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Carbon and Nitrogen exchange in the tripartite symbiosis of beech, ectomycorrhizal fungi and soil bacteria – Universität Innsbruck

Seminar of the Department of Microbiology

Carbon and Nitrogen exchange in the tripartite symbiosis of beech, ectomycorrhizal fungi and soil bacteria

Christina Kaiser – Centre for Microbiology and Environmental Systems Science, Division of Terrestrial Ecosystem Research, University of Vienna, Austria

20.06.2024, 11:00 - Hybrid

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Abstract

Christina Kaiser

Almost all higher land plants deliver recently assimilated carbon (C) to their associated mycorrhizal fungi and receive nutrients in return. Whether this exchange is reciprocal, that is, whether fungi that deliver more nutrients receive more C in return and vice versa, is still under debate. In addition, very little is known about how mycorrhizal fungi interact with free-living soil saprotrophs: Do mycorrhizal fungi pass on the C received from plants to soil saprotrophs to stimulate the decomposition of organic matter, thereby adding another partner to the resource supply chains of the mycorrhizal symbiosis?
We approached these questions in dual stable-isotope labeling experiments where young, ectomycorrhizal beech trees were exposed to a 13CO2 enriched atmosphere, while their mycorrhizal partners received 15N-labelled amino acids and ammonium. We traced added 13C and 15N through the system using isotope ratio mass spectrometry (IRMS) to analyze 13C and 15N in bulk material (EA-IRMS), dissolved organic matter and microbial biomass (LC-IRMS) and 13C incorporation into phospholipid fatty acids (PLFAs, GCIRMS). In addition, we visualized the spatial distribution of 13C and 15N at the microscale across the plant-fungus interface in root cross sections, and at the fungi-bacteria interface at the surface of mycorrhizal hyphae using nanoscale secondary ion mass spectrometry (NanoSIMS).
While we only found weak signs of reciprocity of C and N exchange between plants and their fungal partners at the scales of root system architecture and across individual root tips, NanoSIMS imaging revealed a highly significant spatial correlation between 13C and 15N within a cross-section of a mycorrhizal root. This indicates that a mechanism at the cellular level exists, that (i) either allows plants to direct their C flow into N-delivering parts of the mycorrhizal hyphal network or (ii) allow the fungus to ‘draw’ more C from the plant (develop a higher sink strength) when it has access to N. We further found, based on 13C-PLFA biomarkers, a rapid transfer of recent photosynthates via ectomycorrhizal hyphae to bacteria in root-distant soil areas, which was suppressed when N availability in the hyphosphere increased. Our findings highlight the potential of NanoSIMS in situ isotope visualization combined with stable isotope tracing for investigating C and N dynamics at the plant-fungal and fungal-bacterial interfaces of the mycorrhizal symbiosis.

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