Carbon isotope fractionation of Thermoanaerobacter kivui in different growth media and at different total inorganic carbon concentration

Abstract Chemolithotrophic homoacetogenic bacteria apparently express a characteristic stable carbon isotope fractionation and may contribute significantly to acetate production in anoxic environments. However, fractionation factors ( e ) in bacterial cultures have rarely been determined and the effect of substrate availability has not been assessed. We therefore studied the kinetic carbon isotope effect in cultures of Thermoanaerobacter kivui grown at 55 °C. The fractionation factor in HCO 3 − buffered medium was ca. 15‰ more negative than that in PO 4 3− buffered medium. To test whether the difference was caused by the initial substrate ratio of H 2 and total inorganic carbon (TIC; 0.5 in HCO 3 − vs. 4.0 in PO 4 3− buffered medium), T. kivui was grown in either [3-(N-morpholino) propanesulfonic acid, MOPS] buffered or PO 4 3− buffered media with different HCO 3 − concentration. Indeed, the fractionation factor became more negative with increasing HCO 3 − concentration and decreasing H 2 /TIC ratio. While pH had only a small effect, the fractionation was generally more negative in MOPS buffered than in phosphate buffered media, indicating that the buffer system also affected fractionation. Collectively, the results show that substrate availability and other environmental factors affect the magnitude of isotope fractionation during acetate production by chemolithotrophic homoacetogenesis.