Photosynthetic fixation

The photoassisted reduction of aqueous carbon dioxide in the presence of inorganic minerals has been examined as a model of prebiological photosynthesis,120 a potential precursor to the photosynthetic fixation of C02 by plants. 

Fermentation of D-xylose by Escherichia coli at pH 5.5 (in bicarbonate buffer) gives more than one mole of lactic acid per mole because of fixation of carbon dioxide by the two-carbon fragment,198(a> an observation that may have an important bearing on theories of photosynthetic fixation of carbon dioxide.198[Pg.223]

The NP cycle is very hard to quantify. Many organisms can make and modify NPs conse-quendy, there are many routes around the cycle. However, the NP cycle could be between 1 and 10 bUhons of tormes per annum because the annual photosynthetic fixation of carbon is about too biUion tormes and the primary producers can often use 1-5% of that carbon to make NPs. Furthermore, some microbes, also producers of NPs, live on the carbon fixed by the primary producers hence, a further fraction of the annual carbon is converted to NPs. The author knows of no estimate of the flow of carbon into NPs in these organisms. 

The emission of C02 from anthropogenic activities (the combustion of C-based fossil fuels, deforestation, combustion of woods) amounts to approximately 7.5 Gtc per year, or about 3.5% of the total amount cycled in the natural cycle. However, as the natural systems are unable to use such C02, this leads to its accumulation into the atmosphere. The assumption that an increase of the concentration of C02 in the atmosphere would have boosted both the photosynthesis and the dissolution into the oceans has not been proven to be true. In fact, the solubility of C02 is governed by complex equilibria, while photosynthetic fixation is limited by several factors so that, under the increase of the atmospheric concentration from 280 ppm of the preindustrial era to the present-day 380 ppm, there has not been any sensible improvement of the uptake. Therefore, under natural conditions the uptake of C02 has reached an equilibrium state, and the further increase in atmospheric concentrations may more likely cause climate changes through the greenhouse effect and destabilization of the thermal structure of the atmosphere, than improve the elimination of C02 from the atmosphere. 

Evans, Buchanan, and Arnon (41a) have recently found that the ferredoxin-dependent pyruvate and a-ketoglutarate synthesizing reactions function in a new carbon cycle for the photosynthetic fixation of C02. The new cycle was named the reductive carboxylic acid cycle, and apart from pyruvate and a-ketoglutarate synthases, it includes certain of the enzymes associated with Krebs citric acid cycle, operating in the synthetic direction. Photoreduced ferredoxin and ATP, formed by photo-... 

The primary engine that drives these ocean interior variations is the photosynthetic fixation of carbon into organic matter by marine phytoplankton in the fight-illuminated upper ocean (euphoric zone) (see Fig. 1.1). Along with carbon, nutrient elements such as nitrogen, phosphorus, iron and many others are taken up and assimilated. Most of the resulting organic matter is either respired or reminerafized... 

It should be noted that reactions (1) and (2) represent the sum of many intermediate reactions. These include the photosynthetic fixation of CO2 as biomass, the secretion of organic and carbonic acids by roots and associated microflora, the decomposition of litter and soil organic matter to form soil CO2, the reaction of CO2 with water to form carbonic acid, the reaction of organic and carbonic acids with calcium and magnesium silicates, and the formation of dissolved calcium, magnesium, and bicarbonate in soil and groundwater. 

The dominant source of organic carbon in seawater is the photosynthetic fixation of C02 by unicellular algae (phytoplankton) in the photic zone. Their growth by cell division is rapid, but the population is kept in balance by grazing species (zooplankton). DeVooys (1979) has discussed the state of the art for determining the rate of primary production of marine biomass. Recent estimates, all based on the take-up of radiocarbon, fall into the... 

Precipitation of carbonate, largely from total dissolved carbon which is present primarily as HCOb", involves a much smaller fractionation of carbon isotopes compared to the photosynthetic fixation of carbon. In fact, the 5 C of calcite is relatively insensitive to changes in temperature (e.g. 0.035%o °C" Emrich et al. 1970) such that carbonate minerals can be used generally to monitor... 

Along with adenosine 5 -triphosphate (ATP), NADPH is an important intermediary in the photosynthetic fixation of carbon dioxide. The protons and electrons react with carbon dioxide via these two mediators to produce sugars. The overall photo-biochemical process taking place in green plants is represented by ... 

It is now clear that every technically and economically feasible source of additional methane must be tapped to meet the growing demand for natural gas. One potentially large-scale source of methane is land- and water-based biomass which can be converted to substitute natural gas (SNG) by a variety of techniques. Because biomass is a renewable nonfossil carbon source that derives its energy from photosynthetic fixation of ambient carbon dioxide, the concept could lead to the development of perpetually available SNG supplies 0). 

Carbon dioxide release. Carbon dioxide that is released as the final mineralization product of biopolymers originates from the renewable carbon source for then-biosynthesis. Photosynthetic fixation of the released carbon dioxide by plants generates renewable carbon sources again. Thns, the carbon flnx in the synthesis and degradation of biopolymers is balanced. PHAs therefore do not contribute to global warming. 

As the bifunctional enzyme that catalyzes the competing reactions of ribulosebisphosphate with either C02 or O2, the respective initinal steps in photosynthetic fixation of CO2 and in photorespiration, ribulosebisphosphate carboxylase/oxygenase [abbreviated as RuBisCO,... 

Phosphoenolpyruvate carboxylase (PEPC) and pyruvate Pi dikinase (PPDK) play significant roles in the photosynthetic fixation of carbon in and CAM plants. PEPC catalyzes the fixation of atmospheric CO2 to phosphoenolpyruvate (1), and PPDK catalyzes the formation of phosphoenolpyruvate, the substrate for PEPC (2). 

Carbon has several isotopes, the most abundant of which has an atomic mass of 12 Daltons and is designated C. The isotope C is a stable, naturally occurring isotope, much less abundant than C, constituting about 1.1% of the carbon in atmospheric CO2 (Winkler and Schmidt, 1980). Photosynthetic fixation of CO2 by plants takes place via three different routes, which depend on the nature of the plant (Brause and Raterman, 1982 Brause et al., 1984 Bricout and Koziet, 1987 Pollard, 1993 van der Merwe, 1992 Winkler and Schmidt, 1980). The higher plants fix CO2 via the carbon cycle to form a three-carbon compound, the C3 pathway. In some... 

Photosynthetic fixation of CO2 is an important form of C. (see Calvin cycle), and C also occurs in fatty acid and purine metabolism (Fig.). C of 5-ami-noimidazole-ribotide to 5-aminoimidazole-carboxa-mide-ribotide does not require preliminary activation of the CO2. 

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