Oxygen transport and cardiovascular responses in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) exposed to acute hypoxia

Summary Responses to acute hypoxia were measured in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) (1–3 kg body weight). Fish were prevented from making swimming movements by a spinal injection of lidocaine and were placed in front of a seawater delivery pipe to provide ram ventilation of the gills. Fish could set their own ventilation volumes by adjusting mouth gape. Heart rate, dorsal and ventral aortic blood pressures, and cardiac output were continuously monitored during normoxia (inhalant water (PO ₂>150 mmHg) and three levels of hypoxia (inhalant water PO ₂130, 90, and 50 mmHg). Water and blood samples were taken for oxygen measurements in fluids afferent and efferent to the gills. From these data, various measures of the effectiveness of oxygen transfer, and branchial and systemic vascular resistance were calculated. Despite high ventilation volumes (4–71·min^-1·kg^-1), tunas extract approximately 50% of the oxygen from the inhalant water, in part because high cardiac outputs (115–132 ml·min^-1·kg^-1) result in ventilation/perfusion conductance ratios (0.75–1.1) close to the theoretically ideal value of 1.0. Therefore, tunas have oxygen transfer factors (ml O₂·min^-1·mmHg^-1·kg^-1) that are 10–50 times greater than those of other fishes. The efficiency of oxygen transfer from water in tunas (65%) matches that measured in teleosts with ventilation volumes and order of magnitude lower. The high oxygen transfer factors of tunas are made possible, in part, by a large gill surface area; however, this appears to carry a considerable osmoregulatory cost as the metabolic rate of gills may account for up 70% of the total metabolism in spinally blocked (i.e., non-swimming) fish. During hypoxia, skipjack and yellowfin tunas show a decrease in heart rate and increase in ventilation volume, as do other teleosts. However, in tunas hypoxic bradycardia is not accompanied by equivalent increases, in stroke volume, and cardiac output falls as HR decreases. In both tuna species, oxygen consumption eventually must be maintained by drawing on substantial venous oxygen reserves. This occurs at a higher inhalant water PO₂ (between 130 and 90 mmHg) in skipjack tuna than in yellowfin tuna (between 90 and 50 mmHg). The need to draw on venous oxygen reserves would make it difficult to meet the oxygen demand of increasing swimming speed, which is a common response to hypoxia in both species. Because yellowfin tuna can maintain oxygen consumption at a seawater oxygen tension of 90 mmHg without drawing on venous oxygen reserves, they could probably survive for extended periods at this level of hypoxia.Abbreviations BP_da, BP_va dorsal, ventral aortic blood pressure - C _aO₂, C _vO₂ oxygen content of arterial, venous blood - DO₂ diffusion capacity - E_b, E_w effectiveness of O₂ uptake by blood, and from water, respectively - Hct hematocrit - HR heart rate - PCO₂ carbon dioxide tension - P _aCO₂, P _vCO₂ carbon dioxide tension of arterial and venous blood, respectively - PO₂ oxygen tension - P _aO₂, P _vO₂, P _iO₂, P _cO₂ oxygen tension of arterial blood, venous blood, and inspired and expired water, respectively - pHa, pHv pH of arterial and venous blood, respectively - P_w—b effective water to blood oxygen partial pressure difference - Pg partial pressure (tension) gradient - cardiac output - R vascular resistance - SV stroke volume - SEM standard error of mean - TO₂ transfer factor - U utilization - _g ventilation volume - O₂ oxygen consumption     

The effect of He−O2 exposure on metabolic rate,thermoregulation and thermal conductance during normothermia and daily torpor

