Specific Oxygen Consumption Rate Research Articles

AEROBIC RESPIRATION RATE AND ANAEROBIC ENZYMATIC ACTIVITY OF PETROLISTHES LAEVIGATUS (ANOMURA, PORCELLANIDAE) UNDER LABORATORY CONDITIONS

Abstract Petrolisthes laevigatus is commonly found in the intertidal zone from southern Peru to southern Chile. Its geographic distribution and its location in the intertidal zone suggest that this species should have ample physiological and biochemical adaptability to environmental change. Several experiments were conducted on P. laevigatus specimens collected in south-central Chile (Coliumo Bay) to determine: (i) the relationship between specific oxygen consumption rate and temperature; (ii) the presence and activity of lactate dehydrogenase (LDH), octopine dehydrogenase (OPDH), strombine dehydrogenase (STRDH), alanopine dehydrogenase (ALPDH), and alcohol dehydrogenase (ADH); and (iii) resistance to hypoxic conditions in sea water. The specific respiration rate at temperatures frequently found in Coliumo Bay was 0.054 and 0.058 ml O2h−1g−1 at 10.5°C and 13°C, respectively. LDH and OPDH activities were found in muscle tissue, heart, and visceral mass (midgut gland, gonad); OPDH presented a lower activity...

Steady-state and transient-state analyses of aerobic fermentation in Saccharomyces kluyveri.

Some yeasts, such as Saccharomyces cerevisiae, produce ethanol at fully aerobic conditions, whereas other yeasts, such as Kluyveromyces lactis, do not. In this study we investigated the occurrence of aerobic alcoholic fermentation in the petite-negative yeast Saccharomyces kluyveri that is only distantly related to S. cerevisiae. In aerobic glucose-limited continuous cultures of S. kluyveri, two growth regimens were observed: at dilution rates below 0.5 h(-1) the metabolism was purely respiratory, and at dilution rates above 0.5 h(-1) the metabolism was respiro-fermentative. The dilution rate at which the switch in metabolism occurred, i.e. the critical dilution rate, was 66% higher than the typical critical dilution rate of S. cerevisiae. The maximum specific oxygen consumption rate around the critical dilution rate was found to 13.6 mmol (g dry weight)(-1) h(-1) and the capacity of the pyruvate dehydrogenase-bypass pathway was estimated to be high from in vitro enzyme activities; especially the specific activity of acetyl-CoA synthetase was much higher than in S. cerevisiae at all tested conditions. Addition of glucose to respiring cells of S. kluyveri led to ethanol formation after a delay of 20-50 min (depending on culture conditions prior to the pulse), which is in contrast to S. cerevisiae that ferments immediately after glucose addition.

Aerobic Respiration Rate and Anaerobic Enzymatic Activity of Petrolisthes Laevigatus (Anomura, Porcellanidae) under Laboratory Conditions

Petrolisthes laevigatus is commonly found in the intertidal zone from southern Peru to southern Chile. Its geographic distribution and its location in the intertidal zone suggest that this species should have ample physiological and biochemical adaptability to environmental change. Several experiments were conducted on P. laevigatus specimens collected in south-central Chile (Coliumo Bay) to determine: (i) the relationship between specific oxygen consumption rate and temperature; (ii) the presence and activity of lactate dehydrogenase (LDH), octopine dehydrogenase (OPDH), strombine dehydrogenase (STRDH), alanopine dehydrogenase (ALPDH), and alcohol dehydrogenase (ADH); and (iii) resistance to hypoxic conditions in sea water. The specific respiration rate at temperatures frequently found in Coliumo Bay was |$0.054 \ {\rm and} \ 0.058 \ {\rm ml \ O}_{2}{\rm h}^{-1}{\rm g}^{-1}$| at 10.5°C and 13°C, respectively. LDH and OPDH activities were found in muscle tissue, heart, and visceral mass (midgut gland, gonad); OPDH presented a lower activity than LDH. Both enzymes presented high activities in comparison to other decapods. No activity was detected for ALPDH, STRDH, and ADH. Crabs subjected to low dissolved oxygen concentrations (i.e., |$0.79 \ {\rm to} \ 0.96 \ {\rm ml \ O}_{2} \ {\rm l}^{-1}$|⁠) died after three hours of exposure, and no oxygen consumption was detected during this period. Accordingly, P. laevigatus is not adapted to long-term conditions of environmental hypoxia. Nevertheless, compared to other decapod species, P. laevigatus can cope well with short-term environmental and/or functional hypoxia due to the high activity levels of LDH and OPDH and the presence of both enzymes in the heart.

