Effects of nitrate on intracellular nitrite and growth of Microcystis aeruginosa

Although nitrate is a macronutrient and can serve as good nitrogen source for many species of phytoplankton, high nitrate concentrations do not benefit the growth of phytoplankton. We hypothesise that algae cultured under high nitrate concentrations can accumulate intracellular nitrite, which is produced by nitrate reductase (NR) and can inhibit the growth of algae. To assess the validity of this hypothesis, Microcystis aeruginosa was grown under different nitrate concentrations from 3.57 to 21.43 mM in low CO₂ and high CO₂ conditions for 15 days. We observed that, with increasing nitrate concentrations, the intracellular nitrite concentrations of the alga increased and the growth rates and photosynthesis declined. When grown under high CO₂ conditions, M. aeruginosa showed lower intracellular nitrite concentrations and higher growth rates and \textP_\textm^\textchla {\text{P}}_{\text{m}}^{{\text{chl}}a} , \textR_\textd^\textchla {\text{R}}_{\text{d}}^{{\text{chl}}a} , α^chla than under low CO₂ conditions. These results suggest that the accumulation of intracellular nitrite could be the cause of inhibition of algal growth under high nitrate concentrations.     

New potent biphalin analogues containing p-fluoro-L-phenylalanine at the 4,4' positions and non-hydrazine linkers

We report the synthesis and the biological evaluation of two new analogues of the potent dimeric opioid peptide biphalin. The performed modification is based on the replacement of two key structural elements of the native biphalin, namely: the hydrazine bridge which joins the two palindromic moieties and the phenylalanine residues at the 4,4′ positions of the backbone. The new analogues 9 and 10 contain 1,2-phenylenediamine and piperazine, respectively, in place of the hydrazidic linker and p-fluoro-l-phenylalanine residues at 4 and 4′ positions. Binding values are: K_\texti^m = 0.51 \textnM K_{\text{i}}^{\mu } = 0.51\,{\text{nM}} and K_\texti^d = 12.8 \textnM K_{\text{i}}^{\delta } = 12.8\,{\text{nM}} for compound 9, K_\texti^m = 0.09 \textnM K_{\text{i}}^{\mu } = 0.09\,{\text{nM}} and K_\texti^d = 0.11 \textnM K_{\text{i}}^{\delta } = 0.11\,{\text{nM}} for analogue 10.     

Kinetics of styrene biodegradation by <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. E-93486

The research into kinetics of styrene biodegradation by bacterial strain Pseudomonas sp. E-93486 coming from VTT Culture Collection (Finland) was presented in this work. Microbial growth tests in the presence of styrene as the sole carbon and energy source were performed both in batch and continuous cultures. Batch experiments were conducted for initial concentration of styrene in the liquid phase changed in the range of 5–90 g m⁻³. The Haldane model was found to be the best to fit the kinetic data, and the estimated constants of the equation were: μ _m = 0.1188 h⁻¹, K _S = 5.984 mg l⁻¹, and K _i = 156.6 mg l⁻¹. The yield coefficient mean value Y_\textxs^\textapp Y_{\text{xs}}^{\text{app}} for the batch culture was 0.72 g_{dry cells weight} (g_substrate)⁻¹. The experiments conducted in a chemostat at various dilution rates (D = 0.035–0.1 h⁻¹) made it possible to determine the value of the coefficient for maintenance metabolism m _d = 0.0165 h⁻¹ and the maximum yield coefficient value Y_\textxs^\textM = 0.913 Y_{\text{xs}}^{\text{M}} = 0.913 . Chemostat experiments confirmed the high value of yield coefficient Y_\textxs^\textapp Y_{\text{xs}}^{\text{app}} observed in the batch culture. The conducted experiments showed high activity of the examined strain in the styrene biodegradation process and a relatively low sensitivity to inhibition of its growth at higher concentrations of styrene in the solution. Such exceptional features of Pseudomonas sp. E-93486 make this bacterial strain the perfect candidate for technical applications.     

