Carbon isotope fractionation in formation of amino acids by photosynthetic organisms.

When inorganic carbon is converted to living matter during photosynthesis, there is an isotope effect and the carbon-containing compounds of cells have a slightly lower C13 concentration than the carbonate and carbon dioxide of the environment.' Craig2 has reviewed the existing data on carbon isotope abundances in nature. Park and Epstein3 4 have recently reported on some carbon isotope fractionations in plants grown in the laboratory. The effect of isotopic substitution on the physicochemical properties of molecules has been well studied for inorganic and nonbiological organic systems.5 The relative rates of photosynthesis of the C13and C'2-containing molecules are of a similar order of magnitude as observed for reactions involving formation or rupture of carbon bonds. Although it may be difficult to achieve precise understanding of the carbon isotope effect in living systems, the more qualitative and descriptive features are interesting and have already been the subject of geochemical speculation. Thus, Epstein and Silverman6 have made extensive measurements on petroleum and petroleum fractions. Since marine organisms tend to have a slightly higher concentration of C13 than nonmarine forms, carbon isotope measurements may be important to investigations of the origin and migration of petroleum. In a study of a Finnish Precambrian formation, Rankama7 observed that a problematic fossil, Coryecium enigmaticum (probable age >1.4 X 109 years) has carbon which is depleted in C13 similar to present organic carbon. Rankama's conclusion concerning early life obtained from this evidence has been discussed by Craig.8 Detailed study of this effect might produce information relevant to photosynthesis, biosynthesis, and comparative biochemistry. Accordingly, we have investigated carbon isotope fractionation in a variety of photosynthetic and nonphotosynthetic organisms, including some grown in the laboratory under controlled conditions and some that had grown in a natural environment. A typical experiment consisted of (1) culturing the organisms, (2) extracting the lipides, (3) hydrolyzing the proteins, (4) separating pure amino acids by ion-exchange chromatography, (5) combusting a portion of the amino acids to carbon dioxide, (6) decarboxylating a portion of the amino acids with ninhydrin and purifying the liberated carbon dioxide, and (7) performing an isotopic analysis on the