Significant increase in recombinant protein production of a virus-infected Sf-9 insect cell culture of low MOI under low dissolved oxygen conditions

Spodoptera frugiperda Sf-9 insect cells were infected with recombinant Autographa californica nuclear polyhedrosis virus at a low multiplicity of infection (MOI) (0.1), and the effect of dissolved oxygen (DO) on the production of a polyhedrin promoter-driven recombinant protein (β-galactosidase), intrinsic proteases (carboxyl and cysteine proteases), and the virus was determined. The DO concentrations used in the present study were 45%, 25%, 5%, and 1.3% of air saturation. At 5% DO the cell growth following viral infection was greatest and β-galactosidase was about 5-fold increased in volumetric yield compared to that at 45% and 25% DO, whereas the growth at 1.3% DO was extremely poor. The virus titer in the medium at 4–8 d post-infection (dpi) was also highest at 5% DO, but the titer was significantly decreased by further increasing the culture time. This was in part attributed to the fact that baculovirus is susceptible to oxidative inactivation under aerobic conditions. The DO dependency of the specific oxygen consumption rate of virus-infected and uninfected Sf-9 cells was expressed by a Monod-type equation. A critical DO, above which the rate of oxygen utilization is not limited by DO, was estimated to be 3.5% of air saturation for virus-infected Sf-9 cells. These results indicated that for a baculovirus-infected Sf-9 insect cell culture of low MOI, the optimal DO was likely to be approximately 5% of air saturation, which is above the critical DO for the infected Sf-9 cells but sufficiently low to reduce the possibility of the oxidative inactivation of virus. For the production of carboxyl and cysteine proteases, the accumulation behavior and concentrations did not significantly vary with DO, except that a peak of cysteine protease activity was observed intracellularly only at 5% DO, coinciding with β-galactosidase production.

Effect of high-cell-density fermentation of Candida utilis on kinetic parameters and the shift to respiro-fermentative metabolism.

Candida utilis NRRLY-900 was grown in a high-cell-density continuous culture without oxygen limitation. Glucose or molasses was used as carbon source at 30 g l(-1) or 100 g (reducing sugars) l(-1). At 30 g glucose l(-1), the dilution rate (D) immediately before the change in respiratory metabolism (Dr) was approximately 0.40 h(-1). At this value of D, the corresponding culture in molasses did not reach the Dr value. When the reducing sugar concentration in the feed was 100 g l(-1), the Dr was 0.15 h(-1) for glucose and 0.3 h(-1) for molasses. When D>Dr, accumulation of ethanol and organic acids occurred, due to physiological changes in C. utilis. The changes observed were a decrease in the biomass yield coefficient per gram of oxygen consumed (YO2) and a sudden increase in the specific oxygen consumption rate (qO2) for each substrate. Therefore, at growth rates above Dr in a high-cell-density culture, C. utilis acquired a flexible catabolism directed towards alternative fermentation routes. The D at which metabolic changes took place seemed to depend on the nature and concentration of the carbon source. Biomass productivity was higher with molasses than with glucose when the fermenter was operated at high D values.

