Organic acid production from CO2/H2 and CO/H2 by mixed-culture anaerobes

The gases CO, CO₂, and H₂ were used as substrates in anaerobic fermentations producing organic acids. Various mixed bacterial sources were used, including sewage sludge digester effluent, rabbit feces, and soil. Nonsterile microorganism selection was carried out using CO₂/H₂ and CO/H₂ as the primary carbon and energy sources. Cultures were grown in specially designed, high-pressure (to 70 psig) flasks. Methanogenic bacteria were eliminated from the cultures. Liquid products of the fermentations were acetic through caproic acids, with the even-numbered acids predominating. Carbon balances showed conclusively that acetic acid was formed from carbon contained in the CO or CO₂ feed gas. Measurements made included rates of acid product formation, cell density, and degree of gas utilization. Limited characterization of the microorganisms was also performed. Production of organic acids by mixed culture inocula from CO₂/H₂ or CO/H₂ had not been reported previously. Application of this work is to the production of organic chemicals from synthesis gas (SNG), produced by the gasification of fossil fuels (peat, lignite, and various ranks of coals), biomass (agricultural and forest residues, and various biomass crops grown expressly for energy recovery), and municipal solid waste.     

Double-mixing kinetic studies of the reactions of methyl isocyanide and CO with diliganded intermediates of hemoglobin: alpha 2CO beta 2 and alpha 2 beta 2CO

Kinetics of the reactions of CO and methyl isocyanide with two diliganded intermediates of hemoglobin, alpha 2CO beta 2 and alpha 2 beta 2CO, have been studied by double-mixing and microperoxidase methods. The valency hybrids were prepared by high-pressure liquid chromatography. The reaction time courses of ligand combination and dissociation with both of the ligands were biphasic, and in CO combination reaction the zero-time amplitudes of the two phases were independent of the protein concentration. In the presence of 2 M urea the reaction time course was clearly dependent on protein concentration, as the zero-time amplitude of the fast phase increased at lower protein concentrations. These two observations indicate that little dissociation of tetramers into dimers occurs in the absence of urea. Consistent with this, the kinetic data for the reactions of CO best fit a reaction model consisting of two tetrameric species not in rapid equilibrium with each other. Various considerations, however, suggest that the reaction model is more appropriately described as 2D in equilibrium R in equilibrium T. The reaction of triliganded species (Hb4(CO)2Me1) with methyl isocyanide was monophasic, and the reaction model suggested a fast T in equilibrium R structural change after the binding of the third ligand. Although the precise structural nature of the two species remains undefined, it is concluded that the biphasicity in the reactions of the two hybrids is characteristic of the diliganded species only and is independent of the nature of the ligand.     

Life with CO or CO2 and H2 as a source of carbon and energy

An account is presented of the recent discovery of a pathway of growth by bacteria in which CO or CO2 and H2 are sources of carbon and energy. The Calvin cycle and subsequently other cycles were discovered in the 1950s, and in each the initial reaction of CO2 involved adding CO2 to an organic compound formed during the cyclic pathway (for example, CO2 and ribulose diphosphate). Studies were initiated in the 1950s with the thermophylic anaerobic organism Clostridium thermoaceticum, which Barker and Kamen had found fixed CO2 in both carbons of acetate during fermentation of glucose. The pathway of acetyl-CoA biosynthesis differs from all others in that two CO2 are combined with coenzyme A (CoASH) forming acetyl CoA, which then serves as the source of carbon for growth. This mechanism is designated the acetyl CoA pathway and some have called it the Wood pathway. A unique feature is the role of the enzyme carbon monoxide dehydrogenase (CODH), which catalyzes the conversion of CoASH, CO, and a methyl group to acetyl CoA, the final step of the pathway. The pathway involves the reduction of CO2 to formate, which then combines with tetrahydrofolate (THF) to form formyl THF. It in turn is reduced to CH3-THF. The methyl is then transferred to the cobalt on a corrinoid-containing enzyme. From there the methyl is transferred to CODH, and CO and CoASH bind with the enzyme at separate sites. Acetyl CoA is then synthesized. CODH would more properly be called carbon monoxide dehydrogenase-acetyl CoA synthase as it catalyzes oxidation of CO to CO2 and the synthesis of acetyl CoA. The solution of the mechanism of this pathway required more than 30 years, in part because the intermediate compounds are bound to enzymes, the enzymes are extremely sensitive to O2 and must be isolated under strictly anerobic conditions, and the role of a corrinoid and CODH was unprecedented. It is now apparent that this pathway occurs (perhaps with some modification) in many bacteria including the methane and sulfur bacteria. In some humans this pathway is catalyzed by the bacteria of the gut and acetate is produced rather than methane; it is calculated that 2.3 x 10(6) metric tons of acetate are formed daily from CO2. A similar synthesis occurs in the hind gut of termites. It is becoming apparent that the acetyl CoA pathway plays a significant role in the carbon cycle.(ABSTRACT TRUNCATED AT 400 WORDS)     

