Stable carbon isotope fractionation of six strongly fractionating microorganisms is not affected by growth temperature under laboratory conditions

Abstract Temperature is the major driving force for many biological as well as chemical reactions and may impact the fractionation of stable carbon isotopes. Thus, a good correlation between temperature and fractionation is observed in many chemical systems that are controlled by an equilibrium isotope effect. In contrast, biological systems that are usually controlled by a kinetic isotope effect are less well studied with respect to temperature effects and have shown contrasting results. We studied three different biological pathways (methylotrophic methanogenesis, hydrogenotrophic methanogenesis, acetogenesis by the acetyl-CoA pathway) which are characterized by very strong carbon isotope enrichment factors (−50‰ to −83‰). The microorganisms ( Methanosarcina barkeri , Methanosarcina acetivorans , Methanolobus zinderi , Methanothermobacter marburgensis , Methanothermobacter thermoautotrophicus , Thermoanaerobacter kivui ) exhibiting these pathways were grown at different temperatures ranging between 25 and 68 °C, and the fractionation factors were determined from 13 C/ 12 C isotope discrimination during substrate depletion and product formation. Our experiments showed that the fractionation factors were different for the different metabolic pathways but were not much affected by the different growth temperatures. Slight variations were well within the standard errors of replication and regression analysis. Our results showed that temperature had no significant effect on the fractionation of stable carbon isotopes during anaerobic microbial metabolism with relatively strong isotope fractionation.