Carbon Isotope Fractionation by Autotrophic Bacteria with Three Different C 02 Fixation Pathways

Carbon isotope fractionation during autotrophic growth of different bacteria which possess different autotrophic C 0 2 fixation pathways has been studied. 13C /l2C-Ratios in the cell carbon of the following bacteria were determined ( C 0 2 fixation pathway suggested or proven in paren­ theses): Alkaligenes eutrophus (reductive pentose phosphate cycle), Desulfobacterium autotrophicum and A cetobacterium w oodii (reductive acetyl-CoA pathway), Desulfobacter hydrogenophilus and Therm oproteus neutrophilus (reductive citric acid cycle). The A ö l3C values, which indicate the p er m ille deviation of the 13C content of cell carbon from that of the CO : used as the sole carbon source, range from — 10%c (reductive citric acid cycle) over — 26%c (reductive pentose phosphate cycle) to —36%c (reductive acetyl-CoA pathway). Acetate formed via the acetyl-CoA pathway by the acetogenic Acetobacterium woodii showed a A 6 I3C = —40%c. These data are discussed in view of the different C 0 2 fixation reactions used by the bacteria and especially with regard to the isotopic composition of sedimentary carbon through time.