Cardio-respiratory responses in two ecologically distinct erythrinids (Hoplias malabaricus and Hoplias lacerdae) exposed to graded environmental hypoxia

Synopsis Ecologically distinct species of Hoplias were studied as to the cardio-respiratory responses to graded hypoxia. Hoplias malabaricus maintained a constant oxygen uptake down to a PiO₂ of 20 mmHg. Oxygen uptake declined markedly at lower PiO₂ and, concomitantly, cardiac frequency decreased. Concurrent reductions of oxygen uptake and heart rate also occurred in Hoplias lacerdae but at the considerably higher PiO₂ of 35 mmHg. These species-specific differences are consistent with the respective habitats: H. malabaricus occurs in stagnant hypoxic water, whereas H. lacerdae inhabits well-oxygenated rivers.     

Oxygen diffusion and dark respiration in aquatic macrophytes

Abstract. The supply of oxygen to respiring shoot tissue was investigated for three submerged macrophytes (Potamogeton crispus L., Egeria densa Planch, and Myriophyllum triphyllum Orchard). For all species, the response of oxygen uptake rates to the external O₂ concentration was a rectangular hyperbola over the range 0–5.0 × 10^?3m³ m^?3. However, the response pattern for material with water-infiltrated lacunar airspaces was non-hyperbolic over this range. The change in response was interpreted as an increased substrate (O₂) limitation, resulting from lower radial diffusion rates within the infiltrated material. Neither the uninfiltrated nor the infiltrated responses obeyed the linear and logarithmic formulae of the type observed for submerged macrophytes by earlier authors. These results suggest that the responses observed are affected by factors such as water velocity, internal restrictions to diffusion and the range of oxygen tensions investigated. Therefore, it is unlikely that one response formula can adequately account for the effects of oxygen concentration on submerged macrophyte oxygen uptake. The lacunar airspaces also represent a possible oxygen source for dark respiration. The consumption of oxygen from the airspaces was investigated by displacing the gas from the lacunae and measuring the subsequent increase in the rate of oxygen assimilation from the external liquid. Approximately 30% of the oxygen consumed by E. densa and P. crispus, and more than 40% of that consumed by M. triphyllum, was derived from the lacunar system. This O₂ supply is a consequence of the higher oxygen concentration in the lacunae than in the external medium, due to the low solubility of oxygen in water. Storage of photosynthetically-produced oxygen in the lacunae could not be identified during a light/dark transient, due to rate changes caused by the effects of light on the respiratory metabolism. However, O₂ partial pressure gradients artificially set up between the lacunae and water equilibrated within an hour, suggesting that excess oxygen would be lost to the water within this time.     

Effect of bioreactor configuration on substrate uptake by cell suspension cultures of the plant Eschscholtzia californica

Summary The uptake of carbohydrates and oxygen by cell suspension cultures of the plant Eschscholtzia californica (California poppy) was studied in relation to biomass production in shake flasks, a 1-1 stirred-tank bioreactor and a 1-1 pneumatically agitated bioreactor. The sequence of carbohydrate uptake was similar in all cases, with sucrose hydrolysis occurring followed by the preferential uptake of glucose. The uptake of fructose was found to be affected by the oxygen supply rate. Carbohydrate utilization occurred at a slower rate in the bioreactors. Apparent biomass yields, Y _X/S, ranged from 0.42 to 0.50 g biomass/g carbohydrate, while true biomass yields, Y _X/S, were about 0.69 g/g. The maintenance coefficient for carbohydrate, m _S, ranged between 0.002 and 0.008 g/dry weight (DW) per hour. The maximum measured specific oxygen uptake rate was 0.56 mmol O₂/g DW per hour and occurred early in the growth stage. The decline in specific uptake rate coincided with a decline in cell viability. The oxygen uptake rate was faster in shake flasks, corresponding to the higher growth rate obtained. The true growth yield on oxygen, YX/O₂, was calculated to range from 0.83 to 1.23 g biomass/g O₂, while the maintenance coefficient, mO₂, ranged from 0.15 to 0.25 mmol O₂/g DW per hour. The growth yields for oxygen determined from the stoichiometry of an elemental balance were within 10% of those calculated from experimental data. Offprint requests to: Raymond L. Legge     

