Optimal Dissolved Oxygen Concentration Research Articles

Effect of regulating dissolved oxygen concentration in pulp with aerated gas on pyrite flotation

Moderate oxidation is beneficial to the flotation of sulfide ores, but the exact dissolved oxygen (DO) concentration suitable for flotation is still unclear. In this work, a self–made vacuum aeration flotation equipment was designed and assembled to accurately control the DO concentration in pulp by adjusting the relative proportion of O2/N2 during aeration, and the effect of DO concentration on the floatability and surface properties of pyrite as well as the interaction between pyrite and xanthate was studied. Microflotation experiments, contact angle, time–of–flight secondary ion mass spectrometry, X–ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electrochemical and localized electrochemical impedance spectroscopy measurements were conducted in this investigation. The results showed that the flotation recovery of pyrite could be remarkably affected by DO concentration in pulp; it increased first and then decreased with the increase of DO concentration; an optimal DO concentration (DO = 3.3 mg/L) was helpful for pyrite flotation recovery; both the formation of dixanthogen and the avoidance of excessive iron oxide/hydroxide formed on pyrite surface were the key to improve the pyrite recovery. This research is expected to be applied in optimizing the flotation separation by DO-pH combined detection and regulation, strengthening the green flotation of sulfide ore with low collectors, and the design and manufacture of new closed and efficient sulfide ore flotation equipment.

RESILIENCE AND PHYSIOLOGICAL RESPONSES OF THE DOMESTICATED ASIAN REDTAIL CATFISH Hemibagrus nemurus TO HYPOXIA CONDITION

Hypoxia is one of the critical issues in aquaculture production systems as it can lead to physiological disturbances in cultured fish. This research aimed to evaluate the tolerance level and physiological responses of domesticated Asian redtail catfish Hemibagrus nemurus reared in various hypoxia conditions. A total of 12 fish/treatment were acclimated to gradually decreased dissolved oxygen treatments until fish experienced aquatic surface respiratory (ASR) and loss of equilibrium (LOE). Cortisol, haemoglobin, and glucose levels were detected in the blood plasma to evaluate the stress response of the fish to hypoxia. The result showed that ASR of H. nemurus was identified at 2.17 ± 0.14 ppm of dissolved oxygen (DO) concentration with the percentage of ASR was 77.67 ± 9.53%, while LOE critical of H. nemurus happened at 0.63 ± 0.15 ppm of DO where 55.56 ± 4.81% of the fish experienced LOE. There were significant differences in the values of physiological parameters (blood cortisol, haemoglobin, and glucose) between control and treatments as fish experienced LOE (P<0.05). In the present study, it was found that the Asian redtail catfish is classified as a hypoxia-sensitive fish group. Tehis finding is valuable information for the rearing and growing of the fish to provide an optimal DO concentration for their growth and survival.

Simultaneous partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for total nitrogen removal under limited dissolved oxygen and completely autotrophic conditions

Sustainable nitrogen removal from wastewater at reduced energy and/or chemical consumptions is challenging. This paper investigated, for the first time, the feasibility of coupled partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for sustainable autotrophic nitrogen removal. With NH4+-N as the only nitrogen-containing compound in the influent, near-complete nitrogen removal (a total of 97.5 % with a maximal total nitrogen removal rate of 6.64 ± 2.68 mgN/L/d) was achieved in a sequencing batch reactor for a 203-d operation without organic carbon source addition and forced aeration. Anammox (predominated by Candidatus Brocadia) and NDFO bacteria (such as Denitratisoma) were successfully enriched, with total relative abundances up to 11.54 % and 10.19 %, respectively. Dissolved oxygen (DO) concentration was a key factor affecting the coupling of multi (ammonia oxidization, Anammox, NDFO, iron-reduction, etc.) bacterial communities, resulting in different total nitrogen removal efficiencies and rates. In batch tests, the optimal DO concentration was 0.50–0.68 mg/L with a maximal total nitrogen removal efficiency of 98.7 %. Fe(II) in the sludge not only competed with nitrite oxidizing bacteria for DO to prevent complete nitrification, but promoted the transcription of NarG and NirK genes (10.5 and 3.5 times higher than the group without Fe(II) addition) as indicated by the reverse transcription quantitative polymerase chain reaction (RT-qPCR), resulting in increased NDFO rate (by 2.7 times) and promoted NO2−-N generated from NO3−-N, which back fed the Anammox process, achieving near-complete nitrogen removal. The reduction of Fe(III) by iron-reducing bacteria (IRB) and hydrolytic and fermentative anaerobes enabled a sustainable Fe(II)/Fe(III) recycling, avoiding the need in continuous Fe(II) or Fe (III) dosage. The coupled system is expected to benefit the development of novel autotrophic nitrogen removal processes with neglectable energy and material consumptions for the treatment of wastewater with low organic carbon and NH4+-N contents in underdeveloped regions, such as decentralized rural wastewaters.

