Amino acid stable isotope fingerprinting places arbuscular mycorrhizal fungi close to other fungal functional groups

Abstract

Arbuscular mycorrhizal (AM) fungi are an important part of the soil microbial community. While their effects on soil properties and nutrient cycles are widely studied, their direct contribution to soil food webs and energy fluxes therein remains poorly explored. Biomass of extraradical mycelium and spores of AM fungi in soil can be an important source of nutrients in AM dominated systems, such as grasslands and forests in both temperate and tropical regions. The compound specific stable isotope analysis of carbon in individual amino acids (CSIA-AA of carbon) is able to distinguish ectomycorrhizal from saprotrophic fungi. Here, we tested whether the method can also separate AM fungi from the other fungal groups by their amino acid isotopic profiles. We measured the δ¹³C values of individual essential amino acids (δ¹³С_eAA values) for three species of AM fungi (Rhizophagus aggregatus, R. intraradices and R. irregularis), grown in vitro in bi-compartmented Petri plates separating the AM fungal-colonized host root from the hyphae and spores, allowing to obtain plant-free AM fungal material. The δ¹³С_eAA based fingerprinting showed a strong difference between AM fungi and plants, and a high overlap of two studied species (R. aggregatus and R. intraradices) with both ectomycorrhizal (ECM) and saprotrophic (SAP) fungi. While still clustering close to fungi in the fingerprinting approach, the δ¹³С_eAA values of one species, R. irregularis, were distinct from the other fungal taxa, suggesting differences in metabolic pathways. Our results highlight that AM fungi are isotopically similar to ECM and SAP fungi and can be differentiated from plants, allowing to integrate AM fungi in the quantification of fungal energy channels in soil food webs. Additionally our data indicate the need for further investigations on the amino acid metabolic processes and stable isotope fractionation across different taxa of AM fungi.