Aerobic composting performance and simulation of mixed sludges

The objectives of this study were twofold: to determine the specific oxygen consumption rate of mixed sludges by means of a respirometry study, and to investigate the applicability of a developed model to a pilot-scale reactor containing nightsoil and brewery sludges. The results of the study revealed the specific oxygen consumption rates to be 87.7 mg O2/kg VS-hr and 117.7 mg O2/kg VS-hr at temperatures of 25 °C and 35 °C, respectively. Oxygen was found to be consumed at a higher rate in the reactor operated at the higher temperature. The computer simulation for temperature variations in the pilot-scale reactor showed a good relationship between the simulated and the measured temperatures. After three days operation, the highest simulated temperature was 65 °C, whereas the highest measured temperature was 60 °C. In addition, results suggest that the model can provide an accurate air supply rate to indicate economical reactor operation.

Continuous stable production of von Willebrand Factor monoclonal antibody in spin filter bioreactor with bleeding technology

The characteristics of two different modes of perfusion culture, intermittent and continuous bleedings, were investigated by culturing the hybridoma cells producing von Willebrand Factor (vWF) monoclonal antibody (McAb) in a 15 L bioreactor without clogging the filter. Both culture methods exhibited similar profiles of cell density and metabolite concentrations during the culture period at the cell concentration of around 1×107 cells/mL. When the perfusion rate was increased, the intermittent bleeding culture showed problems of ammonia accumulation and decrease of cell viability. The continuous bleeding culture exhibited higher physiological activity than that of the intermittent bleeding culture in terms of nutrient consumption and metabolite production kinetics. But the analysis of specific oxygen consumption rate showed that the specific oxygen consumption rate of intermittent bleeding culture was similar to that of exponential growth phase. The continuous bleeding culture showed higher specific vWF McAb productivity and cumulative production than those of the intermittent bleeding culture. Finally we proved the possibility of long-term operation of continuous bleeding culture and produced approximately 40 g of vWF McAb in a 15 L bioreactor after one-month operation.

Noninvasive oxygen measurements and mass transfer considerations in tissue culture flasks

Murine hybridomas were cultivated in tissue culture flasks. Dissolved oxygen tensions in the gas and liquid phases during cell growth were monitored. Oxygen levels were measured noninvasively by interrogating an oxygen-sensitive patch mounted on the interior surface of the tissue culture flask with an optrode from outside the tissue culture flask. Readings were made in tissue culture flasks with caps both cracked open and completely closed. Although the oxygen in the gas phase remained near atmospheric oxygen levels in both flasks, over time the liquid-phase oxygen tension at the bottom of the flasks reached zero during cell growth in both the open and closed tissue culture flasks. These results suggest that the widespread practice of cracking open tissue culture flask caps during cell growth with a view to supplying adequate oxygen to cells is ineffective and probably unnecessary.The mass transfer characteristics of the tissue culture flask were also studied. The dominant resistance to oxygen mass transfer to the sensor and the cells was through the liquid media. The mass transfer rates through the liquid layer under standard laboratory conditions were found to be greater than those predicted by diffusion alone. This suggests that mixing at a microscale occurs. Volumetric and specific oxygen consumption rates were also calculated from the sensor data. These consumption rates were comparable with values published elsewhere. (c) 1996 John Wiley & Sons, Inc.

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, μmax, measured in the batch culture agrees with the dilution rate at which washout occurs in continuous cultures. The maximum oxygen consumption rate, qO2,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.

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in continuous cultures

The oxidation and growth kinetics of ferrous iron with Thiobacillus ferrooxidans in continuous cultures was examined at several total iron concentrations. On-line off-gas analyses of O2 and CO2 were used to measure the oxygen and carbon dioxide consumption rates in the culture. Off-line respiration measurements in a biological oxygen monitor (BOM) were used to measure directly the maximum specific oxygen consumption rate, qO2,max, of cells grown in continuous culture. It was shown that these reproducibly measured values of qO2,max vary with the dilution rate. The biomass-specific oxygen consumption rate, qO2, is dependent on the ratio of the ferric and ferrous iron concentrations in the culture. The oxidation kinetics was accurately described with a rate equation for competitive ferric iron inhibition, using the value of qO2,max measured in the BOM. Accordingly, only the kinetic constant Ks/K i needed to be fitted from the measurements. A new method was introduced to determine the steady-state kinetics of a cell suspension in a batch culture that only takes a few hours. The batch culture was set up by terminating the feeding of a continuous culture at its steady state. The kinetic constant K s/K i determined in this batch culture agreed with the value determined in continuous cultures at various steady states.