Isolating, identifying and evaluating of oil degradation strains for the air-assisted microbial enhanced oil recovery process.

Due to the inefficient reproduction of microorganisms in oxygen-deprived environments of the reservoir, the applications of microbial enhanced oil recovery (MEOR) are restricted. To overcome this problem, a new type of air-assisted MEOR process was investigated. Three compounding oil degradation strains were screened using biochemical experiments. Their performances in bacterial suspensions with different amounts of dissolved oxygen were evaluated. Water flooding, microbial flooding and air-assisted microbial flooding core flow experiments were carried out. Carbon distribution curve of biodegraded oil with different oxygen concentration was determined by chromatographic analysis. The long-chain alkanes are degraded by microorganisms. A simulation model was established to take into account the change in oxygen concentration in the reservoir. The results showed that the optimal dissolved oxygen concentration for microbial growth was 4.5~5.5mg/L. The main oxygen consumption in the reservoir happened in the stationary and declining phases of the microbial growth systems. In order to reduce the oxygen concentration to a safe level, the minimum radius of oxygen consumption was found to be about 145m. These results demonstrate that the air-assisted MEOR process can overcome the shortcomings of traditional microbial flooding techniques. The findings of this study can help for better understanding of microbial enhanced oil recovery and improving the efficiency of microbial oil displacement.

Persephonella atlantica sp. nov.: How to adapt to physico-chemical gradients in high temperature hydrothermal habitats

A novel thermophilic, microaerophilic and anaerobic, hydrogen- sulphur- and thiosulphate-oxidising bacterium, designated MO1340T, was isolated from a deep-sea hydrothermal chimney collected from the Lucky Strike hydrothermal vent field on the Mid-Atlantic Ridge. Cells were short, motile rods of 1.4–2.2μm length and 0.5–0.8μm width. Optimal growth was observed for a NaCl concentration of 2.5 % (w/v) at pH 6.5. As for other members of the genus Persephonella, strain MO1340T was strictly chemolithoautotrophic and could oxidise hydrogen, elemental sulphur or thiosulphate using oxygen as electron acceptor. Anaerobic nitrate reduction using hydrogen could also be performed. Each catabolic reaction had a different optimal growth temperature (65 to 75°C) and an optimal dissolved oxygen concentration (11.4 to 119.7 μM at 70°C for aerobic reactions) that varied according to the electron donors utilised. These experimental results are consistent with the distribution of these catabolic substrates along the temperature gradient observed in active hydrothermal systems. They strongly suggest that this adaptive strategy could confer a selective advantage for strain MO1340T in the dynamic part of the ecosystem where hot, reduced hydrothermal fluid mixes with cold, oxygenated seawater. Phylogenetic analysis indicated that strain MO1340T was a member of the genus Persephonella within the order Hydrogenothermales as it shared a 16S rRNA gene sequence similarity <95.5 % and ANI respectively 75.66 % with closest described Persephonella (P. hydrogeniphila 29WT). On the basis of the physiological and genomic properties of the new isolate, the name Persephonella atlantica sp. nov. is proposed. The type strain is MO1340T (=UBOCC-M-3359T =JCM 34026T).

