Determination of carbon dioxide evolution rate using on-line gas analysis during dynamic biodegradation experiments

Respirometry is a precious tool for determining the activity of microbial populations. The measurement of oxygen uptake rate is commonly used but cannot be applied in anoxic or anaerobic conditions or for insoluble substrate. Carbon dioxide production can be measured accurately by gas balance techniques, especially with an on-line infrared analyzer. Unfortunately, in dynamic systems, and hence in the case of short-term batch experiments, chemical and physical transfer limitations for carbon dioxide can be sufficient to make the observed carbon dioxide evolution rate (OCER) deduced from direct gas analysis very different from the biological carbon dioxide evolution rate (CER).To take these transfer phenomena into account and calculate the real CER, a mathematical model based on mass balance equations is proposed. In this work, the chemical equilibrium involving carbon dioxide and the measured pH evolution of the liquid medium are considered. The mass transfer from the liquid to the gas phase is described, and the response time of the analysis system is evaluated.Global mass transfer coefficients (K(L)a) for carbon dioxide and oxygen are determined and compared to one another, improving the choice of hydrodynamic hypotheses. The equations presented are found to give good predictions of the disturbance of gaseous responses during pH changes.Finally, the mathematical model developed associated with a laboratory-scale reactor, is used successfully to determine the CER in nonstationary conditions, during batch experiments performed with microorganisms coming from an activated sludge system. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 243-252, 1997.     

Modeling and experimental validation of carbon dioxide evolution in alkalophilic cultures

The chemical reactions involving carbon dioxide in mineral culture media are considered. A mathematic model is set up, based on published data, which is valid at pH values below 9, and in which the nonideality of the solution is taken into account. The crucial parameter is the constant expressing the equilibrium between carbon dioxide and bicarbonate, K(1).The reactions were studied in three different aqueous solutions: water, mineral salt medium, and a suspension with nongrowing bacterial cells. For each situation, three methods were compared for the determination of the bicarbonate concentration in the solution: equilibrium state total carbon analysis, dynamic monitoring of the rate of acid or alkali addition, and dynamic measurement of the carbon dioxide gas phase mole fraction.In a batch-stirred tank reactor, the equilibrium constant K(1) agreed with the published value, and the three bicarbonate analysis methods give the same results. If the nonideality is not taken into account, the result significantly differed from the published value and is likely to be incorrect.A real alkalophilic process, using Acinetobacter calcoaceticus in a continuous stirred tank reactor at steady state, also gave results that are in accord with the literature. However, the results do not allow validation of the equation expressing the nonideality.The steady state in the batch system and in continuous culture can be well described with the mathematical model. However, in the transient state there are some unexplained differences between simulation and measurement.     

The active transport of oxygen and carbon dioxide into the swim-bladder of fish

The active transport of oxygen and carbon dioxide into the swim-bladder of fish is discussed. The rete mirabile is a capillary network which is involved in the gas secretion into the bladder. The rete is regarded as a counter-current multiplier. Lactic acid which is produced in the gas gland generates in the rete single concentrating effects for oxygen and carbon dioxide; i.e., for equal partial pressures the concentrations of the gases in the afferent rete capillaries are higher than those in the efferent ones. The single concentrating effects were calculated from measurements of sea robin blood (Root, 1931). The multiplication of these effects within the rete for different rete lengths and different transport rates was numerically evaluated. The calculated O₂ and CO₂ pressures in the bladder are in good agreement with the experimental results of Scholander and van Dam (1953). The descent velocities at equilibrium between bladder pressure and hydrostatic pressure are discussed for fishes with different rete lengths.     

Biocatalytic carbon capture via reversible reaction cycle catalyzed by isocitrate dehydrogenase

The practice of carbon capture and storage (CCS) requires efficient capture and separation of carbon dioxide from its gaseous mixtures such as flue gas, followed by releasing it as a pure gas which can be subsequently compressed and injected into underground storage sites. This has been mostly achieved via reversible thermochemical reactions which are generally energy-intensive. The current work examines a biocatalytic approach for carbon capture using an NADP(H)-dependent isocitrate dehydrogenase (ICDH) which catalyzes reversibly carboxylation and decarboxylation reactions. Different from chemical carbon capture processes that rely on thermal energy to realize purification of carbon dioxide, the biocatalytic strategy utilizes pH to leverage the reaction equilibrium, thereby realizing energy-efficient carbon capture under ambient conditions. Results showed that over 25 mol of carbon dioxide could be captured and purified from its gas mixture for each gram of ICDH applied for each carboxylation/decarboxylation reaction cycle by varying pH between 6 and 9. This work demonstrates the promising potentials of pH-sensitive biocatalysis as a green-chemistry route for carbon capture.     