Metabolic adaptations to hypoxia of two species of polychaeta,Nephtys ciliata and Nephtys hombergii

Summary The respiratory responses to declining oxygen tension in Nephtys ciliata and N. hombergii were investigated using flow respirometry. N. hombergii showed a better regulation of oxygen uptake during declining oxygen tension than N. ciliata. N. ciliata showed no difference in regulatory ability when individuals of different sizes were compared. It is argued that the difference in regulating ability can partly explain the distribution pattern of these two species in Århus Bight, Denmark.Abbreviations B2 regulation index - B2 mean regulation index - C_eO2 excurrent oxygen concentration - C _iO2 incurrent oxygen concentration; dw dry weight - PO ₂ partial pressure (oxygen) - VO₂ oxygen uptake - VO₂ relative oxygen uptake - V_W water flow     

Oxygen transfer to mixed cultures in tower systems

A modified dynamic method is introduced to determine the oxygen transfer coefficient, K_L a, in aerobic fermentation systems which are not mechanically agitated. The dissolved oxygen concentration is measured continuously following a step down or a step up in aeration rate. The response curve is analyzed to obtain the value of K_La Experiments were carried out at several different air flow rates using mixed culture in concurrent tower fermentors with motionless mixers. The effect of sieve trays and Koch motionless mixers on oxygen transfer was investigated using a 3 in. diameter column. The values of K_L aobtained at the bottom of each column were found to be higher than those obtained at the top. Comparison of the results showed that the values ofK_L a were higher when the Koch mixers were used than when the sieve trays were employed. The oxygen uptake rate by the organisms rX, is also calculated by using the K_L a values obtained. They compare favorably withthe experimentally measured values.     

Variation of ADM1 by using temperature-phased anaerobic digestion (TPAD) operation

The objective of the study was to examine the application of the Anaerobic Digestion Model No. 1 (ADM1) developed by the IWA task group for mathematical modelling of anaerobic process. Lab-scale temperature-phased anaerobic digestion (TPAD) process were operated continuously, and were fed with co-substrate composed of dog food and flour. The model platform implemented in the simulation was a derivative of the ADM1. Sensitivity analysis showed that k_m.process (maximum specific uptake rate) and K_S.process (half saturation value) had high sensitivities to model components. Important parameters including maximum uptake rate for propionate utilisers (k_m.pro) and half saturation constant for acetate utilisers (K_S.ac) in the thermophilic digester and maximum uptake rate for acetate utilisers (k_m.ac) in the mesophilic digester were estimated using iterative methods, which optimized the parameters with experimental results. Simulation with estimated parameters showed good agreement with experimental results in the case of methane production, uptake of acetate, soluble chemical oxygen demand (SCOD) and total chemical oxygen demand (TCOD). Under these conditions, the model predicted reasonably well the dynamic behavior of the TPAD process for verifying the model.     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.     

Respiratory and digestive responses of postprandial Dungeness crabs, Cancer magister, and blue crabs, Callinectes sapidus, during hyposaline exposure

Respiratory responses and gastric processing were examined during hyposaline exposure in two crab species of differing osmoregulatory ability. The efficient osmoregulator, Callinectes sapidus, displayed an immediate increase in oxygen uptake when exposed to low salinity in isolation. In contrast, the weak osmoregulator, Cancer magister, showed no change in oxygen uptake upon acute exposure (<6 h), but slight increases in oxygen uptake tended to occur over longer time scales (12–24 h). These changes were likely attributable to an increase in avoidance activity after 6 h hyposaline exposure. Following feeding in 100% SW, oxygen uptake doubled for both species and remained elevated for 15 h. When postprandial crabs were exposed to low salinities, C. sapidus were able to sum the demands of osmoregulation and digestion. Thus, gastric processes continued unabated in low salinity. Conversely, postprandial C. magister prioritized responses to low salinity over those of digestion, resulting in a decrease in oxygen uptake when exposed to low salinity. This decrease in oxygen uptake corresponded to a reduction in the rate of contraction of the pyloric stomach and a subsequent doubling of gastric evacuation time. The current study is one of the few to illustrate how summation or prioritization of competing physiological systems is manifested in digestive processes.     