Combination of cascade and feed-forward constrained control for stable partial nitritation with biomass retention

Ammonium removal is a key step in wastewater treatment which can be accomplished biologically. An interesting process option for this purpose is coupling partial nitritation with the Anammox process. The goal of the partial nitritation process is to convert half of the ammonium in the influent stream into nitrite, so both can be later converted into dinitrogen gas by the Anammox reaction. To obtain a stable partial nitritation, ammonium oxidizing bacteria (AOB) have to prevail over nitrite oxidizing bacteria (NOB) so as to avoid further conversion of nitrite into nitrate. The dissolved oxygen concentration is a key variable for the functional group selection. In this study, a constrained combination of cascade and feedforward control is proposed for reactors with biomass retention, aimed at suppressing unwanted NOB while keeping a nitrite:ammonium ratio suitable for coupling with Anammox. The master controller, aimed to regulate this effluent ratio, generates the set-point for the dissolved oxygen concentration slave controller. In addition to the cascade controller feedback loop, a feed-forward controller calculates the optimal dissolved oxygen concentration based on the current influent stream flow rate and concentrations. The resulting dissolved oxygen concentration set-point is compared to constraints that guarantee the suppression of NOB and survival of AOB. The proposed control strategy is simple to apply in common wastewater treatment plants with biomass retention. A sensitivity analysis is performed to assess the effect of model parameters uncertainty on the controller constraints and to determine which parameters need to be identified with more precision to avoid instability or poor results. The performance and the effect of the uncertainty of the most sensitive parameters on the proposed control algorithm are assessed through simulation using realistic streams as inputs of the process.

Evaluations of biofilm thickness and dissolved oxygen on single stage anammox process in an up-flow biological aerated filter

The overall influence of dissolved oxygen (DO) concentrations and biofilm thicknesses on single stage anammox process performance has been evaluated in this study. Results indicated the biofilm displayed a rapid initial increase and followed by a relatively slower formation rate during the operational period. The optimal DO concentration could be determined from a variety of biofilm thicknesses and as well the best biofilm thickness was required among different DO levels. In our lab-scale single stage anammox reactor with a constant hydraulic retention time of 1.0h and influent ammonium of 400mgL−1, an optimal nitrogen removal capacity was acquired (TN removal loading of 2.18kg-Nm−3d−1) at the DO level of 0.6mgL−1 and biofilm thickness of 700μm. Species identification showed that Nitrosomonas related aerobic ammonium-oxidizing bacteria (AerAOB) and Candidatus Brocadia fulgida-like anaerobic ammonium-oxidizing bacteria (AnAOB) were the predominant functional bacteria mixed together with each other and exhibited no distinct niche. However, AerAOB exhibited higher biodiversity at the thinner biofilm while AnAOB showed a stable but lower biodiversity. Moreover, the population of AnAOB was smaller along with more scattered cells at the thinner biofilm while they trended to form specific irregular cauliflower-like zooglea as biofilm thickness increased.

Effects of dissolved oxygen on microbial community of single-stage autotrophic nitrogen removal system treating simulating mature landfill leachate

The performance of four identical sequencing biofilm batch reactors (SBBR) for autotrophic nitrogen removal was investigated with 2000mg/L ammonia-containing mature landfill leachate at 30°C. The main objective of this study was to evaluate the effects of dissolved oxygen (DO) on the performance and microbial community of single-stage nitrogen removal using anammox and partial nitritation (SNAP) system. At an applied load of 0.5kgNm−3d−1, average total nitrogen removal efficiency (TNRE) above 90% was long-term achieved with an optimal DO concentration of 2.7mg/L. The microelectrode-measured profiles showed the microenvironments inside the biofilms. 16S ribosomal Ribonucleic Acid (rRNA) amplicon pyrosequencing and denaturing gradient gel electrophoresis (DGGE) were used to analyze the microbial variations of different DO concentrations and different positions inside one reactor.