Physiological characterization of ‘stay green’ wheat cultivars during the grain filling stage under field growing conditions

Rye (Secale cereale L.) chromosome arm 1RS could delay leaf senescence, and change in H₂O₂ content is a useful index for weighing the ability to delay the senescence. Two wheat cultivars, Chuannong12 (CN12) and Chuannong 18 (CN18), harboring the wheat–rye 1BL/1RS translocated chromosome were investigated for H₂O₂ change and physiological index after flowering under field conditions, and MY11, the agronomical parent of both CN12 and CN18, was used as the control. A combined change in the peak value of CdSe/ZnS quantum dot (QD) fluorescence and morphological observation indicated that the H₂O₂ contents in CN12 and CN18 were generally lower than that in MY11. They both had higher values for net photosynthetic rate (P _n), stomatal conductance (G _s), F_\textv /F_\textm^￠ F_{\text{v}} /F_{\text{m}}^{\prime } F_\textv^￠ /F_\textm^￠ F_{\text{v}}^{\prime } /F_{\text{m}}^{\prime } , and photochemical quenching of PSII (qP) than MY11 only in the late measurement stage. Some small differences were also observed, such as CN12 and CN18 wheat cultivars having higher and longer photosynthetic competence than MY11 during the grain filling stage, which perhaps resulted from a mechanism for removing oxidative species, especially H₂O₂.     

Influence of {\text{NH - }}{{\text{S}}^\gamma } bonding interactions on the structure and dynamics of metallothioneins

Mammalian metallothioneins ( \textM₇^\textIIMTs {\text{M}}_7^{\text{IIMTs}} ) show a clustered arrangement of the metal ions and a nonregular protein structure. The solution structures of Cd₃-thiolate cluster containing β-domain of mouse β-MT-1 and rat β-MT-2 show high structural similarities, but widely differing structure dynamics. Molecular dynamics simulations revealed a substantially increased number of \textNH - \textS^g {\text{NH - }}{{\text{S}}^\gamma } hydrogen bonds in β-MT-2, features likely responsible for the increased stability of the Cd₃-thiolate cluster and the enfolding protein domain. Alterations in the \textNH - \textS^g {\text{NH - }}{{\text{S}}^\gamma } hydrogen-bonding network may provide a rationale for the differences in dynamic properties encountered in the β-domains of MT-1, -2, and -3 isoforms, believed to be essential for their different biological function.     

Physiological optimization underlies growth rate-independent chlorophyll-specific gross and net primary production

Characterization of physiological variability in phytoplankton photosynthetic efficiencies is one of the greatest challenges in assessing ocean net primary production (NPP) from remote sensing of surface chlorophyll (Chl). Nutrient limitation strongly influences phytoplankton intracellular pigmentation, but its impact on Chl-specific NPP (NPP ^*) is debated. We monitored six indices of photosynthetic activity in steady-state Dunaliella tertiolecta cultures over a range of nitrate-limited growth rates (μ), including photosynthetic efficiency of PSII (F _v/F _m), O₂-based gross and net production, 20 min and 24 h carbon assimilation, and carbon- and μ-based NPP. Across all growth rates, O₂-based Chl-specific gross primary production ( GPP_\textO2 ^* GPP_{{{\text{O}}_{2} }}^{*} ), NPP ^*, and F _v/F _m were constant. GPP_\textO2 ^* GPP_{{{\text{O}}_{2} }}^{*} was 3.3 times greater than NPP ^*. In stark contrast, Chl-specific short-term C fixation showed clear linear dependence on μ, reflecting differential allocation of photosynthate between short-lived C products and longer-term storage products. Indeed, ¹⁴C incorporation into carbohydrates was five times greater in cells growing at 1.2 day⁻¹ than 0.12 day⁻¹. These storage products are catabolized for ATP and reductant generation within the period of a cell cycle. The relationship between Chl-specific gross and net O₂ production, short-term ¹⁴C-uptake, NPP ^*, and growth rate reflects cellular-level regulation of fundamental metabolic pathways in response to nutrient limitation. We conclude that growth rate-dependent photosynthate metabolism bridges the gap between gross and net production and resolves a controversial question regarding nutrient limitation effects on primary production measures.     

Parameterization of chlorophyll a-specific absorption coefficients and effects of their variations in a highly eutrophic lake: a case study at Lake Kasumigaura, Japan

The chlorophyll a-specific absorption coefficient ( a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) ) in a highly eutrophic lake can show characteristics distinct from that in the ocean due to the differences in the structure and composition of phytoplankton. In this study, investigated the variation of a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) in Lake Kasumigaura, a highly eutrophic lake in Japan, in association with the package effect and the effect of accessory pigments, and carried out the parameterization of a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) . Although a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) did not vary spatially, it did show significant temporal variation, with a particularly high value after spring-bloom. This high a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) in spring was attributed to a lower package effect and a higher proportion of carotenoid than the other samples. Although the value of a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) was correlated with the concentration of chlorophyll-a (Chl-a), the correlation coefficient was lower than those reported in the ocean. Some lake-water samples showed variations of the package effect and the effect of accessory pigments that were independent of the concentration of Chl-a, and these independent variations resulted in the weak correlation between a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) and the concentration of Chl-a. Together, these results suggest that the factors controlling a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) in highly eutrophic lakes are distinct from that in ocean samples.     