Photosynthetic Capacities and Growth Characteristics of Nicotiana tabacum (cv. Xanthi) Cell Suspension Cultures

Photoheterotrophic growth of cell suspensions of Nicotiana tabacum L. (cv. Xanthi) in organic culture medium enriched in sucrose (30 g per liter) showed a classical sigmoid growth curve. The cells developed functional chloroplast structures during the exponential growth phase, when their chlorophyll content increased steadily. A limited drop (30%) in the chlorophyll amount and structural changes of the plastids (starch accumulation) were observed during the lag phase. The measurements of photosynthetic capacities (O₂ evolution and CO₂ fixation) during the growth cycle revealed changes in the photosynthetic ratio (O₂/CO₂), which was near 1 during the lag and stationary phases and near 2 during exponential growth. During exponential growth there was also a rapid NO₃^? uptake. Analysis of label distribution among the products of ¹⁴CO₂ fixation showed that both CO₂ assimilation pathways, linked to the ribulose-biphosphate carboxylase (the autotrophic pathway) and to phosphoenolpyruvate carboxylase (the non-autotrophic pathway) were operative with an important increase of the capacity of the latter during the exponential growth phase. Maximum rate of oxygen evolution, either endogenous or with p-benzoquinone as Hill reagent, as well as the increased CO₂ Fixation capacity via the non-autotrophic pathway during the exponential phase were concomitant with a high cyanide inhibited O₂ uptake.     

Hydrolase production by Aspergillus niger in solid-state cultivation

Summary A production of macerating enzymes which liquefy and hydrolyze the mandarin orange peel was studied in a solid state cultivation of Aspergillus niger on wheat bran substrate. Solid state cultivation in a 2 drum fermenter capable of interchangeable operation under dynamic or static conditions were carried out maintaining the moisture content of the substrate at 32, 39, 46, 56, 67, and 74%. Biomass grown on the solid substrate was estimated on the basis of a constant value of glucosamine content of A. niger, 50 mg glucosamine/g cell. A linear relationship between oxygen uptake rate and growth rate observed in all the experiments gave an oxygen growth yield, Y_X/O, of 28.5 g cell/mol O₂. The rate of macerating enzyme formation was also in proportion to the growth rate irrespective of the difference of the moisture content of the substrate.The enzyme accumulation on the solid substrate, the growth rate and oxygen uptake rate were maximum when the moisture content of the substrate was maintained at ca. 56% ascending from 32 to 56 and descending from 56 to 74.     

A mathematical approach for the estimation of biomass production rate in solid state fermentation

The oxygen uptake rate (OUR) was studied in a solid state fermentation process of dried citrus peel with the strain Aspergillus niger QH-2 in order to obtain the growth estimation of the microorganism in the system. The relationship between OUR, the maintenance coefficient (m) and the yield for oxygen consumption Y_O2 allows the estimation of the biomass rate if we consider that both parameters are not constants in some periods of the process. It was estimated that in the first 24th the strain has an specific growth rate of 0.174 h^?1 with values for Y_O2 and m in the order of 2.84 g-cell/g-oxygen and 0.006 g-oxygen/g-cell ·h respectively.     

Transfer and consumption of oxygen during the cultivation of the ectomycorrhizal fungus Rhizopogon nigrescens in an airlift bioreactor

The study had the objective of examining the aspects involved in the cultivation of ectomycorrhizal fungi for the production of commercially sustainable inoculant to attend the demands of the seedling nursery industry. It focused on certain parameters, such as the oxygen consumption levels, during the cultivation of the ectomycorrhizal fungus Rhizopogon nigrescens CBMAI 1472, which was performed in a 5-L airlift bioreactor. The dynamic method was employed to determine the volumetric coefficient for the oxygen transfer (k _L a) and the specific oxygen uptake rate (Q _O2 ). The results indicate that specific growth rates (μ _X ) and oxygen consumption decline rapidly with time, affected mainly by increases in biomass concentration (X). Increases in X are obtained primarily by increases in the size of pellets that are formed, altering, consequently, the cultivation dynamics. This is the result of natural increases in transferring resistance that are observed in these environments. Therefore, to avoid critical conditions that affect viability and the productivity of the process, particular settings are discussed.