Treatment performance and microbial diversity under dissolved oxygen stress conditions: Insights from a single stage IFAS reactor treating municipal wastewater

In the present study, assessment of a single stage integrated fixed film activated sludge (IFAS) reactor treating municipal wastewater and subjected to various dissolved oxygen (DO) stresses, is done in terms of treatment performance and changes in bio-chemical characteristics of activated sludge. Results obtained from experimental revealed that the DO concentrations of 0.5, 2.5, and 4.5mgL−1 affected significantly the performance of IFAS reactor. The optimal DO concentrations for the efficient removals of organics, nitrification, denitrification, and total nitrogen were recorded as 4.5, 4.5, 2.5, and 2.5mgL−1, respectively. Biological phosphorus removal (BPR) efficiency of pilot deteriorated significantly at high DO (4.5mgL−1) levels. Insignificant variation in SVI values (190–245mLg−1) was observed at different DO phase experiments. The macromolecular composition) of activated sludge was also determined in terms of lipids, proteins and polysaccharides content. Moreover, activated sludge characterization results revealed that the dominance of micro fauna (ciliated protozoa) and microorganisms (gram characteristics) was different at different DO levels. Overall, the optimal DO concentration was suggested as 4.5mgL−1 for the IFAS system as it not only achieved high organics removal efficiency and but also minimized the sludge production with high sludge retention time.

Impact of coexistence of flocs and biofilm on performance of combined nitritation-anammox granular sludge reactors

Nitrogen (N) removal from high-strength wastewater can be accomplished in single-stage combined nitritation-anammox reactors with suspended growth biomass composed of floccular sludge, granular sludge, or of any mix of these 2 different sludge fractions. To date, the influence of floccular biomass on granular sludge reactor performance and stability has not been investigated experimentally or numerically. To address this knowledge gap, two 1D multi-species models were developed in Aquasim to assess the importance of small levels of flocs in putatively granular sludge combined nitritation-anammox reactors for different bulk oxygen concentrations and organics loads. The models included the growth and decay of aerobic ammonium-oxidizing organism (AOO), nitrite-oxidizing organisms (NOO), heterotrophic organisms (OHO), and anammox organisms (AMO) in exclusively granular sludge reactors, and in granular sludge reactors with small levels (∼5% of total biomass) of flocs. While maximum N removal efficiencies were similar for both model structures, floc addition led to a lower optimal dissolved oxygen concentration (DO) as well as a narrower maximum N removal peak, suggesting that small levels of floccular material may decrease process robustness to bulk oxygen changes. For some DO levels, this led to drastic efficiency drops. Furthermore, floc addition also led to substantial segregation in activity and microbial population distribution, with AOO, NOO and OHO concentrated in flocs and AMO concentrated in granules. Increased organic loading (COD:N = 4:3) improved maximum N removal efficiency in both model structures, but yielded substantially different predictions for optimal DO setpoint and process robustness to variations in DO. Taken together, our results indicate that even small levels of floccular biomass in biofilm reactors can have profound implications for reactor performance and optimization and for segregation of linked microbial processes, and suggest that the common practice of neglecting small levels of floccular material in biofilm models and in practice may lead to erroneous predictions.

Inhibition of Tumor Growth in a Mouse Xenograft Model by the Humanized Anti-HGF Monoclonal Antibody YYB-101 Produced in a Large-Scale CHO Cell Culture

The humanized anti-hepatocyte growth factor (HGF) monoclonal antibody (mAb) YYB-101 is a promising therapeutic candidate for treating various cancers. In this study, we developed a bioprocess for large-scale production of YYB-101 and evaluated its therapeutic potential for tumor treatment using a xenograft mouse model. By screening diverse chemically defined basal media formulations and by assessing the effects of various feed supplements and feeding schedules on cell growth and antibody production, we established an optimal medium and feeding method to produce 757 mg/l of YYB-101 in flask cultures, representing a 7.5-fold increase in titer compared with that obtained under non-optimized conditions. The optimal dissolved oxygen concentration for antibody production was 70% pO2. A pH shift from 7.2 to 7.0, rather than controlled pH of either 7.0 or 7.2, resulted in productivity improvement in 5 L and 200 L bioreactors, yielding 737 and 830 mg/ml of YYB-101, respectively. The YYB-101 mAb highly purified by affinity chromatography using a Protein A column and two-step ion exchange chromatography effectively neutralized HGF in a cell-based assay and showed potent tumor suppression activity in a mouse xenograft model established with human glioblastoma cells.

