fractionation of carbon isotopes in oxygenic photosynthesis


  • The overall equation for the light-dependent reactions is generally:[11] Overview of the Calvin cycle and carbon fixation C3 Pathway The light-independent reactions undergo
    the Calvin-Benson cycle, in which the energy from NADPH and ATP is used to convert carbon dioxide and water into organic compounds via the enzyme RuBisCO.

  • A prevailing model for fractionation of atmospheric CO2 in plants combines the isotope effects of the carboxylation reaction with the isotope effects from gas diffusion into
    the plant in the following equation:[16] Where: δ13Csample is the delta-value of the organism for 13C composition ca is the partial pressure of CO2 in the external atmosphere, and ci is the partial pressure of CO2 in the intercellular spaces.

  • [8][10] This is called a “delta value” and can be calculated from the formula below: Photosynthesis reactions The chemical pathway of oxygenic photosynthesis fixes carbon
    in two stages: the light-dependent reactions and the light-independent reactions.

  • [15] 13C fractionation model In addition to the discriminating effects of enzymatic reactions, the diffusion of CO2 gas to the carboxylation site within a plant cell also
    influences isotopic fractionation.

  • [1] The δ13C of C3 plants depends on the relationship between stomatal conductance and photosynthetic rate, which is a good proxy of water use efficiency in the leaf.

  • In C3 plants A C3 plant uses C3 carbon fixation, one of the three metabolic photosynthesis pathways which also include C4 and CAM (described below).

  • [36] The biochemical characteristics of phytoplankton are similar to C3 plants, whereas the gas exchange characteristics more closely resemble the C4 strategy.

  • In phytoplankton In contrast to terrestrial plants, where CO2 diffusion in air is relatively fast and typically not limiting, diffusion of dissolved CO2 in water is considerably
    slower and can often limit carbon fixation in phytoplankton.

  • [29] This pathway differs from C4 photosynthesis because CAM plants separate carbon by storing fixed CO2 in vesicles at night, then transporting it for use during the day.

  • [37] More specifically, phytoplankton improve the efficiency of their primary carbon-fixing enzyme, RuBisCO, with carbon concentrating mechanisms (CCM), just as C4 plants
    accumulate CO2 in the bundle sheath cells.

  • [1] Oxygenic photosynthesis takes place in plants and microorganisms through different chemical pathways, so various forms of organic material reflect different ratios of
    13C isotopes.

  • These plants are called “C3” due to the three-carbon compound (3-Phosphoglyceric acid, or 3-PGA) produced by the CO2 fixation mechanism in these plants.

  • This pathway allows C4 photosynthesis to efficiently shuttle CO2 to the RuBisCO enzyme and maintain high concentrations of CO2 within bundle sheath cells.

  • [13][14] RuBisCO causes a kinetic isotope effect because 12CO2 and 13CO2 compete for the same active site and 13C has an intrinsically lower reaction rate.

  • Organic carbon contains less of the stable isotope Carbon-13, or 13C, relative to the initial inorganic carbon from the atmosphere or water because photosynthetic carbon fixation
    involves several fractionating reactions with kinetic isotope effects.

  • This model, derived ab initio, generally describes fractionation of carbon in the majority of plants, which facilitate C3 carbon fixation.

  • In contrast, C3 plants directly perform the Calvin Cycle in mesophyll cells, without making use of a CO2 concentration method.

  • The adaptations of C4 plants provide an advantage over the C3 pathway, which loses efficiency due to photorespiration.

  • Carbon isotope measurement Carbon on Earth naturally occurs in two stable isotopes, with 98.9% in the form of 12C and 1.1% in 13C.

  • [33] Isotope fractionation of 13C by phytoplankton photosynthesis is affected by the diffusion of extracellular aqueous CO2 into the cell, the RuBisCO-dependent cell growth
    rate, and the cell geometry and surface area.

  • This pathway converts inorganic carbon dioxide from the atmosphere or aquatic environment into carbohydrates, using water and energy from light, then releases molecular oxygen
    as a product.

  • The isotope fractionations in C3 carbon fixation arise from the combined effects of CO2 gas diffusion through the stomata of the plant, and the carboxylation via RuBisCO.

  • This process alone is similar to that of C4 plants and yields characteristic C4 fractionation values of approximately -11‰.

  • [22][27] Isotopic fractionation differs between C4 carbon fixation and C3, due to the spatial separation in C4 plants of CO2 capture (in the mesophyll cells) and the Calvin
    cycle (in the bundle sheath cells).

  • The lighter isotope has a higher energy state in the quantum well of a chemical bond, allowing it to be preferentially formed into products.

  • Different organisms fix carbon through different mechanisms, which are reflected in the varying isotope compositions across photosynthetic pathways (see table below, and explanation
    of notation in “Carbon Isotope Measurement” section).

  • [4] In CAM plants Plants that use Crassulacean acid metabolism, also known as CAM photosynthesis, temporally separate their chemical reactions between day and night.

  • [19] C3 plants with high water-use efficiency tend to be less fractionated in 13C (i.e., δ13C is relatively less negative) compared to C3 plants with low water-use efficiency.

  • [1][8] The ratio between these isotopes varies in biological organisms due to metabolic processes that selectively use one carbon isotope over the other, or “fractionate”
    carbon through kinetic or thermodynamic effects.

  • This carbon is released to the Calvin cycle during the day, when stomata are closed to prevent water loss, and the light reactions can drive the necessary ATP and NADPH production.

  • Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes
    of carbon.

  • [19] In C4 plants C4 plants have developed the C4 carbon fixation pathway to conserve water loss, thus are more prevalent in hot, sunny, and dry climates.

  • [2] The wide range of variation in delta values expressed in C3 plants is modulated by the stomatal conductance, or the rate of CO2 entering, or water vapor exiting, the small
    pores in the epidermis of a leaf.

  • [1] RuBisCO enzyme catalyzes the carboxylation of CO2 and the 5-carbon sugar, RuBP, into 3-phosphoglycerate, a 3-carbon compound through the following reaction: The product
    3-phosphoglycerate is depleted in 13C due to the kinetic isotope effect of the above reaction.

  • The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism.

  • [6] These combined effects provide δ13C values for CAM plants in the range of -10 to -20‰.


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