Haemoglobin function and respiratory status of the Port Jackson shark,<Emphasis Type="Italic"> Heterodontus portusjacksoni</Emphasis>, in response to lowered salinity

Haemoglobin function and respiratory status of sub-adult sharks, Heterodontus portusjacksoni was investigated for up to 1 week following transfer from 100% to either 75% or 50% seawater. Metabolic rates were unusually low and arterial–venous differences in blood O₂ small. Haemodilution from osmotic inflow lowered haematocrit and reduced blood O₂ content by up to 50%. There was no change in O₂ consumption rate, blood O₂ partial pressure, cardiac output, or the arterial-venous O₂ content difference, and thus O₂ delivery was maintained. Ventilation was acutely elevated but returned to normal within 24 h. The O₂ delivery to the tissues was facilitated by decreased blood O₂-affinity that could not be simply ascribed to changes in the osmolyte concentration. The Hb was unaffected by changes in intra-erythrocyte fluid urea or trimethylamine-N-oxide (TMAO) but was sensitive to changes in NaCl. The Bohr shifts in whole blood were low and there was little role for pH in modulating O₂ transport. Venous Hb saturation remained close to 65%, at the steepest part of the in vivo O₂ equilibrium curve, such that O₂ unloading could be facilitated by small reductions in pressure without increasing cardiac or ventilatory work. H. portusjacksoni tolerated 50% seawater for at least 1 month, but there was little evidence of respiratory responses being adaptive which instead appeared to be consequential on changes in osmotic and ionic status.Abbreviations a–v arterial–venous - CO ₂ CO₂ content - C _a O ₂ content of O₂ in arterial blood - C _v O ₂ content of O₂ in venous blood - %E branchial O₂ extraction efficiency - f _v ventilatory frequency - GTP guanosine triphosphate - Hct haematocrit - [Hb] haemoglobin concentration - ITP inosine triphosphate - met[Hb] methaemoglobin - oxygen consumption - NTP nucleoside triphosphate - OEC oxygen equilibrium curve - P _a O ₂ partial pressure of O₂ in arterial blood - P _e O ₂ partial pressure of expired O₂ - P _i O ₂ partial pressure of inspired O₂ - P _in O ₂ inflow partial pressure of O₂ - PO ₂ partial pressure of O₂ - P _out O ₂ outflow partial pressure of O₂ - pH _a arterial blood pH - pH _pl whole blood pH - PV plasma volume - P _v CO ₂ partial pressure of CO₂in venous blood - P _v O ₂ partial pressure of O₂in venous blood - cardiac output - SW seawater - TMAO trimethylamine-N-oxide - ventilation volume Communicated by G. Heldmaier     

On-line estimation of O<Subscript>2</Subscript> production,CO<Subscript>2</Subscript> uptake,and growth kinetics of microalgal cultures in a gas-tight photobioreactor

Growth of the green algae Chlamydomonas reinhardtii and Chlorella sp. in batch cultures was investigated in a novel gas-tight photobioreactor, in which CO₂, H₂, and N₂ were titrated into the gas phase to control medium pH, dissolved oxygen partial pressure, and headspace pressure, respectively. The exit gas from the reactor was circulated through a loop of tubing and re-introduced into the culture. CO₂ uptake was estimated from the addition of CO₂ as acidic titrant and O₂ evolution was estimated from titration by H₂, which was used to reduce O₂ over a Pd catalyst. The photosynthetic quotient, PQ, was estimated as the ratio between O₂ evolution and CO₂ up-take rates. NH₄ ⁺, NO₂ ⁻, or NO₃ ⁻ was the final cell density limiting nutrient. Cultures of both algae were, in general, characterised by a nitrogen sufficient growth phase followed by a nitrogen depleted phase in which starch was the major product. The estimated PQ values were dependent on the level of oxidation of the nitrogen source. The PQ was 1 with NH₄ ⁺ as the nitrogen source and 1.3 when NO₃ ⁻ was the nitrogen source. In cultures grown on all nitrogen sources, the PQ value approached 1 when the nitrogen source was depleted and starch synthesis became dominant, to further increase towards 1.3 over a period of 3–4 days. This latter increase in PQ, which was indicative of production of reduced compounds like lipids, correlated with a simultaneous increase in the degree of reduction of the biomass. When using the titrations of CO₂ and H₂ into the reactor headspace to estimate the up-take of CO₂, the production of O₂, and the PQ, the rate of biomass production could be followed, the stoichiometrical composition of the produced algal biomass could be estimated, and different growth phases could be identified.     