Metal centers in the anaerobic microbial metabolism of CO and CO2

Carbon dioxide and carbon monoxide are important components of the carbon cycle. Major research efforts are underway to develop better technologies to utilize the abundant greenhouse gas, CO(2), for harnessing 'green' energy and producing biofuels. One strategy is to convert CO(2) into CO, which has been valued for many years as a synthetic feedstock for major industrial processes. Living organisms are masters of CO(2) and CO chemistry and, here, we review the elegant ways that metalloenzymes catalyze reactions involving these simple compounds. After describing the chemical and physical properties of CO and CO(2), we shift focus to the enzymes and the metal clusters in their active sites that catalyze transformations of these two molecules. We cover how the metal centers on CO dehydrogenase catalyze the interconversion of CO and CO(2) and how pyruvate oxidoreductase, which contains thiamin pyrophosphate and multiple Fe(4)S(4) clusters, catalyzes the addition and elimination of CO(2) during intermediary metabolism. We also describe how the nickel center at the active site of acetyl-CoA synthase utilizes CO to generate the central metabolite, acetyl-CoA, as part of the Wood-Ljungdahl pathway, and how CO is channelled from the CO dehydrogenase to the acetyl-CoA synthase active site. We cover how the corrinoid iron-sulfur protein interacts with acetyl-CoA synthase. This protein uses vitamin B(12) and a Fe(4)S(4) cluster to catalyze a key methyltransferase reaction involving an organometallic methyl-Co(3+) intermediate. Studies of CO and CO(2) enzymology are of practical significance, and offer fundamental insights into important biochemical reactions involving metallocenters that act as nucleophiles to form organometallic intermediates and catalyze C-C and C-S bond formations.     

Changes in breath 13CO2/12CO2 consequent to exercise and hypoxia

Because the natural enrichment of carbohydrate with 13C is greater than that of lipid, we hypothesized that the natural enrichment of exhaled CO2 with 13C (EN) could be used to gauge endogenous substrate utilization in exercising human subjects. To test this, EN and the respiratory exchange ratio (R) which equals the respiratory quotient (RQ) in the steady state, were measured simultaneously in seven subjects. Rest and exercise protocols, performed under conditions of room air (sea level) and hypoxic (inspired O2 fraction = 0.15) breathing, were chosen to cause a variety of patterns of oxidative substrate utilization. Work rates were performed both below and above the subject's lactate threshold (LT). Work above the LT was expected to cause the greatest increase in EN reflecting greater utilization of glucose. There was significant intersubject (P less than 0.05) but not intrasubject variability in resting EN. By 40 min of exercise, EN increased significantly (P less than 0.05) over resting values in all exercise protocols during both room air and hypoxia conditions. In the room air studies, we found no difference in EN during the below-LT work, even though there were significant increases in O2 uptake (VO2). In contrast, above-LT work resulted in significantly greater increases in EN by 20 and 40 min of exercise (P less than 0.05). Contrary to our expectations, we observed no separate effect by hypoxia on the EN during exercise. Both EN and R tended to increase from rest to exercise, but during exercise there was no overall correlation between R and the EN. EN reflects changes in endogenous substrate utilization over relatively long periods of time such as at rest, but delays in the appearance of 13CO2 at the mouth due to dilution in body CO2 pools, and possibly isotopic fractionation, preclude the usefulness of EN as an indicator of endogenous fuel mix during short-term exercise.     