Recently it was proposed that the low metabolic rate during torpor may be better explained by the reduction of thermal conductance than the drop of body temperature or metabolic inhibition. We tested this hypothesis by simultaneously measuring body temperature and metabolic rate as a function of ambient temperature in both torpid and normothermic stripe-faced dunnarts, Sminthopsis macroura (Marsupialia; approx. 25 g body mass), exposed to either air or He–O₂ (21% oxygen in helium) atmospheres. He–O₂ exposure increases the thermal conductance of homeothermic mammals by about twofold in comparison to an air atmosphere without apparent side-effects. Normothermic S. macroura exposed to He–O₂ increased resting metabolic rate by about twofold in comparison to that in air because of the twofold increase in apparent thermal conductance. Torpid S. macroura exposed to He–O₂ at ambient temperatures above the set-point for body temperature showed a completely different metabolic response. In contrast to normothermic individuals, torpid individuals significantly decreased or maintained a similar metabolic rate as those in air although the apparent thermal conductance in He–O₂ was slightly raised. Moreover, the metabolic rate during torpor was only a fraction of that of normothermic individuals although the apparent thermal conductance differed only marginally between normothermia and torpor. Our study shows that a low thermal conductance is not the reason for the low metabolic rates during torpor. It suggests that interrelations between metabolic rate and body temperature of torpid endotherms above the set-point for body temperature differ fundamentally from those of normothermic and homeothermic endotherms.Abbreviations T _a ambient temperature - T _b body temperature - BMR basal metabolic rate - C apparent thermal conductance - He–O ₂ 21% oxygen in helium - MR metabolic rate - MSe mean square-error - RMR festing metabolic rate - TMR metabolic rate during torpor - T difference T _b-T _a - TNZ thermoneutral zone - T _set set-point for body temperature - O ₂ rate of oxygen consumption     

Autonomic nervous control of blood pressure and heart rate during hypoxia in the cod,Gadus morhua

Summary The autonomic nervous and possible adrenergic humoral control of blood pressure and heart rate during hypoxia was investigated in Atlantic cod. The oxygen tension in the water was reduced to 4.0–5.3 kPa (i.e.. P_wO₂=30–40 mmHg), and the fish responded with an immediate increase in ventral and dorsal aortic blood pressure (P _va P _da), as well as a slowly developing bradycardia. The plasma concentrations of circulating catecholamines increased during hypoxia with a peak in the plasma level of noradrenaline occurring before the peak for adrenaline. Bretylium was used as a chemical tool to differentiate between neuronal and humoral adrenergic control of blood pressure and heart rate (f _H) during hypoxia. The increase in P _va and P _da in response to hypoxia was strongly reduced in bretylium-treated cod, which suggests that adrenergic nerves are responsible for hypoxic hypertension. In addition, a small contribution by circulating catecholamines to the adrenergic tonus affecting P _va during hypoxia was suggested by the decrease in P _va induced by injection of the -adrenoceptor antagonist phentolamine. The cholinergic and the adrenergic tonus affecting heart rate were estimated by injections of atropine and the -adrenoceptor antagonist sotalol. The experiments demonstrate an increased cholicholinergic as well as adrenergic tonus on the heart during hypoxia.     

The Effects of Hypoxia on Three Sympatric Shark Species: Physiological and Behavioral Responses

Behavioral and physiological responses to hypoxia were examined in three sympatric species of sharks: bonnethead shark Sphyrna tiburo, blacknose shark, Carcharhinus acronotus, and Florida smoothhound shark, Mustelus norrisi, using closed system respirometry. Sharks were exposed to normoxic and three levels of hypoxic conditions. Under normoxic conditions (5.5–6.4mg l^–1), shark routine swimming speed averaged 25.5 and 31.0cm s^–1 for obligate ram-ventilating S. tiburo and C. acronotus respectively, and 25.0cm s^–1 for buccal-ventilating M. norrisi. Routine oxygen consumption averaged about 234.6 mg O₂kg^–1h^–1 for S. tiburo, 437.2mg O₂kg^–1h^–1 for C. acronotus, and 161.4mg O₂ kg^–1 h^–1 for M. norrisi. For ram-ventilating sharks, mouth gape averaged 1.0cm whereas M. norrisi gillbeats averaged 56.0 beats min^–1. Swimming speeds, mouth gape, and oxygen consumption rate of S. tiburo and C. acronotus increased to a maximum of 37–39cm s^–1, 2.5–3.0cm and 496 and 599mg O₂ kg^–1 h^–1 under hypoxic conditions (2.5–3.4mg l^–1), respectively. M. norrisi decreased swimming speeds to 16cm s^–1 and oxygen consumption rate remained similar. Results support the hypothesis that obligate ram-ventilating sharks respond to hypoxia by increasing swimming speed and mouth gape while buccal-ventilating smoothhound sharks reduce activity.     