Variations of vinblastine accumulation and redox state affected by exogenous H<Subscript>2</Subscript>O<Subscript>2</Subscript> in <Emphasis Type="Italic">Catharanthus roseus</Emphasis> (L.) G. Don

Effects of exogenous H₂O₂ application on vinblastine (VBL) and its precursors, vindoline (VIN), catharanthine (CAT) and α-3′,4′-anhydrovinblastine (AVBL), were measured in Catharanthus roseus seedlings in order to explore possible correlation of VBL formation with oxidative stress. VBL accumulation has previously been shown to be regulated by an in vitro H₂O₂-dependent peroxidase (POD)-like synthase. Experimental exposure of plants to different concentrations of H₂O₂ showed that endogenous H₂O₂ and alkaloid concentrations in leaves were positively elevated. The time-course variations of alkaloid concentrations and redox state, reflected by the concentrations of H₂O₂, ascorbic acid (AA), oxidative product of glutathione (GSSG) and POD activity, were significantly altered due to H₂O₂ application. The further correlation analysis between alkaloids and redox status indicated that VBL production was tightly correlated with redox status. These results provide a new link between VBL metabolisms and redox state in C. roseus.     

Characterization and application of an optical sensor for quantification of dissolved O2 in shake-flasks

On-line measurement of dissolved O₂ in shake-flasks was realized via immobilized sensor spots containing a fluorophore with an O₂-dependent luminescent decay time. An unaffected sensor signal during 80 autoclaving cycles suggests multi-usage of sensor equipped shake-flasks. The sensor had a response time of 6 s. Quantification of gas-liquid mass transfer revealed maximum k_La values of 150 h^–1, from which maximum O₂ transfer capacity of 33 mM h^–1 was calculated. Liquid volume and shaking frequency have a strong influence on k_La. Exemplified by cultivations of Corynebacterium glutamicum the importance of shaking rate for O₂ supply of bacterial cultures is shown. Sampling of microbial cultures with intermittent shaking of a few minutes can cause O₂ limitation. Based on the results of this work a simple and straightforward tool is now available for accurate O₂ sensing in shake-flasks, which are widely used in microbial cultivations.     

纳米铁氧化物对Pelotomaculum schinkii培养系丙酸厌氧降解产甲烷的影响

微生物能利用导电材料进行电子传递,提高种间电子传递效率。铁基纳米导电物质可以加速土壤及厌氧消化系统中微生物间的种间电子传递,促进有机废弃物的产甲烷过程。前期获得了厌氧丙酸富集培养系,互营丙酸氧化菌（Pelotomaculum schinkii）在培养系中占优势,本研究考察了10~4 000 mg/L 纳米铁氧化物对丙酸降解产甲烷过程的作用及微生物的影响。结果表明,低浓度的铁基纳米材料对丙酸降解有一定的促进作用,而高浓度会抑制产甲烷：10~1 000 mg/L纳米Fe₃O₄对产甲烷无明显影响,1 500~4 000 mg/L最大产甲烷速率抑制了26%~80%,延滞期增加了174%~222%;10~200 mg/L纳米Fe₂O₃使最大产甲烷速率提高了21%~29%,1 500~4 000 mg/L最大产甲烷速率抑制了48%~58%,延滞期增加了29%~85%。微生物群落解析结果表明,与对照相比,10~1 000 mg/L纳米Fe₂O₃使P. schinkii相对丰度略有增加,而4 000 mg/L纳米₃O₄/Fe₂O₃使P. schinkii的相对丰度下降了70.7%和55.9%,说明高浓度纳米铁氧化物会抑制P. schinkii的活性,导致丙酸降解及产甲烷速率降低。     

Enhancement of xylitol productivity and yield using a xylitol dehydrogenase gene-disrupted mutant of Candida tropicalis under fully aerobic conditions

The effects of glycerol and the oxygen transfer rate on the xylitol production rate by a xylitol dehydrogenase gene (XYL2)-disrupted mutant of Candida tropicalis were investigated. The mutant produced xylitol near the almost yield of 100% from d-xylose using glycerol as a co-substrate for cell growth and NADPH regeneration: 50 g d-xylose l⁻¹ was completely converted into xylitol when at least 20 g glycerol l⁻¹ was used as a co-substrate. The xylitol production rate increased with the O₂ transfer rate until saturation and it was not necessary to control the dissolved O₂ tension precisely. Under the optimum conditions, the volumetric productivity and xylitol yield were 3.2 g l⁻¹ h⁻¹ and 97% (w/w), respectively.     

