Carbon dioxide fixation in trypanosomatids.

Fixation of carbon dioxide has been demonstrated for extracts from Crithidia fasciculata, Trypanosoma mega and Trypanosoma brucei brucei bloodstream and culture forms. The enzymes involved in this fixation were found to be ADP-stimulated phosphoenolpyruvate carboxykinase (E.C. 4.1.1.32), 'malic' enzyme (E.C. 1.1.138-40) and pyruvate carboxylase (E.C. 6.4.1.1). The subcellular localization of these enzymes has been investigated in all three organisms. Products of short and long term fixation experiments were separated and identified. The importance of carboxylation reactions is discussed in relation to the maintenance of oxidized and reduced coenzyme levels.     

Carbon dioxide supersaturation in Florida lakes

We examined data on CO₂ and related limnological and geographic information from a sample of 948 Florida freshwater lakes. The objectives for this study were (1) to determine the partial pressures of carbon dioxide (ρCO₂) in the surface waters of a large sample of Florida lakes, (2) to determine if several limnological or geographic factors are related to levels of ρCO₂ in Florida lakes, and (3) to estimate the net annual rate of loss of CO₂ to the atmosphere from the freshwater lakes of Florida. The calculated ρCO₂ for the lakes in our sample range from 0 to 81,000 μatm, with a mean of 3,550 μatm, a median of 1,030 μatm, and a geometric mean of 1,270 μatm. About 87% of the Florida lakes were supersaturated with CO₂. There were statistically significant correlations between values for ρCO₂ and several water chemistry variables; however, the R ² values were small and accounted for only a small portion of the variance. In general the ρCO₂ values were higher in the lakes with low alkalinities and low contents of dissolved salts. The best predictor of ρCO₂ is pH, with an R ² of 0.82 for a polynomial relationship. The ρCO₂ values tend to decrease from northwest to southeast across the state of Florida, which corresponds to the gradients we found for pH, alkalinity, and specific conductance. The average areal rate of carbon emission from the Florida lakes was 328 g C m⁻² y⁻¹, and the total carbon loss for the lakes and ponds of Florida was 2.0 Tg y⁻¹. This amounts to about 2% of the total carbon emissions from all the lakes of the world as estimated by previous studies. Handling editors: Darren Bade     

Carbon dioxide signalling in plant leaves

The role of carbon dioxide (CO(2)) as a signal in biochemical regulation networks of plants is fathomed. Transport mechanisms of CO(2) and HCO3- are surveyed, which are the prerequisite for signalling. A CO(2) sensor is not known to date, but any reaction where CO(2)/HCO3- is a substrate can be a candidate. Carbon concentrating mechanisms, e.g., in higher plants C(4)-photosynthesis and crassulacean acid metabolism (CAM), generate high internal CO(2) concentrations, important for photosynthesis, but also as a basis for signalling via diffusion of CO(2). Spatiotemporal dynamics of desynchronization/synchronization of photosynthetic activity over leaves can be followed by chlorophyll fluorescence imaging. One example of desynchronization is based on patchiness of stomatal opening/closing in heterobaric leaves due to anatomic constraints of lateral CO(2) diffusion. During CAM, largely different internal CO(2) concentrations prevail in the leaves, offering opportunities to study the effect of lateral diffusion of CO(2) in synchronizing photosynthetic activity over the entire leaves.     

Carbon dioxide fixation in Pyrobotrys stellata

The green alga Pyrobotrys stellata Korshik., an obligate phototroph, is unable to utilise carbon dioxide for growth, although assimilation of acetate is dependent on the photosynthetic process. The incorporation of ¹⁴CO₂ from ¹⁴C-bicarbonate into the cells of P. stellata is only 3% of that in Chlorella pyrenoidosa Chick. The activity of the key enzyme of the Calvin cycle, ribulose-1-5-diphosphate carboxylase, is very low in P. stellata, being only 7% of that in C. pyrenoidosa. The determination of the products of ¹⁴CO₂ fixation in intact cells confirms that ribulose-1-5-diphosphate activity is very low in P. stellata, since little carbon-14 is found in 1–3 diphosphoglyceric acid, the product of carboxylation of ribulose-1-5-diphosphate. It is concluded that the inability of P. stellata to utilize carbon dioxide for growth in the light is probably the result of the low ribulose-1-5-diphosphate carboxylase activity in the organism.     

Carbon dioxide transport through membranes

Several membrane channels, like aquaporin-1 (AQP1) and the RhAG protein of the rhesus complex, were hypothesized to be of physiological relevance for CO(2) transport. However, the underlying assumption that the lipid matrix imposes a significant barrier to CO(2) diffusion was never confirmed experimentally. Here we have monitored transmembrane CO(2) flux (J(CO2)) by imposing a CO(2) concentration gradient across planar lipid bilayers and detecting the resulting small pH shift in the immediate membrane vicinity. An analytical model, which accounts for the presence of both carbonic anhydrase and buffer molecules, was fitted to the experimental pH profiles using inverse problems techniques. At pH 7.4, the model revealed that J(CO2) was entirely rate-limited by near-membrane unstirred layers (USL), which act as diffusional barriers in series with the membrane. Membrane tightening by sphingomyelin and cholesterol did not alter J(CO2) confirming that membrane resistance was comparatively small. In contrast, a pH-induced shift of the CO(2) hydration-dehydration equilibrium resulted in a relative membrane contribution of about 15% to the total resistance (pH 9.6). Under these conditions, a membrane CO(2) permeability (3.2 +/- 1.6 cm/s) was estimated. It indicates that cellular CO(2) uptake (pH 7.4) is always USL-limited, because the USL size always exceeds 1 mum. Consequently, facilitation of CO(2) transport by AQP1, RhAG, or any other protein is highly unlikely. The conclusion was confirmed by the observation that CO(2) permeability of epithelial cell monolayers was always the same whether AQP1 was overexpressed in both the apical and basolateral membranes or not.     