Evaluation of a Two-Site, Three-Barrier Model for Permeation in CaV3.1 (α1G) T-Type Calcium Channels: Ca2+, Ba2+, Mg2+, and Na+

We explored the ability of a two-site, three-barrier (2S3B) Eyring model to describe recently reported data on current flow through open Ca_V3.1 T-type calcium channels, varying Ca²⁺ and Ba²⁺ over a wide range (100 nm–110 mm) while recording whole-cell currents over a wide voltage range (−150 mV to +100 mV) from channels stably expressed in HEK 293 cells. Effects on permeation were isolated using instantaneous current–voltage relationships (IIV) after strong, brief depolarizations to activate channels with minimal inactivation. Most experimental results were reproduced by a 2S3B model. The model described the IIV relationships, apparent affinities for permeation and block for Ca²⁺ and Ba²⁺, and shifts in reversal potential between Ca²⁺ and Ba²⁺. The fit to block by 1 mm  \textMg²⁺_\texti {\text{Mg}}^{2+}_{\text{i}} was reasonable, but block by \textMg²⁺_\texto {\text{Mg}}^{2+}_{\text{o}} was described less well. Surprisingly, fits were comparable with strong ion–ion repulsion, with no repulsion, or with intermediate values. With weak repulsion, there was a single high-affinity site, with a low-affinity site near the cytoplasmic side of the pore. With strong repulsion, the net charge of ions in the pore was near +2 over a relatively wide range of concentration and voltage, suggesting a knockoff mechanism. With strong repulsion, Ba²⁺ preferred the inner site, while Ca²⁺ preferred the outer site, potentially explaining faster entry of Ni²⁺ and other pore blockers when Ba²⁺ is the charge carrier.     

3D-QSAR studies of triazolopyrimidine derivatives of <Emphasis Type="Italic">Plasmodium falciparum</Emphasis> dihydroorotate dehydrogenase inhibitors using a combination of molecular dynamics,docking, and genetic algorithm-based methods

A series of 35 triazolopyrimidine analogues reported as Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) inhibitors were optimized using quantum mechanics methods, and their binding conformations were studied by docking and 3D quantitative structure–activity relationship studies. Genetic algorithm-based criteria was adopted for selection of training and test sets while maintaining structural diversity of training and test sets, which is also very crucial for model development and validation. Both the comparative molecular field analyses ( q_\textLOO² = 0.841 q_{\text{LOO}}^2 = 0.{841} , r_\textncv² = 0.99 r_{\text{ncv}}^2 = 0.{99} ) and comparative molecular similarity indices analyses ( q_\textLOO² = 0.757 q_{\text{LOO}}^2 = 0.{757} , r_\textncv² = 0.943 r_{\text{ncv}}^2 = 0.{943} ) show excellent correlation and high predictive power. Furthermore, molecular dynamics simulations were performed to explore the binding mode of the two of the most active compounds of the series, 10 and 14. Harmonization in the two simulation results validate the analysis and therefore applicability of docking parameters based on crystallographic conformation of compound 14 bound to receptor molecule. This work provides useful information about the inhibition mechanism of this class of molecules and will assist in the design of more potent inhibitors of PfDHODH.     

HNCA-TOCSY-CANH experiments with alternate <Superscript>13</Superscript>C-<Superscript>12</Superscript>C labeling: a set of 3D experiment with unique supra-sequential information for mainchain resonance assignment

