1e:T50b,Mycorrhizal fungi form extensive mycelia in soil and play significant roles in most soil ecosystems. The estimation of their biomasses is thus of importance in order to understand their possible role in soil nutrient processes. For arbuscular mycorrhizal (AM) fungi the signature fatty acid 16:1g5 provides a new and promising tool for the estimation of AM fungal biomass in soil and roots. For ectomycorrhizal fungi 18:2g6,9 dominates among the fatty acids and can be used as an indicator of mycelial biomass of these fungi in soil in experimental systems. In biomass estimation primarily the phospholipid fatty acids (PLFAs) are suitable. Through the use of specific PLFAs it is possible to study interactions between mycorrhizal mycelia and bacteria in soil as well as between AM fungal mycelia and mycelia of saprophytic and parasitic fungi in soil and in roots. AM fungi, in particular, store a large proportion of their energy as lipids and by using the signature fatty acids it is possible to determine the relation between membrane and storage lipids, which could be an indication of energy storage levels. Various aspects of how the fatty acid signatures can be used for studies related to questions of biomass distribution and nutritional status of mycorrhizal fungi are discussed.1f:T470,• A large-scale tree-girdling experiment enabled estimates in the field of the contribution of extramatrical mycelium of ectomycorrhizal (ECM) fungi to soil microbial biomass and by ECM roots and fungi to production of dissolved organic carbon (DOC). • Tree-girdling was made early (EG) or late (LG) during the summer to terminate the flow of photosynthate to roots and ECM fungi. Determination of microbial C (C_micr ) and microbial N in root-free organic soil was performed by using the fumigation-extraction technique; extractable DOC was determined on unfumigated soil. • Soil C_micr was 41% lower on LG than on control plots 1 month after LG, whereas at the same time (that is, 3 months after EG), the C_micr was 23% lower on EG than on control plots. Extractable DOC was 45% lower on girdled plots than control plots. • Our results, which are of particular interest as they were obtained directly in the field, clearly demonstrate the important contribution by extramatrical ECM mycelium to soil microbial biomass and by ECM roots to the production of DOC, a carbon source for other microbes.20:T409,Arbuscular mycorrhiza (AM) symbioses contribute to global carbon cycles as plant hosts divert up to 20% of photosynthate to the obligate biotrophic fungi. Previous studies suggested carbohydrates as the only form of carbon transferred to the fungi. However, de novo fatty acid (FA) synthesis has not been observed in AM fungi in absence of the plant. In a forward genetic approach, we identified two Lotus japonicus mutants defective in AM-specific paralogs of lipid biosynthesis genes (KASI and GPAT6). These mutants perturb fungal development and accumulation of emblematic fungal 16:1ω5 FAs. Using isotopolog profiling we demonstrate that ¹³C patterns of fungal FAs recapitulate those of wild-type hosts, indicating cross-kingdom lipid transfer from plants to fungi. This transfer of labelled FAs was not observed for the AM-specific lipid biosynthesis mutants. Thus, growth and development of beneficial AM fungi is not only fueled by sugars but depends on lipid transfer from plant hosts.

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