CO2, dark fixation

C02 Free Air. Effects of external CO2 concentrations on acidification and deacidification during CAM have been discussed in detail by Wolf (1960). It is now well established that C02-free air inhibits nocturnal malic acid synthesis and accumulation (Kluge, 1968 b, 1969 b see also Wolf, 1960). It is reasonable to assume that this effect is due to the lack of substrate for CO2 dark fixation. However, with C02-free air malic acid accumulation at night is not prevented completely (see for example Kluge, 1969 b) because of endogenous (respiratory) CO2, which may serve as the substrate of dark CO2 fixation. 

Deleens,E., Garnier-Dardart, . Carbon isotope composition of biochemical fractions isolated from leaves of Bryophyllum daigremontianum Berger, a plant with Crassulacean Acid Metabolism Some physiological aspects related to CO2 dark fixation. Planta 139, 214-248 (1977)... 

Zabka,G.G., McMahon, E. Relationships among CO2 dark-fixation succulence, flowering, and organic acid formation in Kalanchoe blossfeldiana var. Tom Thumb. Can. J. Bot. 3, 447 52 (1965)... 

Other sand-based systems using COi pulse-chase procedures have been used to produce carbon budgets for Festuca ovina and Plantago lanceolata seedlings (30) and white lupin (Lupimis albiis) (31). Significantly, CO2 pulse labeling of proteoid roots of white lupin under phosphate-deficient conditions showed that high levels of dark fixation of COi by the roots took place and that 66% of this root-fixed carbon was exuded from the roots (31). Clearly, dark fixation of CO2 by roots and subsequent rhizodeposition is an area that deserves further study in the future. 

Carbon dioxide is also fixed in the dark by photosynthetic organisms by the so-called Wood-Werkman reaction (Wood and Stjemholm, 1962). The CO2 assimilated, however, rarely exceeds that formed by dark respiration i.e. there is no net CO2 uptake. On the other hand, the amount of organic carbon derived from CO2 may be as high as 30% in heterotrophic bacteria and 90% in mixotrophic organisms. In the natural environment, non-photo-synthetic CO2 fixation by these organisms, together with the above-mentioned dark fixation by photosynthetic organisms, may under some condi-... 

Chemosynthesis the process of dark fixation of CO2 into cellular material coupled to oxidation of inorganic compounds. 

Even when the main source of cell carbon is not CO2 (heterotrophic growth), CO2 fixation reactions are still important as the dark fixation reactions, essential for the operation of many biosynthetic pathways. In both autotrophic and heterotrophic growth, CO2 can also play a catalytic role (Raven, 1974). 

It is now generally accepted that dark fixation of COj is the key reaction in CAM. Virtually all experiments conducted to date substantiate the hypothesis that malate is the first and primary stable product of CO2 fixation in CAM. Thurlow and Bonner and Bonner and Bonner in 1948 were perhaps the first to show dark " C02 fixation into malate as well as other organic acids including citrate, isocitrate, fumarate, and succinate. Yet the equilibration of malate- C with the other organic acids was slow. Subsequently Thomas and Ranson (1954) reported the stoichiometry between CO2 fixation and acid synthesis. 

As previously indicated, during dark fixation of CO2, no radioactive carbohydrate products are formed only organic and amino acids are significantly labeled. In cactus root tips, after a 2-h dark fixation period, 80% of the was in organic acids (Ting and Dugger, 1966) with a distribution of 66.4% in malate, 11.4% in citrate, and trace amounts in fumarate, succinate, and isocitrate. Because the tissue was actively respiring (about 200 pi g fr.wt h ), the data were taken to indicate compartmentation of the products outside the mitochondria. 

Cumulative gross dark fixation, respiration, and calculated net dark CO2 fixation integrated from onset of dark period to maximum rate of gross dark fixation. 

Kaplan et al. (1976 a, b) also reported net CO2 fixation calculated from gross dark CO2 fixation less dark respiration as a function of the preceding light intensity. When light increased from 3.2 to 12 mW cm", dark fixation of CO2 by... 

Dittrich, . Equilibrium of label in malate during dark fixation of CO2 in Kalanchoe fedt-schenkoi. Plant Physiol. 58,288-291 (1976)... 