     

Influence of oxygen starvation on the respiratory capacity of Penicillium chrysogenum

Summary The sensitivity of Penicillium chrysogenum to oxygen starvation and to azide was investigated on cells taken from different phases of a penicillin process. These treatments caused irreversible inhibition of the oxygen uptake rate during the tropophase while the idiophase cells were resistant to either treatment. The azide resistance is supposed to depend only on the dissociation of the hydrazoic acid when the process pH increases, but the shift from sensitive to resistant cells with respect to oxygen starvation was neither caused by the pH-change nor by the glucose limitation. Inactivation (I _OUR, percentage) of tropophase cells followed 1st order kinetics according to ln (1-I _OUR/100)^-1=0.048·t, where t is given in minutes. This means that the 1st minute of oxygen starvation may reduce the metabolic capacity by 4.7%.     

Oxygen reduction in a plastoquinone pool of isolated pea thylakoids

Oxygen uptake in isolated pea thylakoids in the presence of an inhibitor of plastoquinol oxidation by b ₆/f-complex dinitrophenylether of 2-iodo-4-nitrothymol (DNP-INT) was studied. The rate of oxygen uptake in the absence of DNP-INT had a distinct maximum at pH 5.0 followed by a decline to pH 6.5 and posterior slow rise, while in the presence of an inhibitor it increased at an increasing pH from 4.5 to 6.5 and then kept close to the rate in its absence up to pH 8.5. Gramicidin D substantially affected the oxygen uptake rate in the absence of DNP-INT, and only slightly in its presence. Such differences pointed to the presence of special oxygen reduction site(s) in photosynthetic electron transport chain `before' cytochrome complex. Oxygen uptake in membrane fragments of Photosystem II (BBY-particles) was low and did not depend on pH. This did not support the participation of Q_B in oxygen reduction in DNP-INT-treated thylakoids. Oxygen uptake in thylakoids in the presence of DNP-INT was inhibited by DCMU as well as by catalase in whole pH range. The catalase effect indicated that oxygen uptake was the result of dioxygen reduction by electrons derived from water, and that H₂O₂ was a final product of this reduction. Photoreduction of Cyt c in the presence of DNP-INT was partly inhibited by superoxide dismutase (SOD), and this pointed to superoxide formation. The latter was confirmed by a rise of the oxygen uptake rate in the presence of ascorbate and by suppression of this rise by SOD. Both tests showed that the detectable superoxide radicals averaged 20–25% of potentially formed superoxide radicals the quantity of which was calculated from the oxygen uptake rate. The obtained data implies that the oxygen reduction takes place in a plastoquinone pool and occurs mainly inside the membrane, where superoxide can be consumed in concomitant reactions. A scheme for oxygen reduction in a plastoquinone pool in thylakoid membranes is proposed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Respiration and nitrogenase activity in nodules ofCasuarina cunninghamiana and cultures ofFrankia sp. HFP020203: Effects of temperature and partial pressure of O2

