Abstract
Data for stable C and N isotope natural abundances of arbuscular mycorrhizal (AM) fungi are currently sparse, as fungal material is difficult to access for analysis. So far, isotope analyses have been limited to lipid compounds associated with fungal membranes or storage structures (biomarkers), fungal spores and soil hyphae. However, it remains unclear whether any of these components are an ideal substitute for intraradical AM hyphae as the functional nutrient trading organ. Thus, we isolated intraradical hyphae of the AM fungus Rhizophagus irregularis from roots of the grass Festuca ovina and the legume Medicago sativa via an enzymatic and a mechanical approach. In addition, extraradical hyphae were isolated from a sand-soil mix associated with each plant. All three approaches revealed comparable isotope signatures of R. irregularis hyphae. The hyphae were 13C- and 15N-enriched relative to leaves and roots irrespective of the plant partner, while they were enriched only in 15N compared with soil. The 13C enrichment of AM hyphae implies a plant carbohydrate source, whereby the enrichment was likely reduced by an additional plant lipid source. The 15N enrichment indicates the potential of AM fungi to gain nitrogen from an organic source. Our isotope signatures of the investigated AM fungus support recent findings for mycoheterotrophic plants which are suggested to mirror the associated AM fungi isotope composition. Stable isotope natural abundances of intraradical AM hyphae as the functional trading organ for bi-directional carbon-for-mineral nutrient exchanges complement data on spores and membrane biomarkers.
Highlights
Natural abundances of stable isotopes are used to trace carbon and both inorganic and organic nitrogen fluxes within plants, fungi and their associations (e.g. Gleixner et al 1993; Gebauer and Dietrich 1993; Courty et al 2015; Chen et al 2019; Saskia Klink and Philipp Giesemann have shared first authorship.Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Giesemann et al 2020; Suetsugu et al 2020)
We report a dual stable 13C and nitrogen (15N) isotope natural abundance approach to separate both soil hyphae and intraradical hyphae from soil or plant contaminants
In this study we present for the first time dual stable 13C and 15N isotope natural abundances of intraradical arbuscular mycorrhizal (AM) fungal hyphae
Summary
Natural abundances of stable isotopes are used to trace carbon and both inorganic and organic nitrogen fluxes within plants, fungi and their associations A few data on isotope natural abundances are available from spores, soil hyphae and biomarkers such as phospholipid fatty acids (PLFAs) or neutral lipid fatty acids (NLFAs) (Allen and Allen 1990; Nakano et al 1999; Courty et al 2011, 2015; Walder et al 2012, 2013; Suetsugu et al 2020) Still, it is unclear whether spores and PLFA/NLFA biomarkers mirror intraradical hyphae, the functional trading organ. 15N-signatures will provide further information on the relative importance of inorganic versus organic nitrogen as nutrient sources for AM fungi To address these uncertainties, in this study we present for the first time dual stable 13C and 15N isotope natural abundances of intraradical AM fungal hyphae. The feasibility of the isolation approach and the quantifiability of intraradical hyphae as the functional nutrient trading organ versus existing approaches using spores or PLFAs/NLFAs as specific fungal components are discussed
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