Described here is a set of three-dimensional (3D) NMR experiments that rely on CACA-TOCSY magnetization transfer via the weak ³ \textJ_{\textCa\textCa} ^{ 3} {\text{J}}_{{{\text{C}}\alpha {\text{C}}\alpha }} coupling. These pulse sequences, which resemble recently described ¹³C detected CACA-TOCSY (Takeuchi et al. 2010) experiments, are recorded in ¹H₂O, and use ¹H excitation and detection. These experiments require alternate ¹³C-¹²C labeling together with perdeuteration, which allows utilizing the small ³ \textJ_{\textCa\textCa} ^{ 3} {\text{J}}_{{{\text{C}}\alpha {\text{C}}\alpha }} scalar coupling that is otherwise masked by the stronger ¹J_CC couplings in uniformly ¹³C labeled samples. These new experiments provide a unique assignment ladder-mark that yields bidirectional supra-sequential information and can readily straddle proline residues. Unlike the conventional HNCA experiment, which contains only sequential information to the ^{1 3} \textC^a ^{ 1 3} {\text{C}}^{\alpha } of the preceding residue, the 3D hnCA-TOCSY-caNH experiment can yield sequential correlations to alpha carbons in positions i−1, i + 1 and i−2. Furthermore, the 3D hNca-TOCSY-caNH and Hnca-TOCSY-caNH experiments, which share the same magnetization pathway but use a different chemical shift encoding, directly couple the ¹⁵N-¹H spin pair of residue i to adjacent amide protons and nitrogens at positions i−2, i−1, i + 1 and i + 2, respectively. These new experimental features make protein backbone assignments more robust by reducing the degeneracy problem associated with the conventional 3D NMR experiments.     

Analysis of the activation mechanism of <Emphasis Type="Italic">Pseudomonas stutzeri</Emphasis> cytochrome <Emphasis Type="Italic">c</Emphasis> peroxidase through an electron transfer chain

The activation mechanism of Pseudomonas stutzeri cytochrome c peroxidase (CCP) was probed through the mediated electrochemical catalysis by its physiological electron donor, P. stutzeri cytochrome c-551. A comparative study was carried out, by performing assays with the enzyme in the resting oxidized state as well as in the mixed-valence activated form, using cyclic voltammetry and a pyrolytic graphite membrane electrode. In the presence of both the enzyme and hydrogen peroxide, the peak-like signal of cytochrome c-551 is converted into a sigmoidal wave form characteristic of an \textE_\textr \textC_\texti^￠ {\text{E}}_{\text{r}} {\text{C}}_{\text{i}}^{\prime } catalytic mechanism. An intermolecular electron transfer rate constant of (4 ± 1) × 10⁵ M⁻¹ s⁻¹ was estimated for both forms of the enzyme, as well as a similar Michaelis–Menten constant. These results show that neither the intermolecular electron transfer nor the catalytic activity is kinetically controlled by the activation mechanism of CCP in the case of the P. stutzeri enzyme. Direct enzyme catalysis using protein film voltammetry was unsuccessful for the analysis of the activation mechanism, since P. stutzeri CCP undergoes an undesirable interaction with the pyrolytic graphite surface. This interaction, previously reported for the Paracoccus pantotrophus CCP, induces the formation of a non-native conformation state of the electron-transferring haem, which has a redox potential 200 mV lower than that of the native state and maintains peroxidatic activity.     

<Emphasis Type="Italic">Iridaea cordata</Emphasis> (Gigartinales,Rhodophyta): responses to artificial UVB radiation

The effects of UVB radiation on the different developmental stages of the carrageenan-producing red alga Iridaea cordata were evaluated considering: (1) carpospore and discoid germling mortality; (2) growth rates and morphology of young tetrasporophytes; and (3) growth rates and pigment content of field-collected plant fragments. Unialgal cultures were submitted to 0.17, 0.5, or 0.83 W m⁻² of UVB radiation for 3 h per day. The general culture conditions were as follows: 12 h light/12 h dark cycles; irradiance of 55 μmol photon^.per square meter per second; temperature of 9 ± 1°C; and seawater enriched with Provasoli solution. All UVB irradiation treatments were harmful to carpospores ( 0.17  \textW \textm ^{- 2} = 40.9 ±6.9% 0.17\;{\text{W}}\,{{\text{m}}^{ - 2}} = 40.9 \pm 6.9\% , 0.5  \textW \textm ^{- 2} = 59.8 ±13.4% 0.5\;{\text{W}}\,{{\text{m}}^{ - 2}} = 59.8 \pm 13.4\% , 0.83  \textW \textm ^{- 2} = 49 ±17.4% 0.83\;{\text{W}}\,{{\text{m}}^{ - 2}} = 49 \pm 17.4\% mortality in 3 days). Even though the mortality of all discoid germlings exposed to UVB radiation was unchanged when compared to the control, those germlings exposed to 0.5 and 0.83 W m⁻² treatments became paler and had smaller diameters than those cultivated under control treatment. Decreases in growth rates were observed in young tetrasporophytes, mainly in 0.5 and 0.83 W m⁻² treatments. Similar effects were only observed in fragments of adult plants cultivated at 0.83 W m⁻². Additionally, UVB radiation caused morphological changes in fragments of adult plants in the first week, while the young individuals only displayed this pattern during the third week. The verified morphological alterations in I. cordata could be interpreted as a defense against UVB by reducing the area exposed to radiation. However, a high level of radiation appears to produce irreparable damage, especially under long-term exposure. Our results suggest that the sensitivity to ultraviolet radiation decreases with increased algal age and that the various developmental stages have different responses when exposed to the same doses of UVB radiation.     