Acid-base and electrolyte regulation,and haemolymph gas transport in crayfish,Astacus astacus,exposed to soft,acid water with and without aluminium

Summary Intermoult crayfish (Astacus astacus) were exposed to acid (pH 4), soft water ([Ca⁺⁺]=100 mol·l^–1) in the absence and presence of aluminium (25 mol·l^–1) for variable time periods (up to 21 days) in order to assess the consequences for acid-base and electrolyte balance and haemolymph gas transport. Haemolymph osmolality and concentration of major ions decreased drastically and to a similar extent in acid and acid-aluminium water. Muscle tissue ion concentrations were, however, regulated at an almost constant level. A severe metabolic acidosis was gradually developed, attaining a haemolymph metabolic acid load of 6–7 mequiv·l^–1 after 12–21 days. The acidosis was partially compensated by ventilatory means, with the postbranchial haemolymph PCO₂ decreasing earlier in acidaluminium-exposed than in acid-exposed specimens. Hyperventilation seemed to be a direct acid-base regulatory response, since the rise in postbranchial PCO₂ had only minimal influence on haemolymph O₂ transport. The Bohr effect of Astacus astacus haemocyanin was low (log P₅₀/GdpH=-0.24), and the mean P₅₀ only increased from 15 to 19 mmHg after 21 days of acid exposure. The decrease in O₂ affinity with decreasing pH was accompanied by a decrease in the cooperativity of O₂ binding. The haemolymph haemocyanin concentration was not affected by acid and acid-aluminium exposure, but decreased after 21 days due to starvation. Muscle tissue aluminium concentrations were unaffected, whereas gill tissue concentrations increased in acid-aluminium exposed crayfish, most likely due to accumulation of aluminium on the gill surface. Mortality was low, and an internal hypoxia and lactacidosis was not developed in either of the experimental groups. This suggests that the gas transfer qualities of the chitincovered gills of crayfish are much less sensitive to acid and acid-aluminium stress than the gills of teleost fish.Abbreviations Hc haemocyanin - SO₂ saturation of Hc with O₂ - P ₅₀ oxygen tension of haemolymph at 50% SO₂ - n ₅₀ Hills coelficient around 50% SO₂     

Homeostatic gas-exchange parameters inferred from 13C/12C in tree rings of conifers

The CO₂ concentration of the atmosphere has increased by almost 30% in the past two centuries, with most of the increase (>5 Pa) during the past 60 years. Controlled environment studies of crop plants dependent on the C₃ photosynthetic pathway indicate that an increase of this magnitude would enhance net photosynthesis, reduce stomatal conductance, and increase the difference in CO₂ concentration across the stomata, i.e., CO₂ concentration outside the leaf to that within (c _a-c _i). Here we report evidence, based on stable isotope composition of tree rings from three species of field-grown, native conifer trees, that the trees have indeed responded. However, rather than increasing c _a-c _i, intercellular CO₂ concentrations have shifted upward to match the rise in atmospheric concentrations, holding c _a-c _i constant. No differences were detected among Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), or western white pine (Pinus monticola). The values of c _a-c _i were inferred from stable carbon isotope ratio (¹³C) of tree ring holocellulose adjusted for the 0.6–2.6 difference between holocellulose and whole sapwood. The cellulose extraction removed contaminants deposited in the tree ring after it formed and the adjustment corrected for the enrichment of cellulose relative to whole tissue. The whole sapwood values were then adjusted for bublished estimates of past atmospheric ¹³CO₂ and CO₂ concentrations. To avoid confounding tree age with CO₂, cellulose deposited by saplings in the 1980s was compared to cellulose deposited in the inner rings of nature trees when the mature trees were saplings, between 1910–1929 and 1941–1970; thus saplings were compared to saplings. In a separate analysis, the juvenile effect, which describes the tendency for ¹³C to increase in the first decades of a tree's life, was quantified independent of source CO₂ effects. This study provides evidence that conifers have undergone adjustments in the intercellular CO₂ concentration that have maintained c _a-c _i constant. Based on these results and others, we suggest that c _a-c _i, which has also been referred to as the intrinsic water-use efficiency, should be considered a homeostatic gas-exchange set point for these conifer species.     

