Can isotopic fractionation during respiration explain the ¹³C‐enriched sporocarps of ectomycorrhizal and saprotrophic fungi?

Abstract

Summary• The mechanism behind the 13 C enrichment of fungi relative to plant materials is unclear and constrains the use of stable isotopes in studies of the carbon cycle in soils.• Here, we examined whether isotopic fractionation during respiration contributes to this pattern by comparing δ 13 C signatures of respired CO 2 , sporocarps and their associated plant materials, from 16 species of ectomycorrhizal or saprotrophic fungi collected in a Norway spruce forest.• The isotopic composition of respired CO 2 and sporocarps was positively correlated. The differences in δ 13 C between CO 2 and sporocarps were generally small, < ±1‰ in nine out of 16 species, and the average shift for all investigated species was 0.04‰. However, when fungal groups were analysed separately, three out of six species of ectomycorrhizal basidiomycetes respired 13 C-enriched CO 2 (up to 1.6‰), whereas three out of five species of polypores respired 13 C-depleted CO 2 (up to 1.7‰; P < 0.05). The CO 2 and sporocarps were always 13 C-enriched compared with wood, litter or roots.• Loss of 13 C-depleted CO 2 may have enriched some species in 13 C. However, that the CO 2 was consistently 13 C-enriched compared with plant materials implies that other processes must be found to explain the consistent 13 C-enrichment of fungal biomass compared with plant materials.

Linking aboveground and belowground food webs through carbon and nitrogen stable isotope analyses

“…For the second mechanism, the selective use of 13 Cenriched components of substrates such as cellulose, which are about 2& higher than bulk wood (Benner et al 1987;Gleixner et al 1993), may contribute to the fungal d 13 C increase (Hobbie et al 2001;Bostrom et al 2008). The selective use of 13 C-enriched C might explain why 13 C enrichment of saprotrophic fungi is greater when the fungi feed on wood than on simple substrates such as glucose.…”

“…Gluconeogenesis, which is the metabolic pathway that generates glucose from non-carbohydrate substrates such as amino acids, has been proposed as the first mechanism (Kohzu et al 2005), because this pathway should favor 13 C-rich monomers for the hexose biosynthesis through aldolase reactions (Schmidt et al 1995). Isotopic fractionation during respiration might also explain fungal 13 C enrichment, but a recent study shows that this process is of minor importance, because both the fungal tissues and the respired CO 2 are enriched in 13 C compared to the plant substrate (Bostrom et al 2008).…”

Biological and physical influences on the carbon isotope content of CO2 in a subalpine forest snowpack, Niwot Ridge, Colorado

“…There is some evidence that different fungal groups have systematically different d 13 C in their respiration (Böstrom et al 2007). Despite the strong possibility of microbial responses to the midwinter and spring melts, we did not observe clear changes in the d 13 C of the apparent respiratory source (Fig.…”

“…6 lends some validity to the steady-state assumption, at least for the apparent respiratory source inferred from snowpack observations. The second assumption is more problematic, since d 13 C of root and heterotrophic respiration are likely different (Böstrom et al 2007;Bowling et al 2008;Fernandez et al 2003;Klumpp et al 2005), and since d 13 C of soil respiration can change over time scales of a few days (Ekblad and Högberg 2001), at least in the summer. The variability in d R*,c shown in Fig.…”

Carbon nutrition of mature green orchid Serapias strictiflora and its mycorrhizal fungus Epulorhiza sp.

“…The 13 C enrichment of mycorrhizal ERM compared to S. strictiflora root tissues (2.3 ‰) reflects its nutritional dependence on orchid roots. This value is comparable to the 1.5‰ enrichment of ectomycorrhizal fungi relative to the fine roots of Pinus sylvestris (Hobbie and Colpaert 2004) and to the mean 2.3 ‰ enrichment calculated from seven different ectomycorrhizal associations (Boström et al 2008). The good ERM development under conditions precluding saprotrophic nutrition (-C 4 -Ben) implies sufficient transfer of carbon for the OM fungus nutrition from the orchid and full fungal biotrophy.…”