Effect of CO2 Concentration During Growth on a CO2 Concentrating Mechanism in White Clover as Predicted from Differential 14CO2/12CO2Uptake

Lehnherr, B. M?chler, F. and N?sberger, J. 1985. Effect of CO₂concentration during growth on a CO₂ concentrating mechanismin white clover as predicted from differential ¹⁴CO₂/¹²CO² uptake.-J. exp. Bot. 36: 1835-1841. White clover was grown at 20 and100 Pa p(CO₂). The CO₂ response of net photosynthesis and differentialuptake of ¹⁴CO₂ and ¹²CO₂ by leaves were measured at varioustemperatures and at various O₂ and CO₂ partial pressures andcompared with predictions from ribulose bisphosphate carboxylase/oxygenasekinetics. Discrepancies between the observed gas exchange characteristicsfor the leaves and those predicted from the enzyme kineticswere interpreted as being due to a CO₂ concentrating mechanism.Plants grown at 20 Pa p(CO₂) showed a higher affinity for CO₂than plants grown at 100 Pa p(CO₂) when measured at 10 ?C. Nodifference in affinity was found at 30 ?C. The postulated CO₂concentrating effect was greater in plants grown at low CO₂than in plants grown at high CO₂ concentration and occurredonly at low temperature and low CO₂ partial pressure. It issuggested that plants grown at the lower CO₂ partial pressurehave a higher affinity for CO₂ due to a more efficient CO₂ concentratingsystem than plants grown at the higher CO₂ partial pressure. Key words: Photosynthesis, CO₂, concentration, RuBP carboxylase/oxygenase     

Flowering time and elevated atmospheric CO2

Flowering is a critical milestone in the life cycle of plants, and changes in the timing of flowering may alter processes at the species, community and ecosystem levels. Therefore understanding flowering-time responses to global change drivers, such as elevated atmospheric carbon dioxide concentrations, [CO(2)], is necessary to predict the impacts of global change on natural and agricultural ecosystems. Here we summarize the results of 60 studies reporting flowering-time responses (defined as the time to first visible flower) of both crop and wild species at elevated [CO(2)]. These studies suggest that elevated [CO(2)] will influence flowering time in the future. In addition, interactions between elevated [CO(2)] and other global change factors may further complicate our ability to predict changes in flowering time. One approach to overcoming this problem is to elucidate the primary mechanisms that control flowering-time responses to elevated [CO(2)]. Unfortunately, the mechanisms controlling these responses are not known. However, past work has indicated that carbon metabolism exerts partial control on flowering time, and therefore may be involved in elevated [CO(2)]-induced changes in flowering time. This review also indicates the need for more studies addressing the effects of global change drivers on developmental processes in plants.     

Resistance to CO2 diffusion in cuticular membranes of amphibious plants and the implication for CO2 acquisition

Cuticular membranes (CMs) were isolated from leaves of amphibious and submerged plants and their CO2 resistances were determined as a contribution to establish quantitatively the series of resistances met by CO2 diffusing from bulk water to the chloroplasts of submerged leaves. The isolation was performed enzymatically; permeabilities were determined and converted to resistances. The range of permeance values was 3 to 43 x 10(-6) m s(-1) corresponding to resistance values of 23 to 295 x 10(3) s m(-1), i.e. of the same order of magnitude as boundary layer resistances. The sum of boundary layer, CM, leaf cell and carboxylation resistances could be contained within the total diffusion resistance as determined from the photosynthetic CO2 affinity of the leaf. From the same species, the aerial leaf CM resistance was always higher than the aquatic leaf CM resistance. In a terrestrial plant, the CM resistance to CO2 diffusion was found lower in leaves developed submerged.     