Mass-spectrometric determination of O2 and CO2 gas exchange in illuminated higher-plant cells

In order to estimate photosynthetic and respiratory rates in illuminated photoautotrophic cells of carnation (Dianthus caryophyllus L.), simultaneous measurements of CO₂ and O₂ gas exchange were performed using ¹⁸O₂, ¹³CO₂ and a mass-spectrometry technique. This method allowed the determination, and thus the comparison, of unidirectional fluxes of O₂ and CO₂. In optimum photosynthetic conditions (i.e. in the presence of high light and a saturating level of CO₂), the rate of CO₂ influx represented 75±5% of the rate of gross O₂ evolution. After a dark-to-light transition, the rate of CO₂ efflux was inhibited by 50% whereas the O₂-uptake rate was little affected. The effect of a recycling of respiratory CO₂ through photosynthesis on the exchange of CO₂ gas was investigated using a mathematical model. The confliction of the experimental data with the simulated gas-exchange rates strongly supported the view that CO₂ recycling was a minor event in these cells and could not be responsible for the observed inhibition of CO₂ efflux. On the basis of this assumption it was concluded that illumination of carnation cells resulted in a decrease of substrate decarboxylations, and that CO₂ efflux and O₂ uptake were not as tightly coupled in the light as in the dark. Furthermore, it could be calculated from the rate of gross photosynthesis that the chloroplastic electron-transport chain produced enough ATP in the light to account for the measured CO₂-uptake rate without involving cyclic transfer of electrons around PS I or mitochondrial supplementation.Abbreviations Chl chlorophyll - K_d permeability coefficient The authors thank Drs A. Vermeglio and P. Thibault, Dépt. de Biologie, CEN-Cadarache, St. Paul Lez Durance, France, for helpful discussions.     

Interactive effect of elevated temperature and O<Subscript>3</Subscript> on antioxidant capacity and gas exchange in <Emphasis Type="Italic">Betula pendula</Emphasis> saplings

We studied the effects of slightly elevated temperature (T), O₃ concentration (O₃) and their combination (T + O₃) on the antioxidant defense, gas exchange and total leaf area of Betula pendula saplings in field conditions. During the second year of the experiment, T enhanced the total leaf area, net photosynthesis (P _n) and maximum capacity of carboxylation, redox state of ascorbate and total antioxidant capacity in the apoplast. O₃ did not affect the total leaf area, but P _n was slightly and g _s significantly reduced. The saplings responded to elevated O₃ level by closing the stomata and by developing leaves with a lower leaf area per mass, rather than by accumulating ascorbate in the apoplast. The effects of T and O₃ on total leaf area and P _n were counteractive. Elevated O₃ reduced the saplings’ ability to utilize the warmer growth environment by increasing the stomatal limitation for photosynthesis and by reducing the redox state of ascorbate in the apoplast in the combination treatment as compared to T alone.     

Ethoxyzolamide repression of the low photorespiration state in two submersed angiosperms