Expression,subcellular localization,and <Emphasis Type="Italic">cis</Emphasis>-regulatory structure of duplicated phytoene synthase genes in melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.)

The emission of molecular iodine (I₂) from the stipe, the meristematic area and the distal blade of the brown macroalga Laminaria digitata (Hudson) Lamouroux (Phaeophyceae) was monitored under low light and dark conditions. Photosynthetic parameters were determined to investigate both the extent of stress experienced by different thallus parts and the effects of emersion on photosynthesis. Immediately after air exposure, intense I₂ emission was detectable from all thallus parts. I₂ emission declined continuously over a period of 180 min following the initial burst, but was not affected by the light regime. The total number of mole of I₂ emitted by stipes was approximately 10 times higher than those emitted from other thallus parts. Initial I₂ emission rates (measured within 30 min of exposure to air) were highest for stipes (median values: 2,999 and 5,222 pmol g⁻¹ dw min⁻¹ in low light and dark, respectively) and lower, by one order of magnitude, for meristematic regions and distal blades. After exposure to air for between 60 and 180 min, I₂ emission rates of all thallus parts were reduced by 70–80%. Air exposure resulted in a decrease of the maximum photosystem II (PSII) efficiency (F _v/F _m) by 3%, and in a 25–55% increase of the effective PSII quantum efficiency ( \Updelta F/F^￠_\textm \Updelta F/F^{\prime}_{\text{m}} ); this was caused by a higher fraction of open reaction centres (q_P), whereas the efficiency of the latter in capturing energy ( F^￠_\textv /F^￠_\textm F^{\prime}_{\text{v}} /F^{\prime}_{\text{m}} ) remained constant. The results indicate the presence of an iodine pool which is easily volatilised and depleted due to air exposure, even under apparently low stress conditions.     

Immobilizing Pt nanoparticles and chitosan hybrid film on polyaniline naofibers membrane for an amperometric hydrogen peroxide biosensor

A convenient and effective way for fabricating amperometric hydrogen peroxide (H₂O₂) biosensor was designed in this paper. First, the polyaniline (PANI) nanofibers membrane with good conductance and high surface area was electropolymerized on a gold electrode surface. Then, Pt nanoparticle (PtNP) was electrochemically deposited on the PANI nanofibers membrane. Finally, the hybrid film of gold nanoparticle, chitosan, and horseradish peroxidase (HRP) was cast onto the modified electrode to form a stable biofunctional film, which was also employed as a protective layer to PtNP. The proposed biosensor exhibited a rapid response to H₂O₂ with the linear range from 7.0 × 10⁻⁶ to 1.4 × 10⁻² M and a detection limit of 2.8 × 10⁻⁶ M (S/N = 3). The sensitivity of 558 μA mM⁻¹ cm⁻² was obtained. The Michaelis–Menten constant, K_\textM^\textapp K_{\text{M}}^{\text{app}} value was 1.90 mM suggesting a high affinity. Moreover, it displayed a good reproducibility and long-term stability.     

Lycopene production from synthetic medium by <Emphasis Type="Italic">Blakeslea trispora</Emphasis> NRRL 2895 (+) and 2896 (−) in a stirred-tank fermenter

The dissolved oxygen tension of 20% of air saturation, pH-shift from 4.0 to 5.5 on day 3, and a moderate shear stress (calculated as an impeller tip speed, V_\texttip = 0. 9 2 6- 2. 1 6 1  \textm/\texts V_{\text{tip}} = 0. 9 2 6- 2. 1 6 1 \, {\text{m}}/{\text{s}} ) were identified to be the key factors in scaling-up the mated fermentation of Blakeslea trispora NRRL 2895 (+) and 2896 (−) for lycopene production from a shake flask to a stirred-tank fermenter. The maximal lycopene production of 183.3 mg/L was obtained in 7.5-L stirred-tank fermenter, and then the mated fermentation process was successfully step-wise scaled-up from 7.5- to 200-L stirred-tank fermenter. The comparability of the fermentation process was well controlled and the lycopene production was maintained during the process scale-up. Furthermore, with the integrated addition of 150 μmol/L abscisic acid on day 3, 0.5 g/L leucine and 0.1 g/L penicillin on day 4, the highest lycopene production of 270.3 mg/L was achieved in the mated fermentation of B. trispora in stirred-tank fermenter.     

