Carbon 13/carbon 12 ratios
The carbon 13 to carbon 12 ratio is an important measure in understanding the isotopic composition of carbon in various environments, primarily in relation to plant photosynthesis. Carbon has three isotopes, with carbon 12 (¹²C) being the most abundant, making up about 99% of natural carbon, while carbon 13 (¹³C) constitutes the remaining 1%. These isotopes differ in neutron count, with ¹²C having six neutrons and ¹³C having seven. During photosynthesis, plants tend to favor the lighter ¹²C over ¹³C, leading to variations in carbon isotope ratios across different plant types.
C3 plants exhibit a greater discrimination against ¹³C, resulting in carbon isotope ratios that can range from -20 to -35 per mil, while C4 plants show less discrimination, with ratios around -8 per mil. Crassulacean Acid Metabolism (CAM) plants can display intermediate ratios. These differences arise from the specific enzymes involved in carbon fixation, particularly Rubisco in C3 plants and PEP carboxylase in C4 and CAM plants. Understanding these ratios can provide insights into plant metabolic processes, ecological dynamics, and even historical climate conditions. The measurement of these isotopic ratios is typically conducted using mass spectrometry, allowing researchers to draw connections between plant types and their respective environments.
Carbon 13/carbon 12 ratios
Categories: Cellular biology; photosynthesis and respiration; physiology
Although the atom is the smallest unit having the properties of its element, atoms are composed of subatomic particles, of which protons, neutrons, and electrons are the most important. All atoms of a given element (in their non-ionic form) have the same number of protons and electrons, but some atoms may have more neutrons than other atoms of the same element. These different atomic forms are called isotopes of the element, and in nature there is a mixture of isotopes for most elements.
Carbon Isotopes
Carbon has an atomic number of 6, meaning that it has six protons. Most carbon atoms also have six neutrons. Because the atomic weight is largely determined by the mass of the protons plus neutrons, this isotope is called carbon 12 (12C); it is the most common form of carbon, accounting for about 99 percent of the carbon in nature. Most of the remaining 1 percent of carbon consists of atoms of the isotope carbon 13 (13C), with seven rather than six neutrons; thus, this isotope is heavier than 12C. A third isotope, carbon 14 (14C), is present in the environment in minute quantities but is not very stable. Consequently, 14C decays spontaneously, giving off radiation, and thus it is a radioactive isotope. Both 12C and 13C are considered stable isotopes.
Stable isotopes are measured using a mass spectrometer, an instrument that separates atoms on the basis of their mass differences. Initially when plant material is combusted, the carbon dioxide (CO2) given off is analyzed by the mass spectrometer for the ratio of carbon 13 to carbon 12 isotopes. This ratio is compared to the ratio of carbon 13 to carbon 12 in an internationally accepted standard of known 13C to 12C ratio and is expressed as the difference between the sample and the standard, minus one. This number is multiplied by one thousand and expressed as a “per mil” (per million parts). In plant matter, this number is always negative. The more negative the ratio of carbon 13 to carbon 12, the less carbon 13 there is present.
C3, C4, and CAM Plants
There are three biochemical pathways of carbon acquisition used by different plant species, and the ratio of carbon 13 to carbon 12 in plant tissues is often a useful means of distinguishing the photosynthetic pathway being used. Most plants photosynthesize by attaching CO2 obtained from the atmosphere onto an organic compound in a single carbon fixation step that is a part of the Calvin cycle. This reaction is catalyzed by an enzyme known as ribulose bisphosphate carboxylase (Rubisco), and the first stable organic product is a three-carbon molecule. Some of this three-carbon compound enters a biochemical pathway leading to sugar formation, and the remainder is used to maintain the Calvin cycle. Such plants are referred to as C3 plants.
Some plants, such as corn and sugarcane, have two carbon fixation steps. Atmospheric CO2 is fixed initially by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), and the first product is a four-carbon organic acid; these plants are known as C4 plants. This product is moved to the interior of the leaf and broken down, releasing CO2 into specialized cells known as Kranz-type bundle sheath cells. Within these cells the CO2 is fixed a second time by the same process used in C3 plants.
A third group of plants, known as CAM plants (for crassulacean acid metabolism), open their stomatal pores (located on the leaf surfaces) primarily at night. CO2 enters and is fixed with the enzyme PEP carboxylase. The organic acid produced is stored in the cell overnight. During the day, the stomata are closed, and the acid is broken down. The CO2 released is then fixed by the same method as in C3 plants.