The effects of time after exposure to acetylene and of nodule excision were examined using a flow-through system. After a transient depression in the rate of acetylene reduction that began about 1.5 min after exposure to acetylene, the rate recovered to 98% of the initial maximum value after 40 min. After nodule excision the rate stabilized to 90% of the initial maximum value observed in the intact plant.Excised nodules, measured at 6-min intervals in a closed system, with frequent changes of the gas mixture, were used for the remaining experiments. Acetylene reduction by the nodules increased rapidly as temperature was increased between 6 and 26°C. Between 26 and 36°C there was relatively little effect of temperature on acetylene reduction.Nodules and cultures ofFrankia were compared with respect to the effect of temperature and pO₂ (partial pressure of oxygen) on oxygen uptake. Cultures ofFrankia were grown on a nitrogen-free medium at either 0.3 kPa O₂ (vesicles absent) or 20 kPa O₂ (vesicles present). Oxygen uptake by nodules (vesicles absent) and by vesicle-containing cultures was strongly dependent on pO₂ at values below 20 kPa. This suggests the presence of a barrier to oxygen diffusion. Oxygen uptake was dependent on temperature as well as on pO₂, but the Q₁₀ was much larger for the cultures than for the nodules. This suggests that vesicles or related structures are not the source of the diffusion barrier in Casuarina nodules. Respiration by cultures ofFrankia lacking vesicles became O₂-saturated at low pO₂ values. Thus these cultures did not have a significant diffusion barrier. From these results it is concluded that nodules ofCasuarina cunninghamiana have a barrier to oxygen diffusion supplied by the host tissue and not byFrankia.     

Benthic community metabolism and trophic conditions of four South American lakes

Oxygen uptake rates of undisturbed sediment columns have been used as an integrative measure of the metabolic activities of benthic communities. Since the intensity of metabolic processes of profundal lake is dependent on the production of organic matter in the pelagic zone, oxygen uptake rates reflect the trophic condition of the whole lake. Four small lakes of central Chile, differing strongly in trophic conditions, provided a possibility to compare benthic oxygen uptake rates, under different oxygen conditions (Quiñenco, Grande, Chica and Lleulleu). Our objective was to establish the relationship between the oxygen uptake rates and bottom characteristics of lakes with different trophic conditions. At 8 mg O₂ l^-1 in the overlying water of the cores studied, the oxygen uptake rates of the sediment were: Quiñenco 51.2–56.0 mg O₂ m² h^-1 (eutrophic), Grande 41.2–46.4 mg O₂ m² h^-1 (mesotrophic), Chica 23.2–18.1 mg O₂ m² h^-1 (mesotrophic) and Lleulleu 11.7–16.0 mg O₂ m² h^-1 (oligotrophic). By exposing the sediments to different oxygen levels in the laboratory, it was found that benthic community metabolism decreased with oxygen concentrations. The slope of regression lines, relating oxygen uptake rates to oxygen concentrations, differed for the different sites investigated, closely related with the trophic conditions of the lakes. It was positively correlated with the organic matter content of the sediment of the cores (r ²= 0.78, p<0,05) and the nutrients of the bottom waters (total-P: r ²= 0.73, p<0,05; total-N: r ²= 0.73, p<0,05), and negatively with the redox potential of the sediments (r ²= 0.88, p<0,05).     

On-line monitoring of responses to nutrient feed additions by multi-frequency permittivity measurements in fed-batch cultivations of CHO cells

Changes in the nutrient availability of mammalian cell cultures are reflected in the β-dispersion parameter characteristic frequency (f _C ) and the on-line dual frequency permittivity signal. Multi-frequency permittivity measurements were therefore evaluated in fed-batch cultivations of two different CHO cell lines. Similar responses to nutrient depletions and discontinuous feed additions were monitored in different cultivation phases and experimental setups. Sudden increases in permittivity and f _C occurred when feed additions were conducted. A constant or declining permittivity value in combination with a decrease in f _C indicated nutrient limitations. f _C correlated well with changes in oxygen uptake rate when cell diameter remained constant, indicating that metabolic activity is reflected in the value of f _C . When significant cell size changes occurred during the cultivations, the analysis of the β-dispersion parameters was rendered complex. For the application of our findings in other systems it will be hence required to conduct additional off-line measurements. Based on these results, it is hypothesized that multi-frequency permittivity measurements can give information on the intracellular or physiological state in fed-batch mode. Similar observations were made when using different cell lines and feeding strategies, indicating that the findings are transferable to other cell lines and systems. The results should lead to an improved understanding of routine fed-batch processes. Additional studies are, however, required to explore how these observations can be used for fed-batch process development and optimization.     