Cytochromes in Thiobacillus tepidarius and the respiratory chain involved in the oxidation of thiosulphate and tetrathionate

Thiobacillus tepidarius was shown to contain cytochrome(s) c with absorption maxima at 421, 522 and 552 nm in room temperature reduced minus oxidized difference spectra, present at 1.1–1.2 nmol per mg dry wt and present in both membrane and soluble fractions of the cell. The membrane-bound cytochrome c (1.75 nmol per mg membrane protein) had a midpoint potential (E_m, pH 7.0) of 337 mV, while the soluble fractions appeared to contain cytochrome(s) c with E_m (pH 7.0) values of about 270 and 360 mV. The organism also contained three distinct membrane-bound b-type cytochromes (totalling 0.33 nmol per mg membrane protein), each with absorption maxima in reduced minus oxidized difference spectra at about 428, 532 and 561 nm. The E_m (pH 7.0) values for the three cytochromes b were 8 mV (47.8% of total), 182 mV (13.7%) and 322 mV (38.5%). No a- or d-type cytochromes were detectable spectrophotometrically in the intact organism or its membrane and soluble fractions. Evidence is presented for both CO-binding and CO-unreactive cytochromes b or o, and CO-binding cytochrome(s) c. From redox effects observed with CO it is proposed that a cytochrome c donates electrons to a cytochrome b, and that a high potential cytochrome b or o may be acting as the terminal oxidase in substrate oxidation. This may be the 445 nm pigment, a photodissociable CO-binding membrane haemoprotein. Substrate oxidation was relatively insensitive to CO-inhibition, but strongly inhibited by cyanide and azide. Thiosulphate oxidation couples directly to cytochrome c reduction, but tetrathionate oxidation is linked (probably via ubiquinone Q-8) to reduction of a cytochrome b of lower potential than the cytochrome c. The nature of possible electron transport pathways in Thiobacillus tepidarius is discussed. One speculative sequence is: c^– b₈ b₁₈₂ c₂₇₀ c₃₃₇ b₃₂₂/c₃₆₀ O₂ Abbreviations E_m midpoint electrode potential - E _inf0 ^sup pH 7, standard electrode potential at pH 7.0 - Q-8 coenzyme Q-8 (ubiquinone-40)     

Stomatal conductance in a tropical xerophilous shrubland at a lava substratum

Diurnal variation in leaf stomatal conductance (g _s) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. Averaged stomatal conductances were 147 and 60.2 (B. cordata), 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m^–2 s^–1 (S. praecox) during the wet and dry season respectively. Leaf water potential () varied in a range of –0.6 to –1.2 (S. praecox), –0.6 to –1.8 (B. cordata) and –0.9 to –3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf–air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating g _s to environmental variables and to accounted for 79%–83% of the variation of g _s in three sites (pooled data); however, the performance of the model was poorer (60%–76%) for individual species from other sites not included in the pooled data.     

Adaptive respiratory significance of organophosphates (ATP & GTP) in air-breathing fishes

The ATP and GTP contents of erythrocytes of Protopterus annectens, Polypterus senegalus and Clarias lazera were assessed in relation to maturation and hypoxia. The steady state normal concentrations of ATP were: 446, 316 and 475 µM/100 ml and those of GTP were 286, 93 and 85 µM/100 ml in Protopterus, Polypterus and Clarias, respectively. The level of ATP decreased and that of GTP increased with increments in body growth, especially during development of the pulmonary organs. The concentration of GTP also increased progressively with hypoxia (O₂: 7.4 to 3.6 mg/l); the level ATP remained almost unaltered. The ratio of GTP increased markedly with increased hypoxia particularly in the adaptive respiratory range of aquatic oxygen concentrations (6.5–4.8 mg/l) when the fish were forced to depart from branchial respiration and became reliant on air-breathing.     

Ecophysiological characterization of carnivorous plant roots: Oxygen fluxes,respiration, and water exudation

Various ecophysiological investigations on carnivorous plants in wet soils are presented. Radial oxygen loss from roots of Droseraceae to an anoxic medium was relatively low 0.02 – 0.07 mol(O₂) m^{– 2} s^–1 in the apical zone, while values of about one order of magnitude greater were found in both Sarracenia rubra roots and Genlisea violacea traps. Aerobic respiration rates were in the range of 1.6 – 5.6 mol kg^–1 (f.m.) s^–1 for apical root segments of seven carnivorous plant species and 0.4 – 1.1 mol kg^–1 (f.m.) s^–1 for Genlisea traps. The rate of anaerobic fermentation in roots of two Drosera species was only 5 – 14 % of the aerobic respiration. Neither 0.2 mM NaN₃ nor 0.5 mM KCN influenced respiration rate of roots and traps. In all species, the proportion of cyanide-resistant respiration was high and amounted to 65 – 89 % of the total value. Mean rates of water exudation from excised roots of 12 species ranged between 0.4 – 336 mm 3 kg^–1 (f.m.) s^–1 with the highest values being found in the Droseraceae. Exudation from roots was insensitive to respiration inhibitors. No significant difference was found between exudation rates from roots growing in situ in anoxic soil and those kept in an aerated aquatic medium. Carnivorous plant roots appear to be physiologically very active and well adapted to endure permanent soil anoxia.     