Nonlinear GPC with In-place Trained RLS-SVM Model for DOC Control in a Fed-batch Bioreactor

In this study, Saccharomyces cerevisiae (baker's yeast) was produced in a fed-batch bioreactor at the optimal dissolved oxygen concentration (DOC) and growth medium temperature. However, it is very difficult to control the DOC using conventional controllers because of the poorly understood and constantly changing dynamics of the bioprocess. A generalized predictive controller (GPC) based on a nonlinear autoregressive integrated moving average exogenous (NARIMAX) model is presented to stabilize the DOC by manipulation of air flow rate. The NARIMAX model is built by an improved recursive least-squares support vector machine, which is trained by an in-place computation scheme and avoids the computation of the inverse of a large matrix and memory reallocation. The proposed nonlinear GPC algorithm requires little preliminary knowledge of the fermentation process, and directly obtains the nonlinear model in matrix form by using iterative multiple modeling instead of linearization at each sampling period. By application of an on-line bioreactor control, experimental results demonstrate the robustness, effectiveness and advantages of the new controller.

Study on Simultaneous Nitrification–Denitrification in Membrane Bioreactor

This study was conducted to evaluate the performance of membrane bioreactors (MBR) for organics and nitrogen removal. The membrane bioreactor was fed with domestic sewage and operated at different contents of dissolved oxygen (DO), different hydraulic retention time (HRT), and various mixed liquor suspended solid (MLSS) concentrations. The results showed that the distribution of DO level in MBR imposed a significant effect on simultaneous nitrification–denitrification (SND), and the optimal DO concentration should be controlled between 0.5 mg/L to1 mg/L. The denitrification was found to show the best performance for total nitrogen (TN) removal when the HRT reached 5 hours. Higher MLSS concentration led to the improvement in TN removal and the optimal MLSS concentration was 9000mg/L. In fact, more than 90% Chemical Oxygen Demand (COD) and ammonia nitrogen (NH3-N) were reduced when the MLSS concentration exceeded 3000mg/L.

Simultaneous removal of organic substances and nitrogen in pilot-scale submerged membrane bioreactors treating digested traditional Chinese medicine wastewater

Influence of dissolved oxygen (DO) concentration on organic substances and nitrogen removal was investigated in pilot-scale submerged membrane bioreactors (SMBRs) degrading digested traditional Chinese medicine (TCM) wastewater. The DO concentration for each SMBR was maintained at 0.6, 1.2, 3.0 and 5.0 mg/L, respectively. Experimental results indicated that DO had an important effect on the chemical oxygen demand (COD) removal and nitrogen conversion. Simultaneous nitrification and denitrification were able to be accomplished in the continuous-aeration SMBRs by controlling ambient DO concentration. It is postulated that, because of the oxygen diffusion limitation, an anoxic micro-zone was formed inside the flocs where the denitrification occurred. From four tested DO levels, the optimal DO concentration favoring COD and total nitrogen removal was found to be 1.2 mg/L. At that DO level, more than 96% COD and 90% total nitrogen were degraded. A model was developed for the SMBR process of degrading the digested TCM effluent, taking into account the biological degradation and membrane separation. The model well fitted the laboratory data, and was able to simulate the removal of COD and nitrogen. It is suggested that the model proposed could reflect and manage the operation of SMBR treating a certain kind of industrial wastewaters.