Effects of temperature and dissolved oxygen on heart rate,ventilation rate and oxygen consumption of spangled perch,Leiopotherapon unicolor (Günther 1859), (Percoidei,Teraponidae)

Summary Heart, ventilation and oxygen consumption rates ofLeiopotherapon unicolor were studied at temperatures ranging from 5 to 35°C, and during progressive hypoxia from 100% to 5% oxygen saturation. Biopotentials recorded from the water surrounding the fish corresponded to ventilation movements, and are thought to originate from the ventilatory musculature. Cardio-respiratory responses to temperature and dissolved oxygen follow the typical teleost pattern, with bradycardia, increased ventilation rate and reduced oxygen consumption occurring during hypoxia. However, ventilation rate did not increase at 15°C and below. Ventilation rate showed a slower response to increasing temperature (normoxic Q₁₀=1.39) than heart rate and oxygen consumption (normoxic Q₁₀=2.85 and 2.38).L. unicolor is unable to survive prolonged hypoxia by utilising anaerobic metabolism, but has a large gill surface area which presumably facilitates oxygen uptake in hypoxic environments. Periodic ventilation during normoxia in restingL. unicolor may improve ventilation efficiency by increasing the oxygen diffusion gradient across the gills.Abbreviations EBG electrobranchiogram - ECG electrocardiogram     

Heterotrophic nitrification and aerobic denitrification inAlcaligenes faecalis strain TUD

Heterotrophic nitrification and aerobic and anaerobic denitrification byAlcaligenes faecalis strain TUD were studied in continuous cultures under various environmental conditions. Both nitrification and denitrification activities increased with the dilution rate. At dissolved oxygen concentrations above 46% air saturation, hydroxylamine, nitrite and nitrate accumulated, indicating that both the nitrification and denitrification were less efficient. The overall nitrification activity was, however, essentially unaffected by the oxygen concentration. The nitrification rate increased with increasing ammonia concentration, but was lower in the presence of nitrate or nitrite. When present, hydroxylamine, was nitrified preferentially. Relatively low concentrations of acetate caused substrate inhibition (K_I=109 M acetate). Denitrifying or assimilatory nitrate reductases were not detected, and the copper nitrite reductase, rather than cytochrome cd, was present. Thiosulphate (a potential inhibitor of heterotrophic nitrification) was oxidized byA. faecalis strain TUD, with a maximum oxygen uptake rate of 140–170nmol O₂·min^-1·mg prot^-1. Comparison of the behaviour ofA. faecalis TUD with that of other bacteria capable of heterotrophic nitrification and aerobic denitrification established that the response of these organisms to environmental parameters is not uniform. Similarities were found in their responses to dissolved oxygen concentrations, growth rate and ammonia concentration. However, they differed in their responses to externally supplied nitrite and nitrate.     

Interactions of cell morphology and transport processes in the lovastatin fermentation

The cholesterol lowering drug, Lovastatin (Mevacor), acts as an inhibitor of HMGCoA reductase, and is produced from an Aspergillus terreus fermentation.Pilot scale studies were carried out in 800 liter fermenters to determine the effects of cell morphology on the oxygen transport properties of this fermentation. Specifically, parallel fermentations giving (i) filamentous mycelial cells, and (ii) discrete mycelial pellets, were quantitatively characterized in terms of broth viscosity, availability of dissolved oxygen, oxygen uptake rates and the oxygen transfer coefficient under identical operating conditions.The growth phase of the fermentation, was operated using a cascade control strategy which automatically changed the agitation speed with the goal of maintaining dissolved oxygen at 50% saturation. Subsequently stepwise changes were made in agitation speed and aeration rate to evaluate the response of the mass transfer parameters (DO, OUR, and k _L a). The results of these experiments indicate considerable potential advantages to the pellet morphology from the standpoint of oxygen transport processes.List of Symbols DO % sat. Dissolved oxygen concentration - k _L a h^–1 Gas-liquid mass transfer coefficient - OUR mmol/dm³h Oxygen uptake rate - P/V KW/m³ Agitator power per unit volume - V _s m/s Superficial air velocity - _app cP Apparent viscosity