page_title: Fractionation of carbon isotopes in oxygenic photosynthesis
hierarchical_section_title: Fractionation of carbon isotopes in oxygenic photosynthesis
caption_attribution_description: English: A simplified model of a chemical reaction with pathways for H and D isotopes of hydrogen. The positions on the energy wells are based on the quantum harmonic oscillator. Note the lower energy state of the heavier isotope and the higher energy state of the lighter isotope. Under equilibrium conditions, the heavy isotope is favored in the products as it is more stable. Under kinetic conditions, like an enzymatic reaction, the lighter isotope is favored because of a lower activation energy.
context_page_description: 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. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated with among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.
  Oxygenic photosynthesis is a metabolic pathway facilitated by autotrophs, including plants, algae, and cyanobacteria. 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.
context_section_description: 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. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated with among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.
  Oxygenic photosynthesis is a metabolic pathway facilitated by autotrophs, including plants, algae, and cyanobacteria. 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.  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. These reactions undergo a kinetic isotope effect because they are limited by overcoming an activation energy barrier. 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). The following sections will outline the different oxygenic photosynthetic pathways and what contributes to their associated delta values.
caption: A simplified model of a chemical reaction with pathways for a light isotope (H) and heavy isotope (D) of hydrogen. The same principle applies for the light isotope 12C and heavy isotope 13C of carbon. The positions on the energy wells are based on the quantum harmonic oscillator. Note the lower energy state of the heavier isotope and the higher energy state of the lighter isotope. Under kinetic conditions, such as an enzymatic reaction with RuBisCO, the lighter isotope is favored because of a lower activation energy.
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