Joshi,G., Dolan,T., Gee,R., Saltman,P. Sodium chloride effect in dark fixation of CO2 by marine and terrestrial plants. Plant Physiol. 37,446 (1962)... 

Kluge, M., Kriebitsch,Ch., v. Willert,D. Dark fixation of CO2 in Crassulacean Acid Metabolism Are two carboxylation steps involved Z. Pflanzenphysiol. 72,460-467 (1974)... 

Kunitake,G., Saltmann,P. Dark fixation of CO2 by succulent leaves conservation of the dark fixed CO2 under diurnal conditions. Plant Physiol. 83,400-403 (1958)... 

Saltman,P., Kunitake,G., Spolter,H., Stitt,C. The dark fixation of CO2 by succulent leaves the first products. Plant Physiol. 31,464-468 (1956)... 

Sutton, B.G., Osmond, C.B. Dark fixation of CO2 by Crassulacean plants. Evidence for a single carboxylation step. Plant Physiol. 50,360-365 (1972)... 

Zabka,G.G., Edelman,J. Pre-illumination as a factor in the dark-fixation of CO2 by Kalanchoe blossfeldiana. Plant Physiol. 36 (1961a)... 

The two- to fourfold variation in sucrose cost per NHj assimilated (Fig. 3B) is not faithfully reflected in respiratory losses of CO2, since dark fixation inputs of CO2 apply to certain compounds (e.g., aspartate and asparagine) but not at all, or to only a minor extent, in the case of others. Net CO2 exchange values therefore range from a net fixation input of 0.58 and 1.08 CO2/NH3 in the case of glutamate and aspartate, respectively, to a net evolution of 0.02 to 0.53 CO2/NH3 for ureide, citrulline, and proline. Based on these calculations the type of products selected by a plant for export of assimilated nitrogen might aJter significantly the apparent respiration status (respiratory quotient) of its assimilatory tissues. 

In contrast to the biological CO2 fixation during the dark reaction of photosynthesis where very low concentrations of CO2 from air can be fixed at room temperature, most reactions with transition-metal complexes or with organic substrates either require high partial pressure of CO2 or high temperatures. An exception was published quite recently, the rapid fixation of CO2 and O2 from air at a palladium(O) complex 10 (Scheme 11) [77]. 

Probably these anomalies of the OM parameters might be connected with the process of chemosynthesis, and the layer of bacterial chemosynthesis should play an important role in the formation of the vertical distribution of nutrient species there. The results of measurements of the dark CO2 fixation [78,79] usually reveal the primary maximum of chemosynthesis (about 0.4-2.0 jiM d x) in a 20-30-m layer below the hydrogen sulfide boundary. The less pronounced secondary maximum is observed about 5-10 m shallower than the hydrogen sulfide boundary and is likely to be connected with nitrification [78]. 

So far, the only process involving CO2 that we have considered in this chapter is photosynthesis. However, we cannot neglect the CO2 produced within mesophyll (and other) cells by respiration and photorespiration. If mitochondrial respiration in leaf cells were the same in the light as in the dark, when it can be readily measured (because of no photosynthesis or photorespiration occurring then), respiration would produce about 5% as much C02 as is consumed by photosynthesis at a moderate PPF. In C3 plants, the rate of light-stimulated production of CO2 by photorespiration at moderate temperatures is often about 30% (range, 15-50%) of the rate of CO2 fixation into photosynthetic products. 

In summary, it is generally assumed that regulation of C4 photosynthesis involves most of the mechanisms discussed earlier for C, photosynthesis. There are, however, a number of specializations in the light-dark activation and deactivation of the enzymes involved in the initial fixation of CO2 in the mesophyll and its release in the bundle sheath. Additional controls are required for the enzymes metabolizing compounds which travel down diffusion gradients between the cell types. 

The dark reaction, known as the Calvin cycle, uses the reducing power of NADPH as well as the free energy stored in the ATP to assimilate carbon dioxide in the form of carbohydrates. The way by which Nature achieves carbon fixation is via the reaction of CO2 with ribulosebiphosphate (RuBP) to give two molecules of 3-phosphoglycerate, a process which is catalyzed by the enzyme RuBP-carboxylase. The phosphogylcerate is converted further to fructose 6-phosphate, the final product of the Calvin cycle. The overall reaction, despite its complex mechanism, corresponds to the simple Eq. (16) above. 

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