Fractionation of carbon isotopes in oxygenic photosynthesis information
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.[1] The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated 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.[2][3]
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.
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.[4] 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.
Different photosynthetic pathways (C3, C4, and CAM) yields biomass with different δ13C values.
Isotope Delta Values of Photosynthetic Pathways
Pathway
δ13C (‰)
C3
-20 to -37[2]
C4
-12 to -16[5]
CAM
-10 to -20[6]
Phytoplankton
-18 to -25[4][7]
^G D Farquhar; J R Ehleringer; Hubick, and K. T. (1989). "Carbon Isotope Discrimination and Photosynthesis". Annual Review of Plant Physiology and Plant Molecular Biology. 40 (1): 503–537. doi:10.1146/annurev.pp.40.060189.002443.
^ abKohn, Matthew J. (2010-11-16). "Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate". Proceedings of the National Academy of Sciences. 107 (46): 19691–19695. Bibcode:2010PNAS..10719691K. doi:10.1073/pnas.1004933107. ISSN 0027-8424. PMC 2993332. PMID 21041671.
^Fry, B.; Sherr, E. B. (1989). "δ13C Measurements as Indicators of Carbon Flow in Marine and Freshwater Ecosystems". Stable Isotopes in Ecological Research. Ecological Studies. Vol. 68. New York, NY: Springer New York. pp. 196–229. doi:10.1007/978-1-4612-3498-2_12. ISBN 9781461281276.
^ abHayes, John (2001-01-01). "Fractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes". Reviews in Mineralogy & Geochemistry. 43 (1): 225–277. Bibcode:2001RvMG...43..225H. doi:10.2138/gsrmg.43.1.225.
^O'Leary, Marion H. (May 1988). "Carbon Isotopes in Photosynthesis". BioScience. 38 (5): 328–336. doi:10.2307/1310735. ISSN 0006-3568. JSTOR 1310735.
^O'Leary, Marion H. (1988). "Carbon Isotopes in Photosynthesis". BioScience. 38 (5): 328–336. doi:10.2307/1310735. JSTOR 1310735.
^Popp, Brian N.; Laws, Edward A.; Bidigare, Robert R.; Dore, John E.; Hanson, Kristi L.; Wakeham, Stuart G. (January 1998). "Effect of Phytoplankton Cell Geometry on Carbon Isotopic Fractionation". Geochimica et Cosmochimica Acta. 62 (1): 69–77. Bibcode:1998GeCoA..62...69P. doi:10.1016/S0016-7037(97)00333-5. ISSN 0016-7037.
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