Dependence of oxygen uptake on ambient PO 2 in isolated perfused frog skin

Summary Rates of O₂ uptake across isolated perfused skin of bullfrogs (Rana catesbeiana) were measured in relation to blood flow at three levels of ambient O₂ tension: normoxia (O₂ tension=152 torr), hypoxia (12% O₂, 87 torr) and hyperoxia (42% O₂, 306 torr). At bulk perfusion rates ranging from 3.4 to 10.1 l·cm^-2·min^-1, O₂ uptake was positively correlated with hemoglobin delivery rate in both normoxia and hyperoxia, but was independent of delivery rate in hypoxia. Mean O₂ uptake in normoxia was 3.8 nmol O₂·cm^-2·min^-1 at a delivery rate of 9.8 nmol·cm^-2·min^-1 and 6.5 nmol O₂·cm^-2·min^-1 at a delivery rate of 28.3 nmol·cm^-2·min^-1. At any given bulk perfusion rate, oxygen uptake averaged about 49% lower in hypoxia than in normoxia, decreasing in proportion to the reduction of O₂ tension difference between medium and blood. In hyperoxia, O₂ uptake did not increase proportionally with the difference in O₂ tension between blood and medium, averaging only 50% higher at a 2.4-fold greater O₂ tension difference. Cutaneous diffusing capacity for O₂ averaged 0.041 nmol O₂·cm^-2·torr^-1·min^-1 during the first hour of perfusion in normoxia, and was not affected by reduction of ambient O₂ tension. The results indicate that cutaneous O₂ uptake in hypoxia is highly diffusion limited, and consequently, increases in cutaneous perfusion can not effectively compensate for reduction of ambient O₂ tension. In hyperoxia, O₂ uptake may be substantially perfusion limited because of reduced blood O₂ capacitance at high O₂ saturations.Abbreviations O₂ capacitance - C _Hb hemoglobin concentration - D diffusing capacity - PO₂ medium-blood PO₂ difference - Hb flow, hemoglobin delivery rate - Hepes N-[2-Hydroxyethyl]piperacine-N-[2 ethanesulfonic acid] - L _diff extent of diffusion limitation - MO₂ oxygen uptake rate - PO₂ oxygen tension - S O₂ saturation     

Effect of changes in water content on photosynthesis,transpiration and discrimination against 13CO2 and C18O16O in Pleurozium and Sphagnum

Photosynthetic gas exchange characteristics of two common boreal forest mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry out from full hydration. Similar patterns of change in CO₂ assimilation with variation in water content occurred for both species. The maximum rates of CO₂ assimilation for Sphagnum (approx. 7 mol m^–2 s^–1) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 mol m^–2 s^–1) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO₂ assimilation declined. In both species total conductance to water vapour (expressed as a percentage of the maximum rates) remained nearly constant at a water content above 9 (fresh weight/dry weight), but below this level declined in a strong linear manner. Short-term, on-line ¹³CO₂ and C¹⁸O¹⁶O discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO₂ to ambient CO₂ partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of ¹³CO₂ discrimination (approx. 15). Discrimination against ¹³CO₂ increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner C¹⁸C¹⁶O discrimination increased from approximately 30 at full hydration in both species to a maximum of 150 in Sphagnum and 90 in Pleurozium, at low water content. The observed changes in C¹⁸O¹⁶O were strongly correlated to predictions of a mechanistic model of discrimination processes. Field measurements of moss water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low water content.     