Apoptosis-resistant E1B-19K-expressing NS/0 myeloma cells exhibit increased viability and chimeric antibody productivity under perfusion culture conditions.

We have shown previously that recombinant NS/0 myelomas expressing sufficient amounts of E1B-19K were resistant to apoptosis occurring in the late phase of batch culture and under stressful conditions such as cultivation in glutamine-free medium or following heat shock. However, no significant increase in monoclonal antibodies (MAb) was observed during the prolonged stationary phase of these batch cultures. Here, we show that E1B-19K can enhance cell survival and improve MAb productivity in high cell density perfusion culture. Typically, lymphoid cells grown under steady state in perfusion exhibit decreasing viabilities with concomitant accumulation of apoptotic cells. By modulating the ability of these cells to resist to induction of apoptosis in low nutrient environment, a 3-fold decrease in specific death rate from 0.22 day-1 for NS/0 control to 0.07 day-1 for E1B-19K cells was achieved, resulting in a significant improvement in cell viability throughout perfusion. E1B-19K cells at the perfusion plateau phase also exhibited a 3-fold reduction in specific growth rate concomitant with a lower percentage of S and higher percentage of G1 phase cells. This was associated with a 40% decrease in specific oxygen consumption rate, likely related to a reduction in the specific consumption rates of limiting nutrient(s). Expression of E1B-19K consequently had a significant impact on the steady-state viable cell density, allowing maintenance of 11.5 x 10(6) E1B-19K cells/mL versus 5.9 x 10(6) control NS/0 cells/mL for the same amount of fresh medium brought into the system (half a volume per day). Whereas MAb concentrations found in perfusion culture of control NS/0 myelomas were almost 3-fold higher than those found in batch culture; in the case of E1B-19K-expressing myelomas, the MAb concentration in perfusion was more than 7-fold higher than in batch. This was attributable to the 2-fold increase in viable cell plateau and to a 40% increase in the perfusion to batch ratio of specific MAb productivity (2.2-fold for E1B-19K myelomas versus 1.6-fold for NS/0 control).

Maintenance energy demand and starvation recovery dynamics of Nitrosomonas europaea and Nitrobacter winogradskyi cultivated in a retentostat with complete biomass retention.

Nitrosomonas europaea and Nitrobacter winogradskyi (strain "Engel") were grown in ammonia-limited and nitrite-limited conditions, respectively, in a retentostat with complete biomass retention at 25 degrees C and pH 8. Fitting the retentostat biomass and oxygen consumption data of N. europaea and N. winogradskyi to the linear equation for substrate utilization resulted in up to eight-times-lower maintenance requirements compared to the maintenance energy demand (m) calculated from chemostat experiments. Independent of the growth rate at different stages of such a retention culture, the maximum specific oxygen consumption rate measured by mass spectrometric analysis of inlet and outlet gas oxygen content always amounted to approximately 45 micromol of O2 mg-1 of biomass-C x h-1 for both N. europaea and N. winogradskyi. When bacteria were starved for different time periods (up to 3 months), the spontaneous respiratory activity after an ammonia or nitrite pulse decreased with increasing duration of the previous starvation time period, but the observed decrease was many times faster for N. winogradskyi than for N. europaea. Likewise, the velocity of resuscitation decreased with extended time periods of starvation. The increase in oxygen consumption rates during resuscitation referred to the reviving population only, since in parallel no significant increase in the cell concentrations was detectable. N. europaea more readily recovers from starvation than N. winogradskyi, explaining the occasionally observed nitrite accumulation in the environment after ammonia becomes available. From chloramphenicol (100 microg x ml-1) inhibition experiments with N. winogradskyi, it has been concluded that energy-starved cells must have a lower protein turnover rate than nonstarved cells. As pointed out by Stein and Arp (L. Y. Stein and D. J. Arp, Appl. Environ. Microbiol. 64:1514-1521, 1998), nitrifying bacteria in soil have to cope with extremely low nutrient concentrations. Therefore, a chemostat is probably not a suitable tool for studying their physiological properties during a long-lasting nutrient shortage. In comparison with chemostats, retentostats offer a more realistic approach with respect to substrate provision and availability.

