Mycol. Res. 99 (2): 143-153 (1995) Printed in Great Britain 143 Mycelial density profiles of fungi on heterogeneous media and their interpretation in terms of nutrient reallocation patterns STEFAN OLSSON Department of Ecology, Microbial Ecology, Helgonavagen 5, 5-223 62 Lund, Sweden The ability to reallocate nutrients by translocation between different parts of the mycelium may have an important influence on pattems 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. The natural habitat of most fungi is heterogeneous with respect to nutrient distribution. An ability to reallocate nutrients effectively between different parts of the mycelium by translocation may therefore aid survival in natural environments (Boddy, 1993). Most studies of translocation in saprotrophic fungi have focussed on translocation of nutrients from an established mycelium into hyphae extending over non-nutrient substrates. The ability to translocate nutrients has been found to correlate, to some extent, with obsenred protoplasmic streaming (Schiitte 1956; Thrower & Thrower, 1968) and has been noted especially among Zygomycetes. This type of translocation may be termed exploratory nutrient allocation since it accompanies exploratory transmigration (Gregory, 1984) and should be distinguished from nutrient reallocation (in line with Boddy, 1993). The latter occurs inside a mycelium when nutrient resources of different quality and composition are utilized in different places. Although, for some fungi, diffusion may be the main mechanism of nutrient reallocation (Olsson & Jennings, 1991a, b), the latter is aided by nutrient uptake in excess of local needs at sites exposed to high nutrient concentrations (Olsson & Jennings, 1991 b: Olsson, 1994). Moreover, work by others (Brownlee & Jennings, 1982), especially with mycorrhiza (Cooper & Tinker, 1978; Finlay & Read, 1986; Finlay, 1992) implies that more active means of translocation could be operative. Theoretically, the mode of translocation may be reflected in characteristic types of colony biomass density profiles when fungi are grown from the centre of plates containing heterogeneous media with opposing gradients of carbon/ energy and minerals. If nutrient reallocation is only by simple diffusion, a peak of biomass density, relative to controls, would be expected in the centre of the plate where the highest concentration of all nutrients coincide. For fungi which actively reallocate all important nutrients, the biomass density profile on gradient plates should resemble profiles on homogeneous media with a much less pronounced peak in the centre where the gradients meet, if any peak at all. More complex pattems would be expected in fungi translocating glucose or one or more of the mineral components on one side of the gradient by diffusion, while translocating all the components on the other side of the gradient actively. The present study involved a survey of nutrient reallocation patterns in a selection of common fungi as inferred from their biomass density profiles when grown on heterogeneous media with opposing nutrient gradients.