Respiratory stimulus in Rhizobium sp. by legume lectins

High molecular weight lectins (> 100 kDa) from seeds of the legumes Canavalia brasiliensis (CnBr), Cratylia floribunda (CFL), Phaseolus vulgaris (PHA) and Vatairea macrocarpa (VML), temporarily stimulate the respiration of Rhizobium tropici-CIAT899 and R. etli-CFN42. These stimulants were significant (P < 0.05) in bacterial suspensions (> 2.85 mg dry biomass ml^–1), having at least 6200 molecules of lectins per bacteria. The VML (20 g ml^–1), induced specific O₂ demand of 2.3–2.5 M O₂ min^–1 mg dry biomass^–1, in CFN42 and CIAT899, respectively. However, CnBr, CFL and PHA induced smaller demands of O₂ (5×), in both strains. The order of affinities of the lectins was approximately VML > PHA > CFL > CnBr, with regard to respiratory stimuli in CIAT899 strain. The co-administration of 10 g VML ml^–1 and 9.8 M galactose, in CIAT899 suspensions, reduced the respiratory stimuli significantly in relation to the treatment with VML alone. These respiratory stimuli, induced by the lectins, increase the significance of the interaction lectin × Rhizobium in terms of bacterial physiology. Its understanding could be important in relation to bacterial symbiotic behaviour.     

The Cytochrome cbo from the Obligate Methylotroph Methylobacillus flagellatus KT Is a Cytochrome c Oxidase

The cbo-type oxidase of Methylobacillus flagellatus KT was purified to homogeneity by preparative native gel electrophoresis, and the kinetic properties and substrate specificity of the enzyme were studied. Ascorbate and ascorbate/N,N,N,N-tetramethyl-p-phenylenediamine (TMPD) were oxidized by cytochrome cbo with a pH optimum of 8.3. With TMPD as an electron donor for the cbo-type oxidase, the optimal pH (7.0 to 7.6) was determined from the difference between respiration rates in the presence of ascorbate/TMPD and only ascorbate. The kinetic constants determined at pH 7.0 were as follows: oxidation by the enzyme of reduced TMPD was characterized by K _M = 0.86 mM and V _max = 1.1 mol O₂/(min mg protein), and oxidation of reduced horse heart cytochrome c was characterized by K _M = 0.09 mM and V _max = 0.9 mol O₂/(min mg protein). Cyanide inhibited ascorbate/TMPD–oxidase activity (K _i = 4.5–5.0 M). The soluble cytochrome c _H (12 kDa), partially purified from M. flagellatus KT, was found to serve as a natural electron donor for the cbo-type oxidase.     

Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C₃, C₃-C₄, C₄-like, and C₄): the O₂ inhibition of photosynthesis measured above the O₂ partial pressure which gives a maximum rate, and O₂- and light-dependent whole-chain electron flow measured at the CO₂ compensation point (). The optimum level of O₂ for maximum photosynthetic rates under atmospheric levels of CO₂ (34 Pa) was lower in C₃ and C₃-C₄ species (ca. 2 kPa) than in C₄-like and C₄ species (ca. 9 kPa). Increasing O₂ partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO₂. This inhibition was calculated as the O₂ inhibition index (_A, the percentage inhibition of photosynthesis per kPa increase in O₂). From measurements of 18 Flaveria species at atmospheric CO₂, the _A values decreased from C₃ (1.9–2.1) to C₃-C₄ (1.2–1.6), C₄-like (0.6–0.8) and C₄ species (0.3–0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at under atmospheric levels of O₂, and increasing O₂ partial pressure at 300 mol quanta·m^–2·s^–1, there was a similar increase in the rate of O₂ evolution associated with whole-chain electron flow (J_{o 2}, calculated from chlorophyll fluorescence analysis) in the C₃ and C₃-C₄ species compared to a much lower rate in the C₄-like and C₄ species. The results indicate that there is substantial O₂-dependent electron flow in C₃ and C₃-C₄ species, reflecting a high level of photorespiration compared to that in C₄-like and C₄ species. Consistent with these results, there was a significant decrease in from C₃ (6–6.2 Pa) to C₃-C₄ (1.0–3.0 Pa), to C₄-like and C₄ species (0.3–0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C₃ and C₃-C₄ species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower values, primarily by refixing photorespired CO₂. The C₄-like and C₄ Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO₂ pump via the C₄ pathway.Abbreviations and Symbols A CO₂ assimilation rate - CE carboxylation efficiency - C_i intercellular CO₂ partial pressure - I_a absorbed PPFD - J_{o 2} oxygen evolution from PSII - PPFD photosynthetic photon flux density (mol · m^–2· s^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - VPD water-vapor pressure difference between the leaf and atmospheric air - CO₂ compensation point - _{CO 2} quantum yield of CO₂ assimilation - _PSII quantum yield of photosystem II - _A O₂ inhibition index for photosynthesis (percentage inhibition of photosynthesis per kPa increase in O₂) This research was supported by the National Science Foundation Grant IBN 9317756 and Equipment (Grant DMB-8515521 and DOE/USDA/NSF Triagency Plnat Biochemistry Research Training Grant Program.     

