particles) considerably stimulated the rate of oxygen consumption. This additional respiration was strongly inhibited by 0.1 mM KCN. The cyanide inhibition curve of endogenous substrate oxidation was slightly biphasic while in the presence of ethanol it became clearly biphasic having K _i values of approx. 0.1 and 0.5 mM. Based on the steady-state cytochrome spectra in the presence of 0.1 mM KCN, we attributed the lower K _i to cytochrome a ₆₀₂. Proteolysis of protoplasts external membrane proteins did not change the rate of endogeneous substrate oxidation but prevented the inhibition of this respiration by low concentrations of KCN and stimulation of oxygen consumption by ethanol. The activity of NAD⁺-dependent ethanol dehydrogenase in the cytoplasm was found to be 520 nmol NADH-x min^–1 x mg^–1 protein. Proteolysis of external membrane proteins apparently inhibits the operation of the cytochrome a ₆₀₂-containing electron transport branch inducing the suppression of electron flow from NADH to oxygen.     

Substrate consumption and excess sludge reduction of activated sludge in the presence of uncouplers: a modeling approach

A mathematical model with a consideration of energy spilling is developed to describe the activated sludge in the presence of different levels of metabolic uncouplers. The consumption of substrate and oxygen via energy spilling process is modeled with a Monod term, which is dependent on substrate and inhibitor. The sensitivity of the developed model is analyzed. Three parameters, maximum specific growth rate (μ _max), energy spilling coefficient (q _max), and sludge yield coefficient (Y _H) are estimated with experimental data of different studies. The values of μ _max, q _max, and Y _H are found to be 6.72 day^-1, 5.52 day^-1, and 0.60 mg COD mg^-1 COD for 2, 4-dinitrophenol and 7.20 day^-1, 1.58 day^-1, and 0.62 mg COD mg^-1 COD for 2, 4-dichlorophenol. Substrate degradation and sludge yield could be predicted with this model. The activated sludge process in the presence of uncouplers that is described more reasonably by the new model with a consideration of energy spilling. The effects of uncouplers on substrate consumption inhibition and excess sludge reduction in activated sludge are quantified with this model.     

Effect of irradiance and temperature on the photosynthesis of a cultivated red alga,Pyropia tenera (= Porphyra tenera), at the southern limit of distribution in Japan

The effect of irradiance and temperature on the photosynthesis of the red alga, Pyropia tenera, was determined for maricultured gametophytes and sporophytes collected from a region that is known as one of the southern limits of its distribution in Japan. Macroscopic gametophytes were examined using both pulse‐amplitude modulated fluorometry and/or dissolved oxygen sensors. A model of the net photosynthesis–irradiance (P‐E) relationship of the gametophytes at 12°C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 46 μmol photons m^?2 s^?1, and the compensation irradiance was 9 μmol photons m^?2 s^?1. Gross photosynthesis and dark respiration for the gametophytes were also determined over a range of temperatures (8–34°C), revealing that the gross photosynthetic rates of 46.3 μmol O₂ mg_chl‐a^?1 min^?1 was highest at 9.3 (95% Bayesian credible interval (BCI): 2.3–14.5)°C, and the dark respiration rate increased at a rate of 0.93 μmol O₂ mg_chl‐a^?1 min^?1°C^?1. The measured dark respiration rates ranged from ?0.06 μmol O₂ mg_chl‐a^?1 min^?1 at 6°C to ?25.2 μmol O₂ mg_chl‐a^?1 min^?1 at 34°C. The highest value of the maximum quantum yield (Fv/Fm) for the gametophytes occurred at 22.4 (BCI: 21.5–23.3) °C and was 0.48 (BCI: 0.475–0.486), although those of the sporophyte occurred at 12.9 (BCI: 7.4–15.1) °C and was 0.52 (BCI: 0.506–0.544). This species may be considered well‐adapted to the current range of seawater temperatures in this region. However, since the gametophytes have such a low temperature requirement, they are most likely close to their tolerable temperatures in the natural environment.     