Analysis of nitrifying bacterial communities in aerobic biofilm reactors with different DO conditions using molecular techniques

In order to assess the relationship between the dissolved oxygen (DO) concentration and the characteristics of nitrifying bacterial communities in an aerobic biofilm reactor, molecular techniques including denaturing gradient gel electrophoresis (DGGE)/cloning based on PCR targeting 16S rRNA and the amoA gene and fluorescence in situ hybridisation (FISH) were conducted. The D-1, D-2, D-3 and D-4 reactors with different DO concentrations (1, 3, 5 and 7 mg/L, respectively) were set up in the thermostat and acclimated. The optimal DO concentration with stable nitrification efficiency was above 5.0 mg/L. As was shown by the results of DGGE and cloning, the community of ammonia-oxidising bacteria (AOB) and the ratio of Nitrosomonas sp. changed only slightly despite their differing nitrification efficiencies. The results of FISH indicated that higher DO concentrations resulted in an increase in AOB and nitrite-oxidising bacteria (NOB), and a reduction in heterotrophic microorganisms. The INT-dehydrogenase activity (DHA) test demonstrated that the activity of AOB decreased with reductions in the DO concentration. This means that the DO concentration does not influence the community of AOB, but rather the activity of AOB. In the relationship between the attached biomass and the nitrification efficiency, only the active biomass affected the nitrification efficiencies.

Oxygen and carbon dioxide kinetic challenges for thermophilic mineral bioleaching processes.

Agitated bacterial tank bioleaching reactors are currently sparged with air to satisfy both oxygen and CO(2) requirements of microbial cells. Under high-sulphide loading conditions, as is the case with high-grade metal sulphide concentrates, the microbial and chemical demand for oxygen is significantly increased during the bioleaching process. Sparging with enriched oxygen gas may offer an alternative process option to increased agitation and sparged aeration, to overcome the mass transfer difficulties at elevated temperatures where thermophilic Archaea, rather than Bacteria, are used. In the case of air sparging, the DO (dissolved oxygen) concentration in tank reactors could not be increased to a point where it would become inhibitory due to the limited oxygen content of air (20.9% O(2)). The use of enriched oxygen in such reactors at large scale does, however, pose its own set of process risks. The first aim of this investigation was, therefore, to determine the effects of various DO concentrations, in both the limiting and inhibitory ranges, on the microbial activity of Sulfolobus sp. U40813, a typical thermophilic mineral-leaching archaeon. Secondly, the effect of CO(2) concentration on the rate of ferrous iron oxidation was investigated. Both the oxygen and CO(2) kinetics were examined in controlled batch cultures at 78 degrees C, using ferrous sulphate and potassium tetrathionate as energy sources. The optimal DO concentration for iron oxidation was found to be between 1.5 and 4.1 mg.l(-1). The use of elevated DO concentrations (above 4.1 mg.l(-1)) inhibited the ferrous oxidation rates. The optimal gas CO(2) concentration for ferrous iron oxidation was found to be in the range 7-17% (v/v). The iron oxidation rates were, however, severely limited at CO(2) concentrations less than 7%, indicating that the CO(2) supply was limiting in this range and inhibited the microbial growth rate.

Sensitivity analysis of a biofilm model describing a one-stage completely autotrophic nitrogen removal (CANON) process.

A mathematical model for nitrification and anaerobic ammonium oxidation (ANAMMOX) processes in a single biofilm reactor (CANON) was developed. This model describes completely autotrophic conversion of ammonium to dinitrogen gas. Aerobic ammonium and nitrite oxidation were modeled together with ANAMMOX. The sensitivity of kinetic constants and biofilm and process parameters to the process performance was evaluated, and the total effluent concentrations were, in general, found to be insensitive to affinity constants. Increasing the amount of biomass by either increasing biofilm thickness and density or decreasing porosity had no significant influence on the total effluent concentrations, provided that a minimum total biomass was present in the reactor. The ANAMMOX process always occurred in the depth of the biofilm provided that the oxygen concentration was limiting. The optimal dissolved oxygen concentration level at which the maximum nitrogen removal occurred is related to a certain ammonium surface load on the biofilm. An ammonium surface load of 2 g N/m2. d, associated with a dissolved oxygen concentration level of 1.3 g O2/m3 in the bulk liquid and with a minimum biofilm depth of 1 mm seems a proper design condition for the one-stage ammonium removal process. Under this condition, the ammonium removal efficiency is 94% (82% for the total nitrogen removal efficiency) (30 degrees C). Better ammonium removal could be achieved with an increase in the dissolved oxygen concentration level, but this would strongly limit the ANAMMOX process and decrease total nitrogen removal. It can be concluded that a one-stage process is probably not optimal if a good nitrogen effluent is required. A two-stage process like the combined SHARON and ANAMMOX process would be advised for complete nitrogen removal.