Carbon dioxide inhibition of yeast growth in biomass production

Saccharomyces cerevisiae was grown under aerobic and substrate-limiting conditions for efficient biomass production. Under these conditions, where the sugar substrate was fed incrementally, the growth pattern of the yeast cells was found to be uniform, as indicated by a constant respiratory quotient during the entire growing period. The effect of carbon dioxide was investigated by replacing portions of the nitrogen in the air stream with carbon dioxide, while maintaining the oxygen content at the normal 20% level, so that identical oxygen transfer rate and atmospheric pressure were maintained for all experiments with different partial pressures of carbon dioxide. Inhibition of yeast growth was negligible below 20% CO₂ in the aeration mixture. Slight inhibition was noted at the 40% CO₂ level and significant inhibition was noted above the 50% CO₂, level, corresponding to 1.6 × 10^?2M of dissolved CO₂ in the fermentor broth. High carbon dioxide content in the gas phase also inhibited the fermentation activity of baker's yeast.     

Isoelectric precipitation of soybean protein using carbon dioxide as a volatile acid

A novel process is presented for the isoelectric precipitation of soy protein, using carbon dioxide as a volatile acid. By contacting a soy meal extract with pressurized carbon dioxide, the solution pH was decreased to the isoelectric region of the soy proteins. Complete precipitation of the precipitable soy proteins could be achieved for protein concentrations up to 40 g/l at pressures less than 50 bar. Isoelectric precipitation with a volatile acid enabled accurate control of the solution pH by pressure and eliminated the local pH overshoot, usual in conventional precipitation techniques. The advantage of the improved precipitation control was reflected by the morphology of the precipitate particles. Protein aggregates formed by CO₂ were perfectly spherical whereas protein precipitated by sulfuric acid had an irregular morphology. The influence of process variables to control particle size is discussed.     

Escherichia coli cad operon functions as a supplier of carbon dioxide

We examined the gene expression of the Escherichia coli cad operon, which consisted of the genes cadB and cadA (lysine decarboxylase), using cells possessing cadB–lacZ fusion gene. The cad operon was expressed when O₂ was limited, and the expression was optimal at pH6.3. The β-galactosidase activity was lowered by the addition of sodium carbonate to the medium. The expression of the cad operon was reduced in cells containing the plasmid-encoding ornithine decarboxylase, which produced carbon dioxide, indicating that the gene expression of the cad operon was regulated by carbon dioxide (or its derivatives). It is known that the Krebs cycle is a major pathway for producing carbon dioxide, and that its activity is repressed when O₂ is limited. Thus, our present results suggested that the physiological role of the cad operon is to supply carbon dioxide when its internal level is lowered under O₂-limiting conditions at a low pH.     

Carbon dioxide efflux accompanies release of fertilization acid from sea urchin eggs

“Fertilization acid” is released from sea urchin eggs upon fertilization and decreases the pH of the surrounding seawater. In bicarbonate-free artificial seawater flushed with nitrogen gas, the pH shift still occurs but returns to the original value in a few minutes, suggesting that the released acid is volatile. A likely candidate for a volatile acid is carbon dioxide released from the eggs. Therefore, the total CO₂ content of seawater was measured pre- and post-fertilization and was found to be correlated stoichiometrically with released proton equivalents, leading to the conclusion that fertilization acid is largely carbon dioxide. Manometric analysis of cell extracts and ashed eggs suggest that the carbon dioxide may be stored in the unfertilized egg as an inorganic carbonate.     

A simple carbon dioxide injection system for photosynthetic studies

A simple carbon dioxide injection system has been developed for the maintenance of CO₂ concentrations in semiclosed cuvette systems suitable for photosynthesis and gaseous pollutant studies. The device injects small volumes of pure carbon dioxide into the cuvette in response to a signal from an infrared gas analyzer.     

Errors in differential infrared carbon dioxide analysis resulting from water vapor

Water vapor was added differentially to the gas streams entering the cells of three makes of differential infrared carbon dioxide analysers. Analyser deflections were compared with those expected from dilution of the carbon dioxide by the additional gas. Tests were made at 0, 365, and 730 cm³ m^–3 concentrations of carbon dioxide, and with the dewpoint in one cell of the analysers held constant at 15, 20, or 25°C. None of the analysers always responded in the ways predicted from dilution. The results showed that errors of a few cm³ m^–3 could occur in estimates of carbon dioxide differentials using the theoretical correction for dilution. Furthermore the amount of error varied with the carbon dioxide range, the difference in water content, and in some cases the dewpoint range.     