Discrimination Against Carbon 13
In terrestrial plants, carbon isotope ratios of photosynthetic tissues vary from –8 per mil to -15 per mil in C4 plants and from –20 per mil to –35 per mil in C3 plants. CAM plants may range from C4-like to C3-like in their carbon isotope ratios. For atmospheric carbon dioxide, the ratio of carbon 13 to carbon 12 is about –8 per mil, and thus C4 plant tissues have slightly less carbon 13 than the air. C3 plants have much less carbon 13 than the air. In other words, during photosynthesis plants tend to discriminate against 13CO2 molecules and more readily fix 12CO2. This discrimination against carbon 13 is even more pronounced in C3 plants.
Discrimination against carbon 13 is attributable to the greater mass of this isotope. One consequence is that 13CO2 does not diffuse as readily to the site of photosynthesis as does 12CO2, which accounts for a small component of discrimination against carbon 13 in all plants. The major difference in carbon isotope ratio between C3 and C4 plants, however, results from a difference in discrimination among the initial carbon-fixing enzymes. In C3 plants, the carbon-fixing enzyme Rubisco results in a –27 per mil discrimination against carbon 13. In C3 plants, this enzyme is present in the cells adjacent to the stomatal pores and thus obtains CO2 more or less directly from the atmosphere. Because carbon 13 is discriminated against, 13CO2 will tend to accumulate, but it readily diffuses out of the leaf when stomatal pores are open.
In C4 plants, on the other hand, the initial carbon-fixing enzyme, PEP carboxylase, discriminates very little against carbon 13. In C4 photosynthesis, the secondary carbon-fixing enzyme, Rubisco, is sequestered in the interior of the leaf in the bundle sheath cells, and the CO2 it fixes is derived from the breakdown of the C4 fixation product. As Rubisco discriminates against 13CO2, this heavier CO2 accumulates within the bundle sheath cells and diffuses out very slowly. As 13CO2 accumulates in the bundle sheath cells, the higher concentration of 13CO2 will overcome the discrimination by the enzyme; in effect, the enzyme will be forced to fix 13CO2, and thus discrimination is minimal. C4 plant tissues consequently have less negative 13CO2/12CO2 ratios.
In typical CAM photosynthesis, the atmospheric CO2 is fixed at night by the enzyme PEP carboxylase, and, as in C4 plants, this enzyme discriminates very little against 13CO2. During the day, the C4 fixation product is broken down, and the CO2 that is released is fixed by Rubisco. This enzyme will discriminate against carbon 13, but because the stomata are closed during the day, 13CO2 will accumulate within the leaf and eventually be fixed. Consequently, little discrimination occurs. Such CAM plants have ratios similar to those of C4 plants. Some CAM plants, however, will open their stomatal pores for varying lengths of time during the day or switch to strictly C3 photosynthesis during certain times of the year. In these plants, the 13CO2/12CO2 ratio will be more similar to that observed for C3 plants.
In aquatic plants, the 13CO2/12CO2 ratio does not indicate the photosynthetic pathway used. C3 aquatic plants frequently will have carbon isotope ratios very similar to that of the source carbon from the water: Because the enzyme Rubisco discriminates against carbon 13, 13CO2 tends to accumulate in the layer of water around the leaf. Because the diffusion of gases in water is very slow, the plant will eventually be forced to fix the 13CO2. Other aspects of the aquatic environment also influence the carbon isotope ratio of aquatic plant tissues.
Bibliography
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Coleman, David C., and Brian Fry, eds. Carbon Isotope Techniques. San Diego: Academic Press, 1991. A reference book with protocols for using carbon isotope tracers in experimental biology and ecology.
Ehleringer, James R., Anthony E. Hall, and Graham D. Farquhar, eds. Stable Isotopes and Plant Carbon/Water Relations. San Diego: Academic Press, 1993. A wide-ranging collection of essays on the theory and uses of stable isoptopes, written from both agricultural and ecological perspectives.
Erez, Jonathan, Anne Bouevitch, and Aaron Kaplan. “Carbon Isotope Fractionation by Photosynthetic Aquatic Microorganisms.” Canadian Journal of Botany 76, no. 6 (June, 1998): 1109-1118. Concludes that the natural fractionation of carbon isotopes is mainly due to the discrimination of ribulose-1,5-bisphosphate carboxylase-oxygenase against carbon 13 during photosynthesis.
Rundel, Phillip W., James R. Ehleringer, and Kenneth A. Nagy, eds. Stable Isotopes in Ecological Research. New York: Springer-Verlag, 1988. Includes twenty-eight chapters describing all aspects of how stable isotopes can be used in ecological studies. The use of stable isotopes other than carbon is discussed. Each chapter includes an extensive bibliography.