High resolution characterization of ectomycorrhizal fungal-mineral interactions in axenic microcosm experiments

Microcosms with Pinus sylvestris seedlings in symbiosis with the fungus mycorrhizal Paxillus involutus were established, and atomic force microscopy (AFM) was used to characterise plant photosynthate-driven fungal interactions with mineral surfaces. Comparison of images of the same area of the minerals before and after mycorrhizal fungal colonization showed extensive growth of hyphae on three different mineral surfaces – hornblende, biotite and chlorite. A layer of biological exudate, or biolayer, covered the entire mineral surface and was composed of globular features of diameter 10–80 nm, and the morphology of the biolayer differed among mineral types. Similar-sized components were found on the fungal hyphae, but with a more elongated profile. Biolayer and hyphae surfaces both appeared to be hydrophobic with the hyphal surfaces yielding higher maximal adhesive interactions and a wider range of values: the mean (± SE) adhesive forces were 2.63 ± 0.03 and 3.46 ± 0.18 nN for biolayer and hypha, respectively. The highest adhesion forces are preferentially localized at the hyphal surface above the Spitzenkörper region and close to the tip, with a mean interaction force in this locality of 5.24 ± 0.49 nN. Biolayer thickness was between 10 and 40 nm. The underlying mineral was easily broken up by the tip, in contrast to the native mineral. These observations of mineral surfaces colonised by mycorrhizal fungus demonstrate how fungal hyphae are able to form a layer of organic exudates, or biolayer, and its role in hyphal attachment and potential weathering of ferromagnesian silicates, which may supply nutrients to the plant.