Strategies for efficient repetitive production of succinate using metabolically engineered Escherichia coli

Escherichia coli AFP111, a pflB, ldhA, ptsG triple mutant of E. coli W1485, can be recovered for additional succinate production in fresh medium after two-stage fermentation (an aerobic growth stage followed by an anaerobic production stage). However, the specific productivity is lower than that of two-stage fermentation. In this study, three strategies were compared for reusing the cells. It was found when cells were aerobically cultivated at the end of two-stage fermentation without supplementing any carbon source, metabolites (mainly succinate and acetate) could be consumed. As a result, enzyme activities involved in the reductive arm of tricarboxylic acid cycle and the glyoxylate shunt were enhanced, yielding a succinate specific productivity above 1 2 5  \textmg  \textg_DCW ^{- 1}  \texth ^{- 1} 1 2 5\;{\text{mg}}\;{\text{g}}_{\rm DCW}^{ - 1} \,{\text{h}}^{ - 1} and a mass yield above 0.90 g g⁻¹ in the subsequent anaerobic fermentation. In addition, the intracellular NADH of cells subjected to aerobic cultivation with metabolites increased by more than 3.6 times and the ratio of NADH to NAD⁺ increased from 0.4 to 1.3, which were both favorable for driving the TCA branch to succinate.     

Oxygen-dependence of metabolic rate in the muscles of craniates

We present evidence that oxygen consumption (V_\textO2 ) (V_{{{\text{O}}_{2} }} ) is oxygen partial pressure (P_\textO2 ) (P_{{{\text{O}}_{2} }} ) dependent in striated muscles and P_\textO2 P_{{{\text{O}}_{2} }} -independent in the vasculature in representatives of three craniate taxa: two teleost fish, a hagfish and a rat. Blood vessel V_\textO2 V_{{{\text{O}}_{2} }} displayed varying degrees of independence in a P_\textO2 P_{{{\text{O}}_{2} }} range of 15–95 mmHg, while V_\textO2 V_{{{\text{O}}_{2} }} by striated muscle tissue slices from all species related linearly to P_\textO2 P_{{{\text{O}}_{2} }} between 0 and 125 mmHg, despite V_\textO2 V_{{{\text{O}}_{2} }} rates varying greatly between species and muscle type. In salmon red muscle, lactate concentrations fell in slices incubated at a P_\textO2 P_{{{\text{O}}_{2} }} of either 30 or 100 mmHg, suggesting aerobic rather than anaerobic metabolism. Consistent with this finding, potential energy, a proxy of ATP turnover, was P_\textO2 P_{{{\text{O}}_{2} }} -dependent. Our data suggest that the reduction in V_\textO2 V_{{{\text{O}}_{2} }} with falling P_\textO2 P_{{{\text{O}}_{2} }} results in a decrease in ATP demand, suggesting that the hypoxic signal is sensed and cellular changes effected. Viability and diffusion limitation of the preparations were investigated using salmon cardiac and skeletal muscles. Following the initial P_\textO2 P_{{{\text{O}}_{2} }} depletion, reoxygenation of the Ringer bathing salmon cardiac muscle resulted in V_\textO2 \texts V_{{{\text{O}}_{2} }} {\text{s}} that was unchanged from the first run. V_\textO2 V_{{{\text{O}}_{2} }} increased in all muscles uncoupled with p-trifluoromethoxylphenyl-hydrazone (FCCP) and 2,4-dinitrophenol (DNP). Mitochondrial succinate dehydrogenase activity, quantified by reduction of 3-(4,5-dimethylthiazol)-2,5-diphenyl-2H-tetrazolium bromide (MTT) to formazan, was constant over the course of the experiment. These three findings indicate that the tissues remained viable over time and ruled out diffusion-limitation as a constraint on V_\textO2 V_{{{\text{O}}_{2} }} .