Proton-motive-force-driven formation of CO from CO2 and H2 in methanogenic bacteria

Cell suspensions of methanogenic bacteria (Methanosarcina barkeri, Methanospirillum hungatei, Methano-brevibacter arboriphilus, and Methanobacterium thermoautotrophicum) were found to form CO from CO2 and H2 according to the reaction: CO2 + H2----CO + H2O; delta G0 = +20 kJ/mol. Up to 15,000 ppm CO in the gas phase were reached which is significantly higher than the equilibrium concentration calculated from delta G0 (95 ppm under the experimental conditions). This indicated that CO2 reduction with H2 to CO is energy-driven and indeed the cells only generated CO when forming CH4. The coupling of the two reactions was studied in more detail with acetate-grown cells of M. barkeri using methanogenic substrates. The effects of the protonophore tetrachlorosalicylanilide (TCS) and of the proton-translocating ATPase inhibitor N,N'-dicyclohexylcarbodiimide (cHxN)2C were determined. TCS completely inhibited CO formation from CO2 and H2 without affecting methanogenesis from CH3OH and H2. In the presence of the protonophore the proton motive force delta p and the intracellular ATP concentration were very low. (cHxN)2C, which partially inhibited methanogenesis from CH3OH and H2, had no effect on CO2 reduction to CO. In the presence of (cHxN)2C delta p was high and the intracellular ATP content was low. These findings suggest that the endergonic formation of CO from CO2 and H2 is coupled to the exergonic formation of CH4 from CH3OH and H2 via the proton motive force and not via ATP. CO formation was not stimulated by the addition of sodium ions.     

Wheat Response to CO2 Enrichment: Growth and CO2 Exchanges at Two Plant Densities

Du Cloux, H. C, André, M., Daguenet, A. and Massinuno,J. 1987. Wheat response to CO₂ enrichment: Growth and CO₂ exchangesat two plant densities.—J. exp. Bot. 38: 1421–1431. The vegetative growth of wheat (Triticum aestivum L., var. Capitole)was followed for almost 40 d after germination in controlledconditions. Four different treatments were carried out by combiningtwo air concentrations of CO₂, either normal (330 mm³ dm ³)or doubled (660 mm³ dm ³) with two plant densities, either 200plants m ² or 40 plants m ². Throughout the experiment the CO₂gas exchanges of each canopy were measured 24 h d¹. These provideda continuous growth curve for each treatment, which were comparedwith dry weights. After a small stimulation at the start (first13 d), no further effect of CO₂ enrichment was observed on relativegrowth rate (RGR). However, RGR was stimulated throughout theexperiment when plotted as a function of biomass. The finalstimulation ol dry weight at 660 mm³ dm ³ CO₂ was a factor of1·45 at high density and 1·50 at low density,contrary to other studies, no diminution of this CO₂ effecton dry weight was observed over time. Nevertheless, at low density,a transient additional enhancement of biomass (up to 1·70)was obtained at a leaf area index (LAI) below 1. This effectwas attributed to a different build up of the gain of carbonin the case of an isolated plant or a closed canopy. In theformer, the stimulation of leaf area and the net assimilationrate are both involved; in the latter the enhancement becomesindependent of the effect on leaf area because the canopy photosynthesisper unit ground area as a function of LAI reaches a plateau. Key words: Triticum aestuum, L. var. Capitole, Vegetative growth, Canopy     

Short-term CO2 exchange response to temperature,irradiance, and CO2 concentration in strawberry