Net photosynthesis in the submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal was inhibited by 21% O₂, but the degree of inhibition was greater for plants in the high than in the low photorespiratory state. Increasing the CO₂ concentration from 50 through 2,500 l l^-1 decreased the O₂ inhibition of the high-photorespiration plants in a competitive manner, but it had no effect on the O₂ inhibition of plants in the low photorespiratory state. Carbonic-anhydrase activity increased by almost threefold with the induction of the low photorespiratory state. Ethoxyzolamide, an inhibitor of carbonic anhydrase, reduced the net photosynthesis of low-photorespiration Myriophyllum and Hydrilla plants by 40%, but their dark respiration was unaffected. This ethoxyzolamide inhibition of net photosynthesis exhibited a competitive response to CO₂ concentration, resulting in a decrease in the apparent affinity of photosynthesis for CO₂. The net photosynthesis of plants in the high photorespiratory state was inhibited only slightly by ethoxyzolamide, and this inhibition was independent of the CO₂ level. Ethoxyzolamide treatment caused an increase in the O₂ inhibition of net photosynthesis of plants in the low photorespiratory state. Ethoxyzolamide increased the low CO₂ compensation points of low-photorespiration Myriophyllum and Hydrilla, but the values for the high-photorespiration plants were unchanged. In comparison, the CO₂ compensation points of the terrestrial plants Sorghum bicolor (C₄), Moricandia arvensis (C₃-C₄ intermediate) and Nicotiana tabacum (C₃) were unaltered by ethoxyzolamide treatment. These data indicate that the low photorespiratory state in Myriophyllum and Hydrilla is repressed by ethoxyzolamide treatment, thus implicating carbonic anhydrase as a component of the photorespiration-reducing mechanism in these plants. The competitive interaction of CO₂ with ethoxyzolamide provides evidence that the low photorespiratory state in submersed angiosperms is the result of some type or types of CO₂ concentrating mechanism. In Myriophyllum it may be via bicarbonate utilization, but in Hydrilla it probably takes the form of an inducible C₄-type system.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose bisphosphate     

Development of cyanide-resistant respiration in mitochondria from potato tubers treated with ethanol,acetaldehyde, and acetic acid

Treatment of intact potato (Solanum tuberosum L.) tubers with acetaldehyde, ethanol or acetic-acid vapors led to a respiratory upsurge which was further increased when the volatiles were applied in 100% O₂. Mitochondria from tubers held in 100% O₂ (O₂ control) displayed a substrate state, state 3, and state 4 in respiration, whereas in mitochondria from the volatile-treated tubers the respiratory rate of the different states was virtually indistinguishable. This respiratory pattern was companied by the development of a cyanide-resistant respiration since these mitochondria exhibited resistance to CN and sensitivity to CN+salicylhydroxamic acid. Acetaldehyde-treated potatoes showed a time-course development (up to 36 h) of cyanide resistance and concomitant sensitivity to salicylhydroxamic acid, indicating the onset of synthetic processes leading to the observed changes in mitochondrial respiration.Abbreviations V total respiration rate - V_cyt velocity of O₂ uptake attributable to cytochrome oxidase - V_alt velocity of O₂ uptake attributable to the alternate oxidase - RCR respiratory control ratio - SHAM salicylhydroxamic acid Paper of the Journal Series, New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Brunswick, N.J., USA     

Veratrol-<Emphasis Type="Italic">O</Emphasis>-demethylase of <Emphasis Type="Italic">Acetobacterium dehalogenans</Emphasis>: ATP-dependent reduction of the corrinoid protein

The anaerobic veratrol O-demethylase mediates the transfer of the methyl group of the phenyl methyl ether veratrol to tetrahydrofolate. The primary methyl group acceptor is the cobalt of a corrinoid protein, which has to be in the +1 oxidation state to bind the methyl group. Due to the negative redox potential of the cob(II)/cob(I)alamin couple, autoxidation of the cobalt may accidentally occur. In this study, the reduction of the corrinoid to the superreduced [Co^I] state was investigated. The ATP-dependent reduction of the corrinoid protein of the veratrol O-demethylase was shown to be dependent on titanium(III) citrate as electron donor and on an activating enzyme. In the presence of ATP, activating enzyme, and Ti(III), the redox potential versus the standard hydrogen electrode (E _SHE) of the cob(II)alamin/cob(I)alamin couple in the corrinoid protein was determined to be −290 mV (pH 7.5), whereas E _SHE at pH 7.5 was lower than −450 mV in the absence of either activating enzyme or ATP. ADP, AMP, or GTP could not replace ATP in the activation reaction. The ATP analogue adenosine-5′-(β,γ-imido)triphosphate (AMP-PNP, 2–4 mM) completely inhibited the corrinoid reduction in the presence of ATP (2 mM).     