Optimal dissolved oxygen concentration for the production of chitinases by Serratia marcescens

Chitinase production by Serratia marcescens 990E was maximal (34 U/ml) for a dissolved oxygen concentration ranging from 20 to 50% saturation. Chitin addition (6 g/l) at the end of fermentation reactivated the growth but did not bring about further chitinase accumulation. Four chitinases with different isoelectric points were separated.

Effect of the levels of dissolved oxygen on the expression of recombinant proteins in four recombinant Escherichia coli strains.

Four recombinant strains of Escherichia coli were examined for the effects of the dissolved oxygen level on the level of biomass, the plasmid content, and the level of recombinant protein at the stationary phase of batch growth. Strains JM101/pYEJ001, and TB-1/pYEJ001 (encoding chloramphenicol acetyltransferase), and strain TB-1/p1034, and TB-1/pUC19 (encoding beta-galactosidase) were grown at the constant dissolved oxygen levels of 0, 50, and 100% air saturation, as well as in the absence of dissolved oxygen control. The biomass of all strains under constant aerobic conditions was 12-36 times higher than that under anaerobic conditions, but was the same as or slightly higher than that without dissolved oxygen control. The plasmid content in all strains under aerobic conditions was 2.9-11.7 times higher than that under aerobic conditions. The optimal dissolved oxygen concentration for the specific activity of recombinant proteins was dependent upon the strain. In no strain were constant aerobic conditions optimal. However, because of the effect on biomass, controlled aerobic conditions were optimal for the volumetric activity of recombinant protein in all but one strain.

The effect of the dissolved oxygen concentration and anabolic limitations on the behaviour of Rhizobium ORS571 in chemostat cultures.

Chemostat cultures of Rhizobium ORS571 limited by the supply of oxygen or an anabolic substrate contained poly-beta-hydroxybutyrate (PHB). Low amounts of PHB (about 10%) were present in ammonia- or nitrate-limited cultures; higher amounts were found in Mg++-limited cultures (about 20%) and in oxygen-limited nitrogen-fixing cultures (37%). A method is described to calculate YATP values (g PHB-free biomass . mol-1 ATP) from the Ysucc values (g dry wt . mol-1 succinate) measured. Ysucc and YATP values in cultures limited by the supply of an anabolic substrate and in the oxygen-limited ammonia-assimilating culture were much lower than the values found in the PHB-free succinate-limited cultures. This shows that uncoupling of growth and energy production occurred. Therefore, H2/N2 ratio (mol hydrogen formed per mol nitrogen fixed) in nitrogen-fixing cultures could not be calculated from the comparison of the YATP value found in the nitrogen-fixing culture and the value found in the corresponding ammonia-assimilating culture. Although the optimal dissolved oxygen concentration (d.o.c.) for nitrogen-fixing cultures of Rhizobium ORS571 is 5 or 10 microM, nitrogen-fixing cultures could be obtained up to a d.o.c. of 40 microM. Not only nitrogenase but also hydrogenase was active at this d.o.c. However, accumulation of PHB (10%) may indicate that cultures grown at unfavourable oxygen concentrations (15-40 microM O2) were N-limited rather than energy-limited, which may be the result of partial inactivation or repression of nitrogenase at a higher d.o.c.