Fungi (Mushrooms)

The Biology of Fungi

While historically categorized as plants, fungi are now known to represent their own biological kingdom. Unlike plants, fungi did not evolve to photosynthesize. They are heterotrophic, like animals, meaning they must consume their nutrition from outside sources, and on the evolutionary tree of life fungi are more closely related to animals than they are to plants.

Fungi are highly diverse and take on a wide variety of forms. Unicellular fungi are known as yeasts and represent ~1% of currently described species, while the rest are multicellular and grow as long, branching filaments called hyphae. Both unicellular and multicellular fungi can be microscopic and are found in a wide array of habitats, such as on the surface of plant leaves, within the colons of mammals, or lurking within soils and bodies of water.

In contrast, macrofungi produce visually macroscopic reproductive structures, generally referred to as mushrooms, when the conditions for reproduction are right. When these organisms are not producing reproductive structures, they exist in their habitats as networks of hyphae, each known as a mycelium. Macrofungi may produce mushrooms a few millimeters to several meters across in size, and display an astonishing diversity of unique forms such as corals, crusts, polypores, puffballs, boletes, jellies, and cups.

Fungi Systematics

Biological systematics is the study of the diversification and classification of living organisms. Modern fungal systematics uses a combination of genetic, morphological, and other characters to understand the diversification of fungi and categorize them taxonomically. Broadly, the kingdom Fungi can be grouped into the basal fungi and the subkingdom Dikarya, which is composed of the phyla Ascomycota and Basidiomycota.

The basal fungi represent a variety of lineages sharing some ancestral fungal traits, and include phyla such as the Glomeromycota, a group of AM mycorrhizal soil fungi, and the Chytridiomycota, a group of aquatic microscopic fungi.

The macrofungi belong primarily to Ascomycota and Basidiomycota, although not all organisms in these two groups are macrofungi. Ascomycota is a