F420H2 oxidase (FprA) from Methanobrevibacter arboriphilus, a coenzyme F420-dependent enzyme involved in O2 detoxification

Cell suspensions of Methanobrevibacter arboriphilus catalyzed the reduction of O₂ with H₂ at a maximal specific rate of 0.4 U (mol/min) per mg protein with an apparent K _m for O₂ of 30 M. The reaction was not inhibited by cyanide. The oxidase activity was traced back to a coenzyme F₄₂₀-dependent enzyme that was purified to apparent homogeneity and that catalyzed the oxidation of 2 F₄₂₀H₂ with 1 O₂ to 2 F₄₂₀ and 2 H₂O. The apparent K _m for F₄₂₀ was 30 M and that for O₂ was 2 M with a V _max of 240 U/mg at 37°C and pH 7.6, the pH optimum of the oxidase. The enzyme did not use NADH or NADPH as electron donor or H₂O₂ as electron acceptor and was not inhibited by cyanide. The 45-kDa protein, whose gene was cloned and sequenced, contained 1 FMN per mol and harbored a binuclear iron center as indicated by the sequence motif H–X–E–X–D–X₆₂–H–X₁₈–D–X₆₀–H. Sequence comparisons revealed that the F₄₂₀H₂ oxidase from M. arboriphilus is phylogenetically closely related to FprA from Methanothermobacter marburgensis (71% sequence identity), a 45-kDa flavoprotein of hitherto unknown function, and to A-type flavoproteins from bacteria (30–40%), which all have dioxygen reductase activity. With heterologously produced FprA from M. marburgensis it is shown that this protein is also a highly efficient F₄₂₀H₂ oxidase and that it contains 1 FMN and 2 iron atoms. The presence of F₄₂₀H₂ oxidase in methanogenic archaea may explain why some methanogens, e.g., the Methanobrevibacter species in the termite hindgut, cannot only tolerate but thrive under microoxic conditions.Dedicated to Hans Schlegel on the occasion of his 80th birthday.     

Gas exchange and blood gases of Puna teal (Anas versicolor puna) embryos in the Peruvian Andes

Oxygen consumption, air cell gases, hematology, blood gases and pH of Puna teal (Anas versicolor puna) embryos were measured at the altitude at which the eggs were laid (4150 m) in the Peruvian Andes. In contrast to the metabolic depression described by other studies on avian embryos incubated above 3700 m, O₂ consumption of Puna teal embryos was higher than even that of some lowland avian embryos at equivalent body masses. Air cell O₂ tensions dropped from about 80 toor in eggs with small embryos to about 45 toor in eggs containing a 14-g embryo; simultaneously air cell CO₂ tension rose from virtually negligible amounts to around 26 torr. Arterial and venous O₂ tensions (32–38 and 10–12 toor, respectively, in 12- to 14-g embryos) were lower than described previously in similarly-sized lowland wild avian embryos or chicken embryos incubated in shells with restricted gas exchange. The difference between air cell and arterial O₂ tensions dropped significantly during incubation to a minimum of 11 torr, the lowest value recorded in any avian egg. Blood pH (mean 7.49) did not vary significantly during incubation. Hemoglobin concentration and hematocrits rose steadily throughout incubation to 11.5 g · 100 ml^-1 and 39.9%, respectively, in 14-g embryos.Abbreviations PO₂ partial pressure gradient of O₂ - BM body mass - D diffusion coefficient - G gas conductance (cm³·s^-1·torr^-1) - conductance to water vapor - IP internal pipping of embryos - P _ACO₂ partial pressure of carbon dioxide in air cell - P _AO₂ partial pressure of oxygen in air cell - P _aCO₂ partial pressure of carbon dioxide in arterial blood - P _aCO₂ partial pressure of oxygen in arteries - P _H barometric pressure (torr) - PCO₂ partial pressure of carbon dioxide - P _IO₂ partial pressure in ambiant air - PO₂ partial pressure of oxygen - P _VCO₂ venous carbon dioxide partial pressure - P _VO₂ mixed venous oxygen partial pressure - SE standard error - VO ₂ oxygen consumption     