Relative importance of short-term environmental interaction and preconditioning to CO₂ exchange response was examined in Fragaria ananasa (strawberry, cv. Quinault). Tests included an orthogonal comparison of 15 to 60-min and 6 to 7-h exposures to different levels of temperature (16 to 32°C), photosynthetically active radiation (PAR, 200 to 800 E m² s^-1), and CO₂ (300 to 600 l/l) on successive days of study. Plants were otherwise maintained at 21°C, 300 E m² s^-1 PAR and 300–360 l/l CO₂ as standard conditions. Treatment was restricted to the mean interval of 14 h daily illumination and the first 3–4 days of each test week over a 12-week cultivation period. CO₂ exchange rates were followed with each step-change in environmental level including ascending/descending temperature/PAR within a test period, initial response at standard conditions on successive days of testing, and measurement at reduced O₂. Response generally supported prior concepts of leaf biochemical modeling in identifying CO₂ fixation as the major site of environmental influence, while overall patterns of whole plant CO₂ exchange suggested additional effects for combined environmental factors and preconditioning. These included a positive interaction between temperature and CO₂ concentration on photosynthesis at high irradiance and a greater contribution by dark respiration at lower PAR than previously indicated. The further importance of estimating whole plant CO₂ exchange from repetitive tests and measurements was evidenced by a high correlation of response to prior treatment both during the daily test period and on consecutive days of testing.Abbreviations C₃ plant a plant in which the product of CO₂ fixation is a 3-carbon acid (3-phosphoglyceric acid) - IRGA intra-red gas analyzer - PAR photosynthetically active radiation - RH relative humidity - RuBisCO ribulose-1,5-bisphosphate carboxylase/oxygenase Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.     

High CO2 partial pressure effects on dark and light CO2 fixation and metabolism in Vicia faba leaves

Stomatal opening on Vicia faba can be induced by high CO₂ partial pressures (10.2%) in dark as well as in light. Stomatal aperture was measured in both cases with a hydrogen porometer. The distribution of ¹⁴C among early products of photosynthesis was studied. Comparisons are made with carboxylations occurring when stomata were open in the dark with CO₂-free air and in light with 0.034% CO₂. Results showed that in high CO₂ partial pressure in light, less radioactivity was incorporated in Calvin cycle intermediates and more in sucrose. carboxylations and photorespiration seemed to be inhibited. In the dark in both CO₂ conditions, ¹⁴C incorporation was found in malate and aspartate but also in serine and glycerate in high CO₂ conditions. In light these changes in metabolic pathways may be related with the deleterious effects recorded on leaves after long-term expositions to high partial pressure of CO₂.Abbreviations DHAP dihydroxyacetone phosphate - PEP phosphonenolpyruvate - PEPCK phosphonenolpyruvatecarboxykinase - PGA 3-phosphoglyceric acid - RUBPc ribulose 1,5-bisphosphate carboxylase     

CeNCl-CeO2 Heterojunction-Modified Ni Catalysts for Efficient Electroreduction of CO2 to CO

Renewable-electricity-powered electrochemical CO₂ reduction (CO₂RR) is considered one of the most promising ways to convert exhaust CO₂ into value-added chemicals and fuels. Among various CO₂RR products, CO is of great significance since it can be directly used as feedstock to produce chemical products through the Fischer–Tropsch process. However, the CO₂-to-CO electrocatalytic process is often accompanied by a kinetically competing side reaction: H₂ evolution reaction (HER). Designing electrocatalysts with tunable electronic structures is an attractive strategy to enhance CO selectivity. In this work, a CeNCl-CeO₂ heterojunction-modified Ni catalyst is successfully synthesized with high CO₂RR catalytic performance by the impregnation-calcination method. Benefiting from the strong electron interaction between the CeNCl-CeO₂ heterojunction and Ni nanoparticles (NPs), the catalytic performance is greatly improved. Maximal CO Faradaic efficiency (FE) is up to 90% at −0.8 V (vs RHE), plus good stability close to 12 h. Detailed electrochemical tests and density functional theory (DFT) calculation results reveal that the introduction of the CeNCl-CeO₂ heterojunction tunes the electronic structure of Ni NPs. The positively charged Ni center leads to an enhanced local electronic structure, thus promoting the activation of CO₂ and the adsorption of ^*COOH.     