Hydrogen Peroxide Inhibits Photosynthetic Electron Transport in Cells of Cyanobacteria

The effect of H₂O₂ on photosynthetic O₂ evolution and photosynthetic electron transfer in cells of cyanobacteria Anabaena variabilis and Anacystis nidulans was studied. The following experiments were performed: 1) directly testing the effect of exogenous H₂O₂; 2) testing the effect of intracellular H₂O₂ generated with the use of methyl viologen (MV); 3) testing the effect of inhibiting intracellular H₂O₂ decomposition by salicylic acid (SA) and 3-amino-1,2,4-triazole (AT). H₂O₂ inhibited photosynthetic O₂ evolution and light-induced reduction of p-benzoquinone (BQ) + ferricyanide (FeCy) in the Hill reaction. The I₅₀ value for H₂O₂ was 0.75 mM. Photosynthetic electron transfer in the cells treated with H₂O₂ was not maintained by H₂O₂, NH₂OH, 1,5-diphenylcarbazide, tetraphenylboron, or butylated hydroxytoluene added as artificial electron donors for Photosystem (PS) II. The H₂O CO₂, H₂O MV (involving PSII and PSI) and H₂O BQ + FeCy (chiefly dependent on PSII) electron transfer reactions were inhibited upon incubation of the cells with MV, SA, or AT. The N,N,N",N"-tetramethyl-p-phenylenediamine MV (chiefly dependent on PSI) electron transfer was inhibited by SA and AT but was resistant to MV. The results show that H₂O₂ inhibits photosynthetic electron transfer. It is unlikely that H₂O₂ could be a physiological electron donor in oxygenic photosynthesis.     

Respiratory behaviour,oxygen consumption and relative dependence on aerial respiration in the African lungfish (Protopterus annectens,owen) and an air-breathing teleost (Clarias lazera,C.).

The relative dependence on branchial and pulmonary organs was studied in the African lungfish P. annectens and in the catfish Clarias lazera. The frequency of pulmonary ventilation varied, in the normal state, with the activity and age of the fish and followed a circadian rhythm. Small specimens of both species exhibited a higher branchial ventilatory rate than older specimens and depended largely on aquatic O₂ uptake (over 85% and 90% in Clarias and Protopterus respectively). The dependence on aerial respiration appeared to develop gradually with age in Clarias but occurred over a limited age-range (200–300 g) in Protopterus. In mature fish (over 400 g), pulmonary respiration constituted 50–60% of the total in Clarias and 80–85% in Protopterus. Partitioning of O₂ uptake between air and water depended on the O₂ content of the water and that of O₂ and CO₂ in the pulmonary organs. Protopterus and Clarias surfaced for air when the O₂ content of the respiratory organs was reduced to 90% and 85% (of that immediately following an air-breath) respectively. An increase in the pulmonary O₂ content lengthened the apnoeic period and reduced pulmonary respiration more markedly in Protopterus than in Clarias whereas an increase of that of CO₂ produced the reverse effects.     

Role of nitrite reductase in the ammonia-oxidizing pathway of <Emphasis Type="Italic">Nitrosomonas europaea</Emphasis>

Metabolism of ammonia (NH₃) and hydroxylamine (NH₂OH) by wild-type and a nitrite reductase (nirK) deficient mutant of Nitrosomonas europaea was investigated to clarify the role of NirK in the NH₃ oxidation pathway. NirK-deficient N. europaea grew more slowly, consumed less NH₃, had a lower rate of nitrite (NO₂ ⁻) production, and a significantly higher rate of nitrous oxide (N₂O) production than the wild-type when incubated with NH₃ under high O₂ tension. In incubations with NH₃ under low O₂ tension, NirK-deficient N. europaea grew more slowly, but had only modest differences in NH₃ oxidation and product formation rates relative to the wild-type. In contrast, the nirK mutant oxidized NH₂OH to NO₂ ⁻ at consistently slower rates than the wild-type, especially under low O₂ tension, and lost a significant pool of NH₂OH–N to products other than NO₂ ⁻ and N₂O. The rate of N₂O production by the nirK mutant was ca. three times higher than the wild-type during hydrazine-dependent NO₂ ⁻ reduction under both high and low O₂ tension. Together, the results indicate that NirK activity supports growth of N. europaea by supporting the oxidation of NH₃ to NO₂ ⁻ via NH₂OH, and stimulation of hydrazine-dependent NO₂ ⁻ reduction by NirK-deficient N. europaea indicated the presence of an alternative, enzymatic pathway for N₂O production.     

