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Isotope Fractionation in Photosynthetic Bacteria during Carbon Dioxide Assimilation ¹

C. Roy Benedict^b

^a Department of Oceanography, Texas A & M University, College Station, Texas 77843, Department of Plant Sciences, Texas A & M University, College Station, Texas 77843

The _PDB¹³C values have been determined for the cellular constituents and metabolic intermediates of autotrophically grown Chromatium vinosum. The isotopic composition of the HCO₃^- in the medium and the carbon isotopic composition of the bacterial cells change with the growth of the culture. The _PDB¹³C value of the HCO₃^- in the media changes from an initial value of –6.6 to +8.1 after 10 days of bacterial growth and the _PDB¹³C value of the bacterial cells change from –37.5 to –29.2 in the same period. The amount of carbon isotope fractionation during the synthesis of hexoses by the photoassimilation of CO₂ has a range of –15.5 at time zero to –22.0 after 10 days. This range of fractionation compares to the range of carbon isotope fractionation for the synthesis of sugars from CO₂ by ribulose 1,5-diphosphate carboxylase and the Calvin cycle.

The amount of carbon isotope fractionation during the synthesis of aspartic acid from CO₂ is –24.9 at time zero and –15.0 after 10 days of bacterial growth. This amount of fractionation is in the range of carbon isotope fractionation for the synthesis of C₄ amino acids by a double carboxylation through ribulose 1,5-diphosphate and phosphoenolpyruvate carboxylase.