The Fungal Habit

Filamentous fungi are immobile but may achieve motility by virtue of apical growth of hyphae from a supporting substratum. The indeterminate and versatile development of fungal mycelia is directed by interactions with the dynamic abiotic and biotic environments, the latter involving both inter- and intra- specific interactions between fungi and between fungi and other organisms. As heterotrophs fungi are nutritional opportunists, having evolved an array of optionally programmed developmental pathways to take advantage of changing local circumstances and resource fluxes (Rayner 1996). Fungi often possess a more restricted degradative repertoire compared with many bacteria, but fungal mycelia possess clear advantages over unicellular or short-chained growth morphs for invading and degrading solid substrata. Fungal mycelia absorb nutrients through a high contact surface area and may penetrate substrata by generation of mechanical pressure and extracellular degradative enzymatic and chemical activity. Mycelia are capable of nutrient translocation to a greater or lesser extent, and therefore have the capacity to support exploration from one nutrient rich source to another, thereby transcending regions of nutrient deprivation.

Highly specialized multihyphal structures such as rhizomorphs and mycelial cords are produced by a variety of wood decay, root pathogenic, and ectomycorrhizal fungi, such as Armillaria spp., Phanerochaete velutina, Hypholoma fasciculare, Phallus impudicus, Tricholomopsis platyphylla, and ectomycorrhizal Leccinum scabrum (see Boddy 1992; 1999; Rayner and Boddy 1988 for review). These linear organs are often evident at the interface of surface litter and soil in boreal, deciduous and tropical woodlands, where they may contribute to stabilizing woodland litter against weather erosion particularly on steep slopes (Lodge and Asbury 1988). Some may even form networks within the tree canopies of tropical forests (Hedger et al. 1993). Cords are organs of migration and exploration, with more efficient rates of biomass deployment and translocation of water and nutrients, compared to a more diffuse hyphal network, allowing the individual to transcend vast distances of often hostile regions or inhospitable nutrient deserts. Cords also provide resistance to the damaging effects produced by the soil microbial and microfaunal communities. The voracious opportunistic scavenging and persistent biomass involved may also facilitate nutrient storage. Cord formers are commonly particularly combative and are capable of producing rapid decay in nature, thereby making significant contributions to nutrient cycling (Boddy 1999; Boddy and Watkinson 1995; Chapela et al. 1988; Coates and Rayner 1985a).

Cord formation may be promoted by resource depletion (low C:N ratios and nutrients such as copper and calcium), recognition or contact with an antagonist, and reduced water potential (Rayner and Webber 1984; Watkinson 1999). Directional growth of cords towards resource baits may possibly be mediated by volatiles or soluble diffusates, and involves coordinated redeployment of mycelial biomass following detection of localized resource base. Different species display different foraging strategies, subject to resource quality, quantity and distribution, biotic factors and microclimate (e.g., Boddy 1999; Donnely and Boddy 1998; Wells et al. 1999; 2001; Zakaria and Boddy 2002). These linear organs can interconnect heterogeneously dispersed nutrient sources to form coordinated resource distribution networks (Boddy 1999), and may translocate different nutrients independently even bi-directionally, e.g., certain ectomycorrhizal cords may translocate carbon from tree roots to the growing mycelium, and mineral nutrients and water in the opposite direction.

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