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STABLE ISOTOPIC EVIDENCE OF CARBON AND NITROGEN USE IN CULTURED ECTOMYCORRHIZAL AND SAPROTROPHIC FUNGI
Citation:
Sanchez, F. S., E. A. Hobbie, AND P T. Rygiewicz. STABLE ISOTOPIC EVIDENCE OF CARBON AND NITROGEN USE IN CULTURED ECTOMYCORRHIZAL AND SAPROTROPHIC FUNGI. Mycological Research. Cambridge University Press, Cambridge, Uk, 108(7):725-736, (2004).
Description:
Stable isotopes in sporocarps have proven useful for inferring ectomycorrhizal or saprotrophic status and understanding carbon (C) and nitrogen (N) utilization. However, greater understanding of processes producing isotopic concentrations is needed. We measured natural abundance of 15N and 13C, and N and C concentrations in mycelia of 10 and seven strains of ectomycorrhizal (EMF) and saprotrophic (SPF) fungi, respectively. Mycelia were grown on three agar media [Modified Melin Norkrans (MMN), MMN- (MMN with malt extract excluded), and Potato Dextrose Agar (PDA)], and then mycelia were subjected to mass spectrometer analysis. Compared with the other EMF, Russula adusta, R. Sororia and Lactarius deliciosus were depleted in 15N on MMN and MMN-, whereas the Russula species were enriched in 15N on PDA. Among SPF, Agaricus subrutilescens displayed 15N patterns similar to the Russula species with respect to media type. EMF mycelia were more enriched in d13C than SPF mycelia on all media. EMF fungi were only 0.4% more negative in d13C than the C source (glucose) used in MMN-. SPF were almost 2.5% depleted relative to this C source. Although most species produced d15N values similar to the media, variable results indicate that d15N signatures do not necessarily reflect d15N signatures of substrates. Isotopic fractionation during uptake or secretion of isotopically-altered compounds may influence fungal d15N signatures, even when source d15N is held constant. The d13C data do not contradict field-based studies suggesting differences in d13C between SPF and EMF arise from differences in source d13C. However, they do suggest that differences in source d13C between the two fungal types calculated using d13C measurements from field samples actually may be greater than the field data would indicate. Using both d13C and %C measurements in cultured fungi may be useful in inferring EMF and SPF status. Biochemical processes within fungi that produce isotopic fractionation need to be elucidated so that field- and laboratory-based measurements enhance our understanding of the physiological ecology of these fungi.