Growth ofClostridium thermoaceticum on H2/CO2 or CO as energy source

The type strain Fontaine ofClostridium thermoaceticum proliferated on H₂/CO₂ as energy source and was culturally adapted to grow on 100% CO in the headspace. The doubling times at 55°C on CO or H₂/CO₂ were 16 and 18 h, respectively. Under these conditions, the substrate-product transformation stoichiometries observed were: 4H₂+2.1CO₂→0.9 acetate and 4CO→2CO₂+1.1 acetate. It is concluded thatC. thermoaceticum has a single carbon growth physiology.     

Soil microorganisms at different CO2 and O2 tensions

Microbial biomass and activity were determined in cambisol incubated under ambient and increased (up to 2.23 mmol/L) CO₂ concentrations. An immediate negative response of the soil microbial community to [CO]₂ increase was observed during the first day with respect to microbial biomass, soil respiration and specific respiration activity (both expressed as CO₂ evolution). In contrast, O₂ consumption was not affected but anabolic utilization of available substrate increased. These phenomena were observed under conditions of increased CO₂ tension but without any change in O₂ concentration.     

Retinoic acid and CO2 laser resurfacing

The purpose of this study was to analyze the effect of retinoic acid on wound healing and depth of injury in an animal skin model resurfaced with a CO2 laser. The dorsal skin of 21 Hartley guinea pigs was divided into halves. One-half received a daily application of 0.05% retinoic acid for 28 days, whereas the other half served as the control. The animals were divided into three treatment groups of seven animals. Group A was laser resurfaced with one pass of the Coherent UltraPulse CO2 laser (300 mJ, 60 W, density 40 percent). Group B received two passes, and group C received three passes. Histologic studies were obtained before laser resurfacing and days 1, 4, and 7 after resurfacing. Depth of injury, thickness, number of squamous cell and granular cell layers, and epithelialization rates were measured. We found that the depth of injury was statistically less in animals pretreated with retinoic acid. Granular cells were thicker and more numerous at day 4 in pretreated animals but similar to controls by day 7. Animals pre-treated with retinoic acid overall seemed to heal wounds earlier. In conclusion, pretreatment with retinoic acid may reduce the depth of injury in laser resurfacing and speed healing rates.     

CO2 exacerbates oxygen toxicity

Reactive oxygen species (ROS) are harmful because they can oxidize biological macromolecules. We show here that atmospheric CO(2) (concentration range studied: 40-1,000 p.p.m.) increases death rates due to H(2)O(2) stress in Escherichia coli in a dose-specific manner. This effect is correlated with an increase in H(2)O(2)-induced mutagenesis and, as shown by 8-oxo-guanine determinations in cells, DNA base oxidation rates. Moreover, the survival of mutants that are sensitive to aerobic conditions (Hpx(-) dps and recA fur), presumably because of their inability to tolerate ROS, seems to depend on CO(2) concentration. Thus, CO(2) exacerbates ROS toxicity by increasing oxidative cellular lesions.     

Effects of temperature and CO2 concentration on photosynthetic CO2 fixation by Chlorella

The rates of photosynthetic ¹⁴CO₂ fixation by Chlorella vulgarisllh, grown under high CO₂, were determined between 4 to 37°Cwith air containing from 300 to 13,000 ppm ¹⁴CO₂. When the CO₂level was increased, both the rate of photosynthesis and theoptimum temperature for maximum photosynthesis increased. Themaximum photosynthetic rate was reached at 12°C with 300ppm ^l4CO₂. Among the photosynthetic products fromed at 300 ppm ¹⁴CO₂, glycolatedecreased greatly when the temperature was raised from 20 to30°C. At 3,000 ppm ¹⁴CO₂ an insignificant amount of glycolatewas formed at all temperatures, whereas ¹⁴C-incorporation intothe insoluble fraction, sucrose, and the lipid fraction wassignificantly higher than at 300 ppm ¹⁴CO₂. The ¹⁴C in sucrosewas greatly increased and the radioactivity in the insolublefraction decreased when the temperature was raised from 28 to36°C. (Received April 8, 1980; )