川西亚高山森林土壤呼吸和微生物生物量碳氮对施氮的响应

随着全球大气氮沉降的明显增加,将有可能显著影响我国西部地区受氮限制的亚高山森林生态系统。土壤微生物是生态系统的重要组成部分,是土壤物质循环和能量流动的重要参与者。由于生态系统类型、土壤养分、氮沉降背景值等的差异,土壤呼吸和土壤生物量碳氮对施氮的响应存在许多不确定性。而施氮会不会促进亚高山森林生态系统中土壤呼吸和微生物对土壤碳氮的固定?基于此假设,选择了川西60年生的四川红杉(Larix mastersiana)亚高山针叶林为研究对象,通过4个水平的土壤施氮控制试验(CK:0 g m~(-2) a~(-1)、N1:2 g m~(-2)a~(-1)、N2:5 g m~(-2) a~(-1)、N3:10 g m~(-2)a~(-1)),监测了土壤呼吸及土壤微生物生物量碳氮在一个生长季的动态情况。结果表明:施氮对土壤呼吸各指标和土壤微生物碳氮都有极显著的影响,施氮能促进土壤全呼吸、自养呼吸、异养呼吸通量和土壤微生物生物量碳氮的增长,施氮使土壤呼吸通量提高了11%—15%,土壤微生物量碳提高了5%—9%,土壤微生物量氮提高了23%—34%。在中氮水平下(5 g m~(-2) a~(-1))对土壤呼吸的促进最显著。相关分析发现,土壤呼吸与微生物生物量碳氮和微生物代谢商极呈显著正相关,微生物量碳氮与土壤温度呈极显著的正相关,与土壤湿度呈极显著负相关。通过一般线性回归拟合土壤呼吸速率与土壤10 cm温湿度的关系,发现土壤呼吸速率与土壤温度呈极显著的正相关,与土壤湿度极显著负相关(P0.001),中氮水平下土壤温度敏感性系数Q_(10)值(7.10)明显高于对照(4.26)。     

The effect of temperature on the respiration rate of meiofauna

Summary The effect of temperature on respiration rate has been established, using Cartesian divers, for the meiofaunal sabellid polychaeteManayunkia aestuarina, the free-living nematodeSphaerolaimus hirsutus and the harpacticoid copepodTachidius discipes from a mudflat in the Lynher estuary, Cornwall, U.K. Over the temperature range normally experienced in the field, i.e. 5–20° C the size-compensated respiration rate (R _c) was related to the temperature (T) in °C by the equation Log₁₀ R _c=-0.635+0.0339T forManayunkia, Log₁₀ R _c=0.180+0.0069T forSphaerolaimus and Log₁₀ R _c=-0.428+0.0337T forTachidius, being equivalent toQ ₁₀ values of 2.19, 1.17 and 2.17 respectively. In order to derive the temperature response forManayunkia a relationship was first established between respiration rate and body size: Log₁₀ R=0.05+0.75 Log₁₀ V whereR=respiration in nl·O₂·ind^-1·h^-1 andV=body volume in nl.TheQ ₁₀ values are compared with values for other species derived from the literature. From these limited data a dichotomy emerges: species with aQ ₁₀2 which apparently feed on diatoms and bacteria, the abundance of which are subject to large short term variability, and species withQ ₁₀1 apparently dependent on more stable food sources.     

Application of High Enzyme Activities Present in Clostridium Thermoaceticum for the Efficient Regeneration of NADPH,NADP+, NADH AND NAD+

Crude extracts of Clostridium thermoaceticum DSM 521 contain various AMAPORs (artificial mediator accepting pyridine nucleotide oxidoreductases). The specific activities of this mixture of AMAPORs is about 8–9 U mg^?1 protein (µmoles mg^?1 min^?1) for NADPH and 3–4 U mg^?1 protein for NADH formation with reduced methylviologen (MV⁺⁺) as electron donor. These AMAPOR-activities are only slightly oxygen sensitive. The reoxidation of NADPH and NADH with carboxamido-methylviologen is catalysed by crude extracts with 2.0 and 1.6 U mg^?1 protein, respectively. The same crude extracts also catalyse the dehydrogenation of reduced pyridine nucleotides with suitable quinones such as anthraquinone-2,6-disulphonate. The reduced quinone can be reoxidised by dioxygen.

The K_m-values of these enzymes for the pyridine nucleotides and also for the artificial electron mediators are in a suitable range for preparative transformations.