Inorganic-carbon transport in some marine eukaryotic microalgae

Inorganic-carbon transport was investigated in the eukaryotic marine microalgaeStichococcus minor, Nannochloropsis oculata and aMonallantus sp. Photosynthetic O₂ evolution at constant inorganic-carbon concentration but varying pH showed thatS. minor had a greater capacity for CO₂ rather than HCO ₃ ^– utilization but forN. oculata andMonallantus HCO ₃ ^– was the preferred source of inorganic carbon. All three microalgae had a low affinity for CO₂ as shown by the measurement of inorganic-carbon-dependent photosynthetic O₂ evolution at pH 5.0. At pH 8.3, where HCO ₃ ^– is the predominant form of inorganic carbon, the concentration of inorganic carbon required for half-maximal rate of photosynthetic O₂ evolution [K _0.5 (CO₂)] was 53 M forMonallantus sp. and 125 M forN. oculata, values compatible with HCO ₃ ^– transport. Neither extra- nor intracellular carbonic anhydrase was detected in these three microalgal species. It is concluded that these microalgae lack a specific transport system for CO₂ but that HCO ₃ ^– transport occurs inN. oculata andMonallantus, and in the absence of intracellular carbonic anhydrase the conversion of HCO ₃ ^– to CO₂ may be facilitated by the internal pH of the cell.     

CP-61,405, a novel polycyclic pyrrolether antibiotic produced byStreptomyces routienii Huang sp. nov.

Summary CP-61,405 (C₂₆H₂₉N₂O₇Na) is a novel polycyclic pyrrolether antibiotic produced by a new species,Streptomyces routienii Huang sp. nov. (ATCC 39446). Recovery, fractionation and purification were achieved using standard procedures. The crystalline form includes the CP-61,405 sodium salt, m.p. 334–335°C, [] _D ^25°C +315°C (c=1, chloroform). The structure is shown below. CP-61,405 was co-produced with the polycyclic ether antibiotics salinomycin and epi-17-deoxy-(0–8)-salinomycin. It exhibited activity in vitro against gram-positive and anaerobic bacteria, efficacy against poultry coccidia and stimulation in vitro of propionic acid production.     

C3−C4 Intermediate species in the genus Flaveria: leaf anatomy,ultrastructure, and the effect of O2 on the CO2 compensation concentration

Leaf anatomical, ultrastructural, and CO₂-exchange analyses of three closely related species of Flaveria indicate that they are C₃–C₄ intermediate plants. The leaf mesophyll of F. floridana J.R. Johnston, F. linearis Lag., and F. chloraefolia A. Gray is typical of that in dicotyledonous C₃ plants, but the bundle sheath cells contain granal, starch-containing chloroplasts. In F. floridana and F. chloraefolia, the chloroplasts and numerous associated mitochondria are arranged largely centripetally, as in the closely related C₄ species, F. brownii A.M. Powell. In F. linearis, fewer mitochondria are present and the chloroplasts are more evenly distributed throughout the bundle sheath cytosol. There is no correlation between the bundle sheath ultrastructure and CO₂ compensation concentration. () values of these C₃–C₄ intermediate Flaveria species. At 21% O₂ and 25°C, for F. chloraefolia, F. linearis, and F. floridana is 23–26, 14–19, and 8–10 l CO₂ l^-1, respectively. The O₂ dependence of is the greatest for F. chloraefolia and F. linearis (similar to that for C₃–C₄ intermediate Panicum and Moricandia species) and the least for F. floridana, whose O₂ response is identical to that for F. brownii from 1.5 to 21% O₂, but greater at higher pO₂. The variation in leaf anatomy, bundle sheath ultrastructure, and O₂ dependence of among these Flaveria species may indicate an active evolution in the pathway of photosynthetic carbon metabolism within this genus.Abbreviations CO₂ compensation concentration - IRGA infrared gas analysis Published as Paper No. 7068, Journal Series, Nebraska Agricultural Experiment Station