Measurement of carbon dioxide compensation points of freshwater algae

A technique is described for the measurement of total dissolved inorganic carbon by acid release as CO₂ followed by its conversion to methane and detection by flame ionization in a modified gas chromatograph. This method was used to determine the dissolved inorganic carbon concentration reached at compensation point when algae were allowed to photosynthesize in a closed system in a buffer at known pH, and the CO₂ compensation point was calculated from this concentration. The CO₂ compensation points of 16 freshwater algae were measured at acid and alkaline pH in air-saturated medium: at acid pH the CO₂ compensation points ranged from 4.8 to 41.5 microliters per liter while at alkaline pH they ranged from 0.2 to 7.2 microliters per liter. Removal of O₂ from the medium caused a slight lowering of compensation point at acid pH but had little effect at alkaline pH. These low, O₂-insensitive compensation points are characteristic of C₄ plants. It is suggested that these low CO₂ compensation points are maintained by an active bicarbonate uptake by algae especially at alkaline pH.     

Carbon dioxide injection method for enhancing hydrogenotrophic denitrification of secondary wastewater effluent in fixed bed reactor

This study presents an optimal injection method for using carbon dioxide as a carbon source for the hydrogenotrophic denitrification of secondary wastewater effluent in a laboratory-scale fixed bed reactor (FBR). The FBR was operated under three conditions: a continuous CO₂ supply, periodic CO₂ supply, and without a CO₂ supply. The continuous operation of the FBR without carbon dioxide injection resulted in an increase in pH to 10 and a noticeable level of nitrite accumulation. The continuous co-injection of carbon dioxide and hydrogen gas decreased the pH to a range of 6 ～ 8, but the denitrification efficiency decreased to 29%. The co-injection of carbon dioxide decreased the maximum dissolved hydrogen concentration and hydrogen mass transfer rate by 25 and 61%, respectively. Compared to the continuous injection method, a periodic injection of carbon dioxide increased the denitrification efficiency from 28.6 to 85% as the hydrogen flow rate and hydraulic retention time (HRT) increased. With the periodic injection of carbon dioxide, the nitrite accumulation appeared to be insignificant as the hydrogen flow rate increased.     

Bioconversion of carbon dioxide to methane using hydrogen and hydrogenotrophic methanogens

Biogas produced from organic wastes contains energetically usable methane and unavoidable amount of carbon dioxide. The exploitation of whole biogas energy is locally limited and utilization of the natural gas transport system requires CO₂ removal or its conversion to methane. The biological conversion of CO₂ and hydrogen to methane is well known reaction without the demand of high pressure and temperature and is carried out by hydrogenotrophic methanogens. Reducing equivalents to the biotransformation of carbon dioxide from biogas or other resources to biomethane can be supplied by external hydrogen. Discontinuous electricity production from wind and solar energy combined with fluctuating utilization cause serious storage problems that can be solved by power-to-gas strategy representing the production of storable hydrogen via the electrolysis of water. The possibility of subsequent repowering of the energy of hydrogen to the easily utilizable and transportable form is a biological conversion with CO₂ to biomethane. Biomethanization of CO₂ can take place directly in anaerobic digesters fed with organic substrates or in separate bioreactors. The major bottleneck in the process is gas-liquid mass transfer of H₂ and the method of the effective input of hydrogen into the system. There are many studies with different bioreactors arrangements and a way of enrichment of hydrogenotrophic methanogens, but the system still has to be optimized for a higher efficiency. The aim of the paper is to gather and critically assess the state of a research and experience from laboratory, pilot and operational applications of carbon dioxide bioconversion and highlight further perspective fields of research.     