Furthermore the crude extract of C. thermoaceticum contains about 2.5 U mg^?1 protein of an NADP⁺-dependent formate dehydrogenase (FDH), which is suitable for NADPH and/or MV⁺⁺ regeneration. The regeneration of MV⁺⁺ with FDH and formate as electron donor proceeds with a specific activity of about 5 U mg^?1 protein of the crude extract. The reduced viologen in turn reduces NAD(P)⁺ by AMAPOR. The formate dehydrogenase is sensitive to oxygen.

Examples of compounds which have been prepared by combination of AMAPORs or formate dehydrogenase with an oxidoreductase are: (S)-3-hydroxycarboxylates, esters of (S)-3-hydroxycarboxylates, (1R,2S)-1-hydroxypropane-tricarboxylate (D_s-(+)-isocitrate), L_s-(-)-isocitrate and 6-phosphogluconate.     

Photosynthetic electron transport in thylakoids from cotton cultivars (Gossypium sp.) differing in tolerance to prometryn

A method to determine photosynthetic electron transport in thylakoid membranes is described for Gossypium barbadense (cv. Pima S-7) and G. hirsutum (cv. DP 5415). These cultivars differed markedly in tolerance to prometryn, a PS II inhibitor. The rates of photosynthetic electron transport obtained were 245 mole oxygen mg^–1 chl h¹. Plant age and leaf size influenced the activity of the thylakoid preparations. Thylakoids from leaves of plants 24 to 37 d and 50–70 mm in diameter had the highest activities; thylakoids from cotyledons, fully expanded leaves and young leaves had low activity. Thylakoids from both species had similar photosynthetic activities and I₅₀'s for prometryn, atrazine and diuron. Thus, tolerance to prometryn was not due to differential binding at D1 protein.Abbreviations PSII photosystem II - DAP day after planting - DQ duroquinone - DBMIB dibromothymoquinone - DMBQ 2,5-dimethyl-p-benzoquinone - I₅₀ concentration to inhibit reaction by 50% - Q_A quinone A - Q_B quinone B     

Respiration Rate of Stream Insects Measured <Emphasis Type="Italic">in situ</Emphasis> Along a Large Altitude Range

Field studies of respiration in stream insects are few in comparison with laboratory studies. To evaluate the influence of temperature and oxygen along altitudinal gradients we measured the respiration rate of fully acclimatized larval Trichoptera, Plecoptera and Ephemeroptera under similar field conditions in streams from 400 to 3800 m above sea level in tropical Ecuador. Mean active respiration rates of the animals at 3800 m were approximately half of those at 400 m. Trichoptera showed a slightly larger difference in respiration with altitude than Ephemeroptera. Comparative respiration measurements at 100 and 50% oxygen saturation indicated that highland animals reduced their oxygen uptake more than their counterparts in the lowland when oxygen availability decreased. The temperature response of respiration calculated between the insect assemblages at different altitudes showed a mean assemblage Q₁₀−value of 1.50. Trichopteran larvae had a slightly stronger temperature response (Q₁₀ of 1.68) than ephemeropterans (Q₁₀ of 1.30). These community Q₁₀-values are considerably lower than the mean value of 2.36 found in single species in the laboratory. The weak community-wide response of respiration to temperature in tropical streams is probably due to full acclimatization of the component species to stable and narrow temperature ranges. Adaptations to the low oxygen availability at high altitude probably consist of a suite of genetic physiological and behavioural features.     

How rapid are the internal reactions of the ubiquinol:cytochrome c 2 oxidoreductase?