Photosynthetic net O<Subscript>2</Subscript> evolution enhancement as a sign of acclimation to phosphorus deficiency in bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) leaves

Primary leaves of bean (Phaseolus vulgaris L.) seedlings cultivated for 14 days in a growth chamber on complete (control) and phosphate deficient (−P) Knop liquid medium were used for measurements. The −P leaves were smaller and showed an increased specific leaf area (SLA). Their inorganic phosphate (P_i) concentration was considerably lowered. They did not show any significant changes in chlorophyll (Chl) (a + b) concentration and in their net CO₂ assimilation rate when it was estimated under the conditions close to those of the seedlings growth. Light response curves of photosynthetic net O₂ evolution (P _NO₂) of the leaves for the irradiation range up to 500 μmol(photon) m⁻² s⁻¹ were determined, using the leaf-disc Clark oxygen electrode. The measurements were taken under high CO₂ concentration of about 1 % and O₂ concentrations of 21 % or lowered to about 3 % at the beginning of measurement. The results obtained at 21 % O₂ and the irradiations close to or higher than those used during the seedlings growth revealed the phosphorus stress suppressive effect on the leaf net O₂ evolution, however, no such effect was observed at lower irradiations. Other estimated parameters of P _NO₂ such as: apparent quantum requirement (QR_A) and light compensation point (LCP) for the control and −P leaves were similar. However, with a high irradiation and lowered O₂ concentration the rate of P _NO₂ for the −P leaves was markedly higher than that for the control, in relation to both the leaf area and leaf fresh mass. This difference also disappeared at low irradiations, but the estimated reduced QR_A values indicate, under those conditions, the increased yield of photosynthetic light reaction, especially in the −P leaves. The presented results confirm the suggestion that during the initial phase of insufficient phosphate feeding the acclimations in the light phase of photosynthesis, both structural and functional appear. They correspond, probably, to the increased energy costs of carbon assimilation under phosphorus stress, e.g. connected with raised difficulties in phosphate uptake and turnover and enhanced photorespiration. Under the experimental conditions especially advantageous for the dark phase of photosynthesis (saturating CO₂ and PAR, low O₂ concentration), those acclimations may be manifested as an enhancement of photosynthetic net O₂ evolution.     

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     

Nucleotide Content Gradients in Maternally and Paternally Inherited Mitochondrial Genomes of the Mussel <Emphasis Type="Italic">Mytilus</Emphasis>

Several studies have shown that in vertebrate mtDNAs the nucleotide content at fourfold degenerate sites is well correlated with the site’s time of exposure to the single-strand state, as predicted from the asymmetrical model of mtDNA replication. Here we examine whether the same explanation may hold for the regional variation in nucleotide content in the maternal and paternal mtDNAs of the mussel Mytilus galloprovincialis. The origin of replication of the heavy strand (O_H) of these genomes has been previously established. A systematic search of the two genomes for sequences that are likely to act as the origin of replication of the light strand (O_L) suggested that the most probable site lies within the ND3 gene. By adopting this O_L position we calculated times of exposure for 0_FD (nondegenerate), 2_FD (twofold degenerate), and 4_FD (fourfold degenerate) sites of the protein-coding part of the genome and for the rRNA, tRNA and noncoding parts. The presence of thymine and absence of guanine at 4_FD sites was highly correlated with the presumed time of exposure. Such an effect was not found for the 2_FD sites, the rRNA, the tRNA, or the noncoding parts. There was a trend for a small increase in cytosine at 0_FD sites with exposure time, which is explicable as the result of biased usage of 4_FD codons. The same analysis was applied to a recently sequenced mitochondrial genome of Mytilus trossulus and produced similar results. These results are consistent with the asymmetrical model of replication and suggest that guanine oxidation due to single-strand exposure is the main cause of regional variation of nucleotide content in Mytilus mitochondrial genomes. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. [Reviewing Editor: Dr. David Pollock]     

