Plant root systems colonized by arbuscular mycorrhizal (AM) fungi have previously been shown to influence soil bacterial populations; however, the direct influence of the AM extraradical mycelium itself on bacterial growth and community composition is not well understood. In this study, we investigated the effects of exudates produced by AM extraradical mycelia on the growth and development of an extracted soil bacterial community in vitro. The chemical composition of the mycelial exudates was analysed using proton nuclear magnetic resonance spectrometry. Following the addition of exudates to a bacterial community extracted from soil, bacterial growth and vitality were determined using a bacterial vitality stain and fluorescence microscopy. Changes in community composition were also analysed at various times over the course of 3 days by terminal restriction fragment length polymorphism analysis, in combination with cloning and sequencing of 16S rRNA genes. Mycelial exudates increased bacterial growth and vitality and changed bacterial community composition. Several Gammaproteobacteria, including a taxon within the Enterobacteriaceae, increased in frequency of occurrence in response to AM mycelial exudates. This study is the first attempt to identify carbohydrates from the extraradical mycelium of an AM fungus, and demonstrates the direct effects of mycelial exudates on a soil bacterial community.

A colorimetric method was developed to permit semi-quantitative measurement of substrate acidification by different ectomycorrhizal and one saprotrophic fungus growing on media containing one of five different minerals. Overall, substrate acidification differed between fungal species and the degree of variation in acidification in response to different minerals was highly species-dependent. Mycena galopus and Cortinarius glaucopus produced the least biomass of all tested species and produced the highest amount of acidification per unit mycelial density. Substrate acidification by C. glaucopus was inversely related to mycelial density, with particularly high acidification at low mycelial density on medium enriched with tri-calcium phosphate. Substrate acidification by M. galopus was constant irrespective of mycelial density and varied only according to mineral treatment, with higher substrate acidification on tri-calcium phosphate compared to the other minerals.

Interactions between mycelia of the ectomycorrhizal fungus Suillus variegatus and the wood decomposing fungus Hypholoma fasciculare were studied in soil microcosms. The ectomycorrhizal mycelium extended from Pinus sylvestris seedlings and the saprotrophic fungus grew out into the soil from birch wood blocks of two alternative sizes. Transfer of 32P between the interacting mycelia was measured non‐destructively using electronic autoradiography. The outcome of the interactions was clearly affected by the size of the wood blocks. In systems with large wood blocks (1.6 cm3) the wood decomposer fungus overgrew the mycelium of the mycorrhizal fungus whereas in systems with small wood blocks (0.44 cm3) the mycorrhizal mycelium overgrew the wood decomposer fungus. The saprotrophic fungus was able to capture significantly more 32P from labelled mycorrhizal mycelium when growing out from larger wood blocks. The mycorrhizal fungus was able to capture significantly more 32P from labelled saprotrophic mycelium when the wood blocks were smaller.