Cycle fixation

Kestens, P.J. (1964). La Perfusion du Foie Isole. Editions Arscia S.A. Bruxelles. Meyer, AJ. Lamers, W.H., Chamulead, R.A.F.M. (1990). Nitrogen metabolism and ornithine cycle fixation. Physiol. Rev. 70, 701-748. 

One of these alternate models, postulated by Gunter Wachtershanser, involves an archaic version of the TCA cycle running in the reverse (reductive) direction. Reversal of the TCA cycle results in assimilation of CO9 and fixation of carbon as shown. For each turn of the reversed cycle, two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus, for every succinate that enters the reversed cycle, two succinates are returned, making the cycle highly antocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways (see Section 20.13), a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. 

The set of reactions that transforms 3-phosphoglycerate into hexose is named the Calvin-Benson cycle (often referred to simply as the Calvin cycle) for its discoverers. The reaction series is indeed cyclic because not only must carbohydrate appear as an end product, but the 5-carbon acceptor, RuBP, must be regenerated to provide for continual COg fixation. Balanced equations that schematically represent this situation are... 

Purple sulfur bacteria fix carbon dioxide using the Calvin-Benson cycle, but green sulfur bacteria use a completely different pathway, the reverse tricarboxylic acid cycle. Other photosynthetic bacteria use still different pathways for CO2 fixation (Perry and Staley, 1997). 

Fig. 10-13. The links between the cycling of C, N, and O2 are indicated. Total primary production is composed of two parts. The production driven by new nutrient input to the euphotic zone is called new production (Dugdale and Goering, 1967). New production is mainly in the form of the upward flux of nitrate from below but river and atmospheric input and nitrogen fixation (Karl et al, 1997) are other possible sources. Other forms of nitrogen such as nitrite, ammonia, and urea may also be important under certain situations. The "new" nitrate is used to produce plankton protoplasm and oxygen according to the RKR equation. Some of the plant material produced is respired in the euphotic zone due to the combined efforts...

Gruber, N. and Sarmiento, J. L. (1997). Global patterns of marine nitrogen fixation and denitrification. Glob. Biogeochem. Cycles 11,235-266. 

Karl, D., Letelier, R., Tupas, L. et al. (1997). The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean. Nature 388, 533-538. 

The global nitrogen cycle is often referred to as the nitrogen cycles, since we can view the overall process as the result of the interactions of various biological and abiotic processes. Each of these processes, to a first approximation, can be considered as a self-contained cycle. We have already considered the biological cycle from this perspective (Fig. 12-1), and now we will look at the other processes, the ammonia cycle, the cycle, and the fixation/denitrification cycle. 

As mentioned previously, the fixation-denitrification cycle (Fig. 12-7) is the most heavily perturbed by humans. This is due to both the increasing use of nitrogenous fertilizers and the planting of nitrogen-fixing plants. One of the... 

Fig. 12-7. Fixation-denitrification cycle. Each arrow represents one flux. The magnitude of the flux is given in Tg N/yr. Where two numbers are given, the top value is the anthropogenic contribution and the lower number is the total flux (natural + anthropogenic).

Galloway J. N. et al. (1995). Nitrogen fixation Anthropogenic enhancement - environmental response. Global Biogeochem. Cycles 9,235-252. 

Granhall, U. (1981). Biological nitrogen fixation in relation to environmental factors and functioning of natural ecosystems. In "Terrestrial Nitrogen Cycles" (F. E. Clark and T. Rosswall, eds). Ecological Bulletin 33, 131-145. Swedish Natural Science Research Council, Stockholm. 

Not surprisingly, only about 20 of the chemical elements found on Earth are used by living organisms (Chapters 3 and 8). Most of them are common elements. Rare elements are used, if at all, only at extremely low concentrations for specialized functions. An example of the latter is the use of molybdenum as an essential component of nitrogenase, the enzyme that catalyzes the fixation of elemental dinitrogen. Because they are composed of common elements, living organisms exert their most profound effects on the cycles of those elements. 

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