The temperature dependence of the partial reactions leading to turn-over of the UQH₂:cyt c ₂ oxidoreductase of Rhodobacter sphaeroides have been studied. The redox properties of the cytochrome components show a weak temperature dependence over the range 280–330 K, with coefficients of about 1 m V per degree; our results suggest that the other components show similar dependencies, so that no significant change in the gradient of standard free-energy between components occurs over this temperature range. The rates of the reactions of the high potential chain (the Rieske iron sulfur center, cytochromes c ₁ and c ₂, reaction center primary donor) show a weak temperature dependence, indicating an activation energy < 8 kJ per mole for electron transfer in this chain. The oxidation of ubiquinol at the Q_z-site of the complex showed a strong temperature dependence, with an activation energy of about 32 kJ mole^–1. The electron transfer from cytochrome b-566 to cytochrome b-561 was not rate determining at any temperature, and did not contribute to the energy barrier. The activation energy of 32 kJ mole^–1 for quinol oxidation was the same for all states of the quinone pool (fully oxidized, partially reduced, or fully reduced before the flash). We suggest that the activation barrier is in the reaction by which ubiquinol at the catalytic site is oxidized to semiquinone. The most economical scheme for this reaction would have the semiquinone intermediate at the energy level indicated by the activation barrier. We discuss the plausibility of this simple model, and the values for rate constants, stability constant, the redox potentials of the intermediate couples, and the binding constant for the semiquinone, which are pertinent to the mechanism of the ubiquinol oxidizing site.Abbreviations (BChl)₂ P870, primary donor of the photochemical reaction center - b/c ₁ complex ubiquinol: cytochrome c ₂ oxidoreductase - cyt b _H cytochrome b-561 or higher potential cytochrome b - cyt b _L cytochrome b-566, or low potential cytochrome b - cyt c ₁, cyt c ₂, cyt c _t cytochromes c ₁ and c ₂, and total cytochrome c (cyt c ₁ and cyt c ₂) - Fe.S Rieske-type iron sulfur center, Q - QH₂ ubiquinone, ubiquinol - Q_z, Q_zH₂, Q_z ^– ubiquinone, ubiquinol, and semiquinone anion of ubiquinone, bound at quinol oxidizing site - Q_z-site ubiquinol oxidizing site (also called Q_o-(outside) - Q_o (Oxidizing) - Q_P (Positive proton potential) site) - Q_c-site uubiquinone reductase site (also called the Q_i-(inside) - Q_R (Reducing), or - Q_N (Negative proton potential) site) - UHDBT 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazol     

Temperature dependence and ontogenetic changes of metabolic rate of an endemic earthworm <Emphasis Type="Italic">Dendrobaena mrazeki</Emphasis>

Ontogenetic changes and temperature dependency of respiration rate were studied in Dendrobaena mrazeki, an earthworm species inhabiting relatively warm and dry habitats in Central Europe. D. mrazeki showed respiration rate lower than in other earthworm species, < 70 μl O₂ g⁻¹ h⁻¹, within the temperature range of 5–35°C. The difference of respiration rate between juveniles and adults was insignificant at 20°C. The response of oxygen consumption to sudden temperature changes was compared with the temperature dependence of respiratory activity in animals pre-acclimated to temperature of measurement. No significant impact of acclimation on the temperature response of oxygen consumption was found. The body mass-adjusted respiration rate increased slowly with increasing temperature from 5 to 25°C (Q₁₀ from 1.2 to 1.7) independently on acclimation history of earthworms. Oxygen consumption decreased above 25°C up to upper lethal limit (about 35°C). Temperature dependence of metabolic rate is smaller than in other earthworm species. The relationships between low metabolic sensitivity to temperature, slow locomotion and reactivity to touching as observed in this species are discussed.     

Diversity of juvenile fish assemblages in the pelagic waters of Lebanon (eastern Mediterranean)

Oxygen consumption rates of nauplii of the brine shrimp Artemia franciscana Kellogg 1906 were determined over a range of salinities from 10 to 110 ppm, in temperatures from 0 to 30°C, using a multi-factorial design. The oxygen micro-sensors employed have a fast response time and are capable of accurately measuring oxygen concentrations at temperatures well below 0°C. Oxygen uptake rate ranged from 0.03 to 0.66 μmol O₂ mg⁻¹ h⁻¹ and was sensitive to changes in both salinity and temperature. Temperature was the dominant factor affecting oxygen consumption rates, which showed a significant increase with increasing temperature. A slight decrease was measured in oxygen consumption with increasing salinity related to differential solubility of oxygen in waters of different salinities. Thermal sensitivity of oxygen consumption determined from calculations of Q ₁₀, indicated physiological adaptation of Artemia nauplii to the ranges of temperatures tested. Handling editor: A. van Kerchove