Involvement of reactive oxygen species during early stages of ectomycorrhiza establishment between <Emphasis Type="Italic">Castanea sativa</Emphasis> and <Emphasis Type="Italic">Pisolithus tinctorius</Emphasis>

Evidence for the participation of reactive oxygen species (ROS) and antioxidant systems in ectomycorrhizal (ECM) establishment is lacking. In this paper, we evaluated ROS production and the activities of superoxide dismutase (SOD) and catalase (CAT) during the early contact of the ECM fungus Pisolithus tinctorius with the roots of Castanea sativa (chestnut tree). Roots were placed in contact with P. tinctorius mycelia, and ROS production was evaluated by determining the levels of H₂O₂ and O₂ ^·− during the early stages of fungal contact. Three peaks of H₂O₂ production were detected, the first two coinciding with O₂ ^·− bursts. The first H₂O₂ production peak coincided with an increase in SOD activity, whereas CAT activity seemed to be implicated in H₂O₂ scavenging. P. tinctorius growth was evaluated in the presence of P. tinctorius-elicited C. sativa crude extracts prepared during the early stages of fungal contact. Differential hyphal growth that matched the H₂O₂ production profile with a delay was detected. The result suggests that during the early stages of ECM establishment, H₂O₂ results from an inhibition of ROS-scavenging enzymes and plays a role in signalling during symbiotic establishment.     

Xylem parenchyma cells deliver the H<Subscript>2</Subscript>O<Subscript>2</Subscript> necessary for lignification in differentiating xylem vessels

Lignification in Zinnia elegans L. stems is characterized by a burst in the production of H₂O₂, the apparent fate of which is to be used by xylem peroxidases for the polymerization of p-hydroxycinnamyl alcohols into lignins. A search for the sites of H₂O₂ production in the differentiating xylem of Z. elegans stems by the simultaneous use of optical (bright field, polarized light and epi-polarization) and electron-microscope tools revealed that H₂O₂ is produced on the outer-face of the plasma membrane of both differentiating (living) thin-walled xylem cells and particular (non-lignifying) xylem parenchyma cells. From the production sites it diffuses to the differentiating (secondary cell wall-forming) and differentiated lignifying xylem vessels. H₂O₂ diffusion occurs mainly through the continuous cell wall space. Both the experimental data and the theoretical calculations suggest that H₂O₂ diffusion from the sites of production might not limit the rate of xylem cell wall lignification. It can be concluded that H₂O₂ is produced at the plasma membrane in differentiating (living) thin-walled xylem cells and xylem parenchyma cells associated to xylem vessels, and that it diffuses to adjacent secondary lignifying xylem vessels. The results strongly indicate that non-lignifying xylem parenchyma cells are the source of the H₂O₂ necessary for the polymerization of cinnamyl alcohols in the secondary cell wall of lignifying xylem vessels.     

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in batch cultures

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in batch cultures was examined, using on-line off-gas analyses to measure the oxygen and carbon dioxide consumption rates continuously. A cell suspension from continuous cultures at steady state was used as the inoculum. It was observed that a dynamic phase occurred in the initial phase of the experiment. In this phase the bacterial ferrous iron oxidation and growth were uncoupled. After about 16 h the bacteria were adapted and achieved a pseudo-steady state, in which the specific growth rate and oxygen consumption rate were coupled and their relationship was described by the Pirt equation. In pseudo-steady state, the growth and oxidation kinetics were accurately described by the rate equation for competitive product inhibition. Bacterial substrate consumption is regarded as the primary process, which is described by the equation for competitive product inhibition. Subsequently the kinetic equation for the specific growth rate, μ, is derived by applying the Pirt equation for bacterial substrate consumption and growth. The maximum specific growth rate, μ _max, measured in the batch culture agrees with the dilution rate at which washout occurs in continuous cultures. The maximum oxygen consumption rate, q _O2,max, of the cell suspension in the batch culture was determined by respiration measurements in a biological oxygen monitor at excess ferrous iron, and showed changes of up to 20% during the course of the experiment. The kinetic constants determined in the batch culture slightly differ from those in continuous cultures, such that, at equal ferric to ferrous iron concentration ratios, biomass-specific rates are up to 1.3 times higher in continuous cultures. Received: 8 February 1999 / Accepted: 17 February 1999