Growth-Dependent Stable Carbon Isotope Fractionation by Basidiomycete Fungi: δ13C Pattern and Physiological Process

We grew 11 basidiomycetes in axenic culture to characterize their physiological capacities to fractionate stable C isotopes. Generally, δ¹³C values of the fungal biomass were (i) enriched in ¹³C relative to the growth medium, (ii) variable among the isolates, and (iii) dependent on the growth rate and growth stage of the fungi. We found a multiphasic dynamic of fractionation for Cryptoporus volvatus and Marasmius androsaceus during various growth stages. The first phase, P1, corresponded to the exponential growth stage and was characterized by an increasing enrichment in ¹³C content of the fungal biomass relative to the growth medium ranging between 4.6 and 6.9‰. The second phase, P2, exhibited a continual depletion in ¹³C of the fungal biomass, with the δ¹³C values of the fungal biomass asymptotically returning to the δ¹³C value of the growth medium at inoculation. The expression of the various fractionation phases was dependent on the amount of low-concentration micronutrients and growth factors added to the growth medium. The onset of P2 occurred at reduced concentrations of these elements. All of the sugars in the growth medium (sucrose, maltose, and glucose) were utilized for growth, indicating that the observed fractionation was not an artifact derived from the preferential use of ¹³C-rich maltose, which was found at low concentrations in the growth medium. In this study, we establish a framework with which to explore the impact of physiological fractionations by fungal interfaces on natural distributions of stable C isotopes.