Natural Abundance of the Stable Isotopes of Carbon in Biological Systems

Abstract

About 99% of all carbon is the 12C isotope while 1% is 13C. The other isotopes of carbon are, by comparison, extremely rare. For instance, only about one atom in every 1012 carbon atoms is 14C. The precise ratio of the isotopes will vary depending on the material analyzed. Limestones, atmospheric C02, marine algae, and land plants each possess characteristic 13C/12C ratios, differing slightly from one another. The lowest ratio so far observed is for carbon from ancient blue-green algal mats (Kaplan and Nissenbaum, 1966) and the highest for carbonate carbon from meteorites (Clayton, 1963). Fractionation of plant carbon is brought about primarily by carbon dioxide assimilation in photosynthesis and is due to preferential utilization of 12C and exclusion of 13C. Curiously enough, it has been found recently (Tregunna et al., 1970) that higher plants which fix carbon dioxide via the Calvin cycle pathway differ in 13C/12C ratios from plants which fix carbon dioxide via the C4-dicarboxylic acid pathway. Thus, it is now possible to determine whether a given sample of sucrose was synthesized in sugarcane or in sugar beet. While it is the aim of this paper to discuss a few recent studies on naturally-occurring 13C/12C ratios of biological materials, it is pertinent to note that the subject has been reviewed from a geochemical viewpoint (Bowen, 1966; Degens, 1969; Kroepelin, 1966; McMullen and Thode, 1963; Schwarcz, 1969). The various isotopic species of an element differ slightly in chemical properties from one another. Just as the chemical properties of different compounds determine their formation under various conditions, so the chemical properties of different isotopic species of the compounds will determine how the isotopic abundances distribute themselves in nature. Thus substitution