Responses of trees to elevated carbon dioxide and climate change

The enhancement in photosynthesis at elevated concentration of carbon dioxide level than the ambient level existing in the atmosphere is widely known. However, many of the earlier studies were based on instantaneous responses of plants grown in pots. The availability of field chambers for growing trees, and long-term exposure studies of tree species to elevated carbon dioxide, has changed much of our views on carbon dioxide acting as a fertiliser. Several tree species showed acclimation or even down-regulation of photosynthetic responses while a few of them showed higher photosynthesis and better growth responses. Whether elevated levels of carbon dioxide can serve as a fertilizer in a changed climate scenario still remains an unresolved question. Forest-Air-Carbon dioxide-Enrichment (FACE) sites monitored at several locations have shown lately, that the acclimation or down regulation as reported in chamber studies is not as wide-spread as originally thought. FACE studies predict that there could be an increase of 23–28% productivity of trees at least till 2050. However, the increase in global temperature could also lead to increased respiration, and limitation of minerals in the soil could lead to reduced responses in growth. Elevated carbon dioxide induces partial closure of leaf stomata, which could lead to reduced transpiration and more economical use of water by the trees. Even if the carbon dioxide acts as a fertilizer, the responses are more pronounced only in young trees. And if there are variations in species responses to growth due to elevated carbon dioxide, only some species are going to dominate the natural vegetation. This will have serious implications on the biodiversity and the structure of the ecosystems. This paper reviews the research done on trees using elevated CO₂ and tries to draw conclusions based on different methods used for the study. It also discusses the possible functional variations in some tree species due to climate change.     

Equilibrium carbon isotope effect on a decarboxylation reaction

Carbon isotope exchange equilibrium between carbon dioxide and the β-carboxyl carbon of D-isocitric acid has been achieved by means of the enzyme isocitrate dehydrogenase. Measurements of the isotopic compositions at these two sites reveal that the carboxyl carbon is enriched in ¹³C relative to carbon dioxide. The isotope exchange equilibrium constant is 1.0027 at 25°, pH 7.5.     

Measurement of dissolved carbon dioxide.

Several probes for measuring dissolved carbon dioxide (CO2) concentration were installed in a 68-litre fermentor and their effectiveness compared. Submerged silastic rubber tubing gave reproducible results over a wide range of operating conditions and was generally superior to all other probes evaluated. The silastic rubber probe was used to compare the partial pressure of CO2 in viscous fermentation media with that in the fermentor exhaust gas. No significant difference was found. Results show that determination of the CO2 partial pressure in the exhaust gas gives an excellent approximation of the partial pressure of dissolved CO2 in the liquid medium, eliminating the need for measurement of CO2 concentration in the broth.     

Ternary effects on the gas exchange of isotopologues of carbon dioxide

The ternary effects of transpiration rate on the rate of assimilation of carbon dioxide through stomata, and on the calculation of the intercellular concentration of carbon dioxide, are now included in standard gas exchange studies. However, the equations for carbon isotope discrimination and for the exchange of oxygen isotopologues of carbon dioxide ignore ternary effects. Here we introduce equations to take them into account. The ternary effect is greatest when the leaf-to-air vapour mole fraction difference is greatest, and its impact is greatest on parameters derived by difference, such as the mesophyll resistance to CO(2) assimilation, r(m) . We show that the mesophyll resistance to CO(2) assimilation has been underestimated in the past. The impact is also large when there is a large difference in isotopic composition between the CO(2) inside the leaf and that in the air. We show that this partially reconciles estimates of the oxygen isotopic composition of CO(2) in the chloroplast and mitochondria in the light and in the dark, with values close to equilibrium with the estimated oxygen isotopic composition of water at the sites of evaporation within the leaf.     

Effect of changes in the pH and carbon dioxide evolution rate on the measured respiratory quotient of fermentations

The total concentration of dissolved carbon dioxide in fermentation broths is one to two orders of magnitude greater than that of oxygen for pH > 6.5. The rate of change in this total concentration can be sufficiently large to produce a discrepancy between the carbon dioxide transfer rate (CTR) across the gas-liquid interface, available from gas analyses, and the carbon dioxide evolution rate (CER) of biomass in the fermentor. The CER is the variable of most interest to fermentation technologists but cannot be measured directly. The CTR is commonly used to yield the measured respiratory quotient (called here the TQ, or transfer quotient). Evaluation of the real underlying respiratory quotient (RQ), however, requiures the unmeasureable CER. Equations defining the problem are presented and are found to accurately predict the discrepancy between the TQ and the RQ in fed-batch fermentations of Escherichia coli. During the exponential growth phase, the TQ is less than the RQ. A changing pH can cause the TQ to be bigger or smaller than the RQ, while pH fluctuations associated with on-off pH controller action make the CTR and hence the TQ noisy. The RQ is estimated on-line during an E. coli fermentation and is shown to be constant during the fermentation, even though the TQ varies greatly. (c) 1992 John Wiley & Sons, Inc.