Carbon dioxide and senescence in cotton plants

Glandless cotton plants (Gossypium hirsutum L. cv. Coker 100) were subjected to the influence of high CO₂-bicarbonate. The content of protein decreased with no accompanying increase in its degraded products. The decrease in protein was correlated with the low content of chlorophyll and also with the reduced activity of carbonic anhydrase. The initiation of these correlations coincided with the time when the control leaves contained the highest enzyme activity during leaf growth. The high concentration of bicarbonate directly restricted the rate of photophosphorylation and that of the Hill reaction in isolated chloroplasts. The amount of ATP in leaves treated in vivo also diminished. High CO₂ as bicarbonate, however, did not directly inhibit the activity of carbonic anhydrase in vitro.     

Carbon dioxide and biearbonate in cell division

Summary The effects of carbon dioxide and of bicarbonate on cell division were studied on synchronized cells of the high-temperature green alga, Chlorella 7-11-05. After 7 hours of growth in nutrient medium in light, cells were centrifuged and resuspended in distilled water or in bicarbonate and placed in darkness. Atmospheric air, or a mixture of carbon dioxide and air, was bubbled through algal suspensions during the dark period. In distilled water cells readily divided in atmospheric air but not in 1% (2.6·10^-4 M) or in higher concentrations of carbon dioxide. The suspension of cells in bicarbonate counteracted the inhibitory action of carbon dioxide. A minimum molar concentration of bicarbonate necessary to counteract the inhibitory effect of carbon dioxide was found to be equal to the molar concentration of carbon dioxide in the suspending fluid. The highest concentration of carbon dioxide, the adverse effect of which could not be balanced by any concentration of bicarbonate, was found to be in the vicinity of 1.3·10^-2 M (50% CO₂ in air). Possible effects on cell division of the change in Ph and the implicated role of carbon dioxide in normal and neoplastic growth were discussed.     

Carbon dioxide fixation in isolated kalanchoe chloroplasts

Chloroplasts isolated from Kalanchoe diagremontiana leaves were capable of photosynthesizing at a rate of 5.4 μmoles of CO₂ per milligram of chlorophyll per hour. The dark rate of fixation was about 1% of the light rate. A high photosynthetic rate was associated with low starch content of the leaves. Ribose 5-phosphate, fructose 1,6-diphosphate, and dithiothreitol stimulated fixation, whereas phosphoenolpyruvate and azide were inhibitors. The products of CO₂ fixation were primarily those of the photosynthetic carbon reduction cycle.     

Carbon dioxide fixation in orchid aerial roots

Acidity fluctuation, CO₂ gas exchange, δ¹³C value, PEP carboxylase and RuBP carboxylase activities in aerial roots of selected thick-leaved orchid hybrids ( Arachnis and Aranthera ) were studied. Both aerial roots and leaves showed acidity fluctuation over a 24 h period. Dark acidification in aerial roots was enhanced at low temperature (15°C). Aerial roots had δ¹³C values close to those of leaves which have been previously demonstrated to possess crassulacean acid metabolism. Variation in δ¹³C values along the length of the roots was observed; the root tip having a less negative δ¹³C value (—13.34%‰) than the older portions of the roots (—14.55%‰). There was no net CO₂ fixation by aerial root, although ¹⁴³²CO₂ fixation was observed in light and in darkness. The pattern of fluctuation in activities of PEP carboxylase and RuBP carboxylase in aerial roots was similar to that obtained for the leaves. In both aerial roots and leaves, PEP carboxylase activity was several times higher than that of RuBP carboxylase.     

Carbon dioxide fixation in green sulphur bacteria

1. About one-third of the CO(2) fixed during photosynthesis by washed suspensions of Chlorobium thiosulfatophilum strain 8346 gave rise to alpha-oxoglutarate and branched-chain oxo acids, mainly beta-methyl-alpha-oxovalerate. Another one-third to one-half gave rise to a polyglucose. 2. The fixation of CO(2) was inhibited by fluoroacetate, increasing concentrations up to 1mm stimulating the accumulation of alpha-oxoglutarate and causing a decrease in the formation of the branched-chain oxo acids and polyglucose. 3. Acetate was converted into the same products as was CO(2). 4. Fluoroacetate (1mm) had a negligible effect on the formation of polyglucose from acetate and caused a slight inhibition of the formation of the branched-chain oxo acids and increased accumulation of alpha-oxoglutarate. 5. Iodoacetate (1mm) strongly inhibited polyglucose formation from acetate and caused accumulation of pyruvate. The formation of the branched-chain oxo acids from acetate was only slightly affected by this inhibitor. 6. Pyruvate can be metabolized by this organism in the presence of a suitable electron donor whether CO(2) is present or not. In the absence of CO(2) pyruvate is converted into polyglucose. 7. The accumulation of oxo acids during CO(2) fixation is completely inhibited by NH(4) (+) ions. The formation of the branched-chain oxo acids is considerably decreased by the presence of isoleucine, leucine or valine, or a mixture of these. 8. CO(2) fixation in two other strains of Chlorobium appears to exhibit a similar pattern to that in C. thiosulfatophilum strain 8346. 9. It is concluded that in washed suspensions, CO(2) is fixed mainly by a mechanism involving the reductive carboxylic acid cycle. Acetate, the product of the cycle, is converted into polyglucose via pyruvate synthase and a reversal of glycolysis or into branched-chain oxo acids by an unknown mechanism.