Figure 2.

Modeling the isotopic microbial fractionation phenomenon dependency on energy and substrate concentrations as derived from our theory. (a) Isotopic microbial fractionation factor (α) as a function of substrate concentration times the square of the catabolic exergy (x axis) as predicted from our microbial growth theory (Equation (3)). (b) Experimental data from Penning et al. (2005) (triangles) obtained with Methanobacterium bryantii; and Valentine et al. (2004) (circles) obtained with Methanothermobacter marburgensis; on ¹³C isotopic fractionation associated with hydrogenotrophic methanogenesis confirming the existence of microbial overfractionation. (c) Experimental data from Kampara et al. (2008) including fractionation factor obtained after 50% (triangles) or 70% (circles) of toluene degradation by Pseudomonas putida under aerobic conditions confirming the existence of microbial underfractionation. (d) Experimental data from Goevert and Conrad (2009); on ¹³C isotopic fractionation associated with aceticlastic methanogenesis by pure cultures of Methanosarcina barkeri and M. acetivorans. Interestingly, in this case, it illustrates the switch between depletion (α>1) and enrichment (α<1) of reaction products in heavy isotope depending on the value of as predicted by our model (see Supplementary Material).