Metabolic responses to different glucose and glutamine levels in baby hamster kidney cell culture

In this work, a BHK21 clone producing a recombinant antibody/cytokine fusion protein was used to study the dependence of cell metabolism on the glucose and glutamine levels in the culture medium. Results obtained indicate that both glucose and glutamine consumptions show a Michaelis-Menten dependence on glucose and glutamine concentrations respectively. A similar dependence is also observed for lactate and ammonia productions. The estimated value of the Michaelis constant for the dependence of lactate production on glucose (KLacGlc) was 1.4 +/- 0.1 mM and for the dependence of ammonia production on glutamine (KAmmGln) was 0.25 +/- 0.11 mM and 0.10 +/- 0.03 mM, at glucose concentrations of 0.28 mM and 5.6 mM respectively. At very low glucose concentrations, the glucose to lactate yield decreased markedly, showing a metabolic shift towards lower lactate production. This metabolic shift was also confirmed by the significant increase in the specific oxygen consumption rate also observed at low glucose concentrations. Although it was highly dependent on glucose concentration, the oxygen consumption also increased with the increase in glutamine concentration. At very low glutamine concentrations, the glutamine to ammonia yield increased, showing a more efficient glutamine metabolism.

Oxygen Consumption Rate and Swimming Efficiency of the Blacknose Shark, Carcharhinus acronotus

Oxygen consumption rates and swimming efficiency of 10 blacknose sharks (4787 cm total length and 0.45-3.51 kg weight), Carcharhinus acronotus, were determined using a closed, circular respirometer. Swimming speed ranged from 20-40 cm sec-1 and averaged 30.8 (? 0.48 SE) cm sec-1. Routine weight specific oxygen consumption rates increased from 278.5 mg 02 kg-' h-1 for a 3.5 kg shark to 486 mg 02 kg-' h-1 for a 0.8 kg shark. The relationship of swimming speed and weight specific oxygen consumption rate for sharks 0.45-0.85 kg was described by the equation log V02 = 0.007 U + 2.38, r2 = 0.31, P = 0.006, where U is swimming speed in cm sec-'. Mean standard metabolic rate, the amount of oxygen consumed at zero activity, was calculated by extrapolating to zero swimming speed and was 239.8 mg 02 kg-l h-1. Total cost of transport (cal g-i km-l), the energy required to travel a given distance at a particular speed, was used as a measure of overall swim efficiency. Highest total cost of transport (1.68 cal g-~ km-') was at 25 cm sec-1 and decreased with increasing speed. Swimming efficiency was optimal at speeds of 3639 cm sec-1. Results suggest blacknose sharks have higher routine oxygen consumption rates and total cost of transport, but the cost of locomotion was similar to other active, tropical sharks. STUDIES on shark oxygen consumption rates

Kinetics of ferrous iron oxidation by Leptospirillum bacteria in continuous cultures.

The oxidation of ferrous iron by Leptospirillum bacteria was studied in a continuous culture in the dilution rate range 0.009-0.077 h-1 and could be described with a rate equation for competitive ferric iron inhibition kinetics in terms of the ferric/ferrous iron ratio in the solution. The ferrous iron oxidation in the continuous culture was followed by means of oxygen and carbon dioxide concentration analyses in reference air and off-gas. From these measurements the oxygen consumption rate, rO2, the carbon dioxide consumption rate, rCO2, the biomass concentration, Cx, and the biomass specific oxygen consumption rate, qO2, in the culture were determined. The ferrous iron concentration in the culture was below accurate levels to determine with the usual titrimetric method and was therefore derived from measuring the solution redox potential. The degree of reduction balance was used to check the theoretically expected relation between the rates of ferrous iron, -rFe2+, oxygen, -rO2, and biomass, rx. The maximum biomass yield and maintenance coefficient on oxygen are Yoxmax = 0.047 mol of C/mol of O2 and mo = 0.057 mol of O2/(mol of C.h). The maximum specific oxygen consumption rate, qO2,max = 1.7 mol of O2/(mol of C.h), the affinity coefficient, Ks/Ki = 0.0005 mol of Fe2+/mol of Fe3+, and the maximum specific growth rate, micromax = 0.069 h-1, Ks/Ki = 0.0004, were fitted from the measured data. For several dilution rates, off-line respiratory measurements with cell suspension from the continuous culture were carried out in dynamic BOM-Eh measurements. The dissolved oxygen and redox potential were measured simultaneously and monitored. The measured value of qO2,max varied between 2.3 and 1.7 mol/(mol of C.h). The value of Ks/Ki = 0.0007 was equal in all experiments. The measured values of qO2 in the continuous culture were well described with the kinetics determined in dynamic BOM-Eh measurements. It was concluded that dynamic BOM-Eh measurements are a convenient method to determine the kinetics of continuous culture grown Leptospirillum bacteria.

Characterization of hematopoietic cell expansion, oxygen uptake, and glycolysis in a controlled, stirred-tank bioreactor system.

Cultures of umbilical cord blood and mobilized peripheral blood mononuclear cells were carried out in a stirred bioreactor with pH and dissolved oxygen control. Expansion of total cells and colony-forming units granulocyte-macrophage was greatly enhanced by the use of a cell-dilution feeding protocol (as compared to a cell-retention feeding protocol). The specific oxygen consumption rate (qO2) for these cultures ranged from 1.7 x 10(-8) to 1.2 x 10(-7) micromol/(cell.h). The maximum in qO2 for each culture closely corresponded with the maximum percentage of progenitor or colony-forming cells (CFCs) present in the culture. The maximum qO2 values are slightly less than those reported for hybridomas, while the lowest qO2 values are somewhat greater than those reported for mature granulocytes. Examination of the ratio of lactate production to oxygen consumption in these cultures suggests that post-progenitor cells of the granulomonocytic lineage obtain a greater portion of their energy from glycolysis than do CFCs. The different metabolic profiles of CFCs and more mature cells suggest that monitoring the uptake or production of oxygen, lactate, and other metabolites will allow estimation of the content of several cell types in culture.

The onset of fermentative metabolism in continuous cultures depends on the catabolite repression properties of saccharomyces cerevisiae

In glucose-limited continuous cultures, a Crabtree positive yeast such as Saccharomyces cerevisiae displays respiratory metabolism at low dilution rates ( D) and respirofermentative metabolism at high D. We hypothesized that the onset of fermentative metabolism is related with the catabolite repression or glucose repression effect. To test this hypothesis, we have investigated the physiological behavior in glucose-limited continuous cultures of S. cerevisiae strain CEN.PK122 and isogenic mutants, snf1 ( cat1) and snf4 ( cat3), defective in proteins involved in the release from glucose repression and the mutant in glucose repression mig1. We analyzed the behavior of the wild type and mutant strains at steady state in chemostat cultures as a function of D. Wild-type cells displayed respiratory metabolism up to a D of 0.2 h −1. snf1 and snf4 mutants started fermenting after a D of 0.1 and 0.15 h −1, respectively. The latter behavior was not due to an impairment of respiration since their specific rate of oxygen consumption was similar or even higher than that shown by the wild type. The snf1 strain displayed much lower yields than the wild type and the other mutants in the whole range of D studied. We conclude that the onset of fermentative metabolism in yeast growing in chemostat cultures is related with glucose repression.

Identification and control of oxidative metabolism in Ssaccharomyces cerevisiae during transient growth using calorimetric measurements.

The objective of this study was to characterize the dynamic adaptation of the oxidative capacity of Saccharomyces cerevisiae to an increase in the glucose supply rate and its implications for the control of a continuous culture designed to produce biomass without allowing glucose to be diverted into the reductive metabolism. Continuous cultures subjected to a sudden shift-up in the dilution rate showed that the glucose uptake rate increased immediately to the new feeding rate but that the oxygen consumption could not follow fast enough to ensure a completely oxidative metabolism. Thus, part of the glucose assimilated was degraded by the reductive metabolism, resulting in a temporary decrease of biomass concentration, even if the final dilution rate was below Dcrit. The dynamic increase of the specific oxygen consumption rate, qO2, was characterized by an initial immediate jump followed by a first-order increase to the maximum value. It could be modeled using three parameters denoted qjumpO2, qmaxO2, and a time constant tau. The values for the first two of the parameters varied considerably from one shift to another, even when they were performed under identical conditions. On the basis of this model, a time-dependent feed flow rate function was derived that should permit an increase in the dilution rate from one value to another without provoking the appearance of reductive metabolism. The idea was to increase the glucose supply in parallel with the dynamic increase of the oxidative capacity of the culture, so that all of the assimilated glucose could always be oxidized. Nevertheless, corresponding feed-profile experiments showed that deviations in the reductive metabolism could not be completely suppressed due to variability in the model parameters. Therefore, a proportional feedback controller using heat evolution rate measurements was implemented. Calorimetry provides an excellent and rapid estimate of the metabolic activity. Satisfactory control was achieved and led to constant biomass yields. Ethanol accumulated only up to 0.49 g L-1 as compared to an accumulation of 1.82 g L-1 without on-line control in the shift-up experiment to the same final dilution rate.

Planktonic bacterial respiration as a function of C:N:P ratios across temperate lakes

Bacterioplankton and total planktonic community respiration were measured in 14 Canadian lakes during the summer. Total planktonic community respiration (O2 consumption and CO2 production) was best predicted by total phosphorus (TP), with respiratory rates increasing at higher TP levels. The bacterioplankton respiration alone was less well linked to nutrient concentrations. Yet, the rates of both planktonic and bacterial CO2 production changed similarly with changes in the C:N ratio, with the bacterial fraction, on average, contributing about 42% of the total planktonic respiratory rates. Bacterial carbon respired was uncoupled from oxygen consumption, with resultant average planktonic respiratory quotient (RQ) close to 2. Bacterioplankton respiratory rates are affected by a negative relationship between specific aerobic respiration and bacterial biomass. Higher bacterial specific oxygen consumption rates (µg O2 cell-1 d-1) were observed with increasing C:N and C:P ratios, suggesting an increase in maintenance cost at the low cell densities observed in oligotrophic waters. Phosphorus appeared as the limiting nutrient in these lakes and determined total planktonic respiratory rates, however, epilimnetic bacterial respiration was mainly related to the DOC: nutrient ratios.

Responses of juvenile rainbow trout, under food limitation, to chronic low pH and elevated summer temperatures, alone and in combination

Rainbow trout were exposed (90 days) in synthetic soft water to sublethal low pH (5.2) and a simulated climate warming scenario (+2°C above the control summer temperature range of 16.5–21° C), alone and in combination, under conditions of limited food (∼4% dry body weight day−1). Weight specific oxygen consumption rates (Mo2) were ∼55% of Mo2(max), in contrast to ∼75% of Mo2(max) found in trout fed an unlimited ration. This is likely due to a reduction in food quantity and thus feeding activity. However, the trout exposed to low pH at control temperatures exhibited higher conversion efficiencies and increased growth. In contrast, trout exposed to +2°C had reduced growth rates. No ionoregulatory disturbance occurred in any treatment, suggesting that this ration was sufficient to provide a replacement salt load in the diet. Energy budgets indicated that the limited ration resulted in a lowered optimum temperature for growth, with a greater proportion of the energy intake dissipated for metabolic expenditure, resulting in reduced conversion efficiencies. A fourfold reduction in faecal and unaccounted energy losses indicated higher absorption efficiencies than in satiation‐fed trout.