The subdiscipline in mycology and ecology that examines community composition and structure; responses, activities, and interactions of single species; and the functions of fungi in ecosystems. These organisms display an extraordinary diversity of ecological interactions and life history strategies, but are alike in being efficient heterotrophs. Fungi, along with bacteria, are the primary decomposers, facilitating the flow of energy and the cycling of materials through ecosystems. See Ecological energetics, Fungi
Fungi occur in many different habitat types—on plant surfaces; inside plant tissues; in decaying plant foliage, bark and wood; and in soil—generally changing in abundance and species composition through successional stages of decomposition. Fungi are also found in marine and aquatic habitats; in association with other fungi, lichens, bacteria, and algae; and in the digestive tracts and waste of animals. Some fungi grow in extreme environments: rock can harbor free-living endolithic fungi or the fungal mutualists of lichens; thermotolerant and thermophilic fungi can grow at temperatures above 45°C (113°F); psychrophilic fungi can grow at temperatures to below -3°C (27°F). Xerotolerant fungi are able to grow in extremely dry habitats, and osmotolerant fungi grow on subtrates with high solute concentrations. Most fungi are strict aerobes, but species of the chytrid Neocallimastix, which inhabit the rumen of herbivorous mammals, are obligate anaerobes. Several aquatic fungi are facultative anaerobes. Many fungi occur as free-living saprobes, but fungi are particularly successful as mutualistic, commensal, or antagonistic symbionts with other organisms. See Population ecology
Fungi possess unique features that affect their capacity to adapt and to function in ecosystems:
1. Fungi are composed of a vegetative body (hyphae or single cells) capable of rapid growth. Hyphae are linear strands composed of tubular cells that are in direct contact with the substrate. The cells secrete extracellular enzymes that degrade complex polymers, such as cellulose, into low-molecular-weight units that are then absorbed and catabolized. Many fungi also produce secondary metabolites such as mycotoxins and plant growth regulators that affect the outcomes of their interactions with other organisms.
2. Filamentous fungi are able to mechanically penetrate and permeate the substrate.
3. Fungi have an enormous capacity for metabolic variety. Fungal enzymes are able to decompose highly complex organic substances such as lignin, and to synthesize structurally diverse, biologically active secondary metabolites. Saprotrophic fungi are very versatile; some are able to grow on tree resins and even in jet fuel.
4. Structural and physiological features of fungi facilitate absorption and accumulation of mineral nutrients as well as toxic elements. The capacity of fungi to absorb, accumulate, and translocate is especially significant ecologically where hyphal networks permeate soil and function as a link between microhabitats.
5. Fungi have the capacity for indeterminate growth, longevity, resilience, and asexual reproduction. The vegetative cells of Eumycota are often multinucleate, containing dissimilar haploid nuclei. This combination of features gives the fungi an unparalleled capacity for adaptation to varying physiological and ecological circumstances and ensures a high level of genetic diversity.
6. Many species of fungi have a capacity to shift their mode of nutrition. The principal modes are saprotrophy (the utilization of dead organic matter) and biotrophy, which is characteristic of parasitic, predacious, and mutualistic fungi (including mycorrhizae and lichen fungi). See Biodegradation, Fungal genetics, Mycorrhizae
Fungi interact with all organisms in ecosystems, directly or indirectly, and are key components in ecosystem processes. As decomposers, fungi are crucial in the process of nutrient cycling, including carbon cycling as well as the mineralization or immobilization of other elemental constituents. As parasites, pathogens, predators, mutualists, or food sources, fungi can directly influence the species composition and population dynamics of other organisms with which they coexist. Fungi may act both as agents of successional change or as factors contributing to resilience and stability. Mycorrhizal fungi function as an interface between plant and soil, and are essential to the survival of most plants in natural habitats.
There are several economically important areas that benefit from application of the principles of fungal ecology: biotechnology, biological control, bioremediation, agriculture, forestry, and land reclamation. With only a small fraction of the total species known, the fungi offer a rich potential for bioprospecting, the search for novel genetic resources with unique, useful biochemical properties. See Ecology, Ecosystem, Fungi, Fungal biotechnology