Carbon Isotope Fractionation of 11 Acetogenic Strains Grown on H2 and CO2

Acetogenic bacteria are able to grow autotrophically on hydrogen and carbon dioxide by using the acetyl coenzyme A (acetyl-CoA) pathway. Acetate is the end product of this reaction. In contrast to the fermentative route of acetate production, which shows almost no fractionation of carbon isotopes, the acetyl-CoA pathway has been reported to exhibit a preference for light carbon. In Acetobacterium woodii the isotope fractionation factor (epsilon) for C-13 and C-12 has previously been reported to be epsilon = -58.6 parts per thousand. To investigate whether such a strong fractionation is a general feature of acetogenic bacteria, we measured the stable carbon isotope fractionation factor of 10 acetogenic strains grown on H-2 and CO2. The average fractionation factor was epsilon(TIC) = -57.2 parts per thousand for utilization of total inorganic carbon and epsilon(acetate) = -54.6 parts per thousand for the production of acetate. The strongest fractionation was found for Sporomusa sphaeroides (epsilon(TIC) = -68.3 parts per thousand), the lowest fractionation for Morella thermoacetica (epsilon(TIC) = -38.2 parts per thousand). To investigate the reproducibility of our measurements, we determined the fractionation factor of 21 biological replicates of Thermoanaerobacter kivui. In general, our study confirmed the strong fractionation of stable carbon during chemolithotrophic acetate formation in acetogenic bacteria. However, the specific characteristics of the bacterial strain, as well as the cultural conditions, may have a moderate influence on the overall fractionation.