Atomic Force Microscopy of the fungi–mineral interface: applications in mineral dissolution, weathering and biogeochemistry

The interaction between mycorrhizal fungi and minerals is of fundamental importance in affecting the geochemical carbon cycle and CO2 concentration in the atmosphere, alongside roles in soil creation and the release of nutrients. The symbiosis between the fungi and the plant, supported by photosynthesis in the host plant, has as one of its key features the interfacial zone where mineral and fungi come into contact. At this interface, the organism exudes a complex mixture of organic acids, chelating molecules, protons, and extracellular polysaccharide. In this review, examples will be given of recent Atomic Force Microscopy experiments to monitor the colonization of phyllosilicate minerals in sterile controlled microcosm environments containing only tree seedlings, mineral chips and mycorrhizal fungi. The surface activity of the colonizing fungal hyphae is extensive and complex. In complementary experiments involving exposure of minerals surfaces to single organic acids, it has been possible to monitor dissolution at the unit cell level and to extract activation energies for specific dissolution processes, for example 49 kJ mol−1 for 100 mM oxalic acid acting upon a biotite sample. The link between these simpler model experiments and the whole microcosm studies is illustrated partly by observations of fungal-colonized mineral surfaces from microcosms after careful removal of the organism and biolayer. These mineral surfaces give clear indications of basal plane modification and fungal weathering.