Mycelial density profiles of fungi on heterogeneous media and their interpretation in terms of nutrient reallocation patterns

Abstract

The ability to reallocate nutrients by translocation between different parts of the mycelium may have an important influence on patterns of fungal colonization in heterogeneous environments. The shape of colony biomass density profiles of colonies grown on media with opposing gradients of carbon and mineral nutrients was used as an indicator of translocating ability and measured using image analysis for non-coloured mycelia or estimated by eye for coloured mycelia. Three principal types of density profile, with or without autolysis in the older mycelium, were predicted and their occurrence noted in 62 test fungi. Type 1 profiles (26 fungi), characterized by peaks of high biomass densities at the centre of the plates and progressively lower biomass densities in both directions from the centre indicated no translocation, other than diffusion, in any direction. Type 2 profiles (13 fungi), characterized by biomass density profiles similar to control plates (without gradients) on half of the plate and progressively decreasing densities, from the centre of the plates, on the other half of the plates indicated active translocation of carbon (glucose) or minerals in one direction, and diffusive translocation of minerals or carbon in the other direction. Type 3 profiles (21 fungi), characterized by biomass density profiles over the whole plate that were similar to profiles on control plates (without gradients) indicated active translocation of all nutrients in both directions. The profiles of two fungi could only tentatively be assigned to type 3. Some fungi, especially amongst those with type 3 profiles, produced more biomass during the same time of incubation and sometimes also at higher densities on gradient plates than on control plates, but most fungi formed less biomass and at lower densities on these plates. Radial extension rates were expected to decrease during incubation for type 1 fungi on gradient plates but no such tendency was found. Most fungi had constant radial extension rates and, unexpectedly, some showed increasing radial extension rates. Six of the 62 tested fungi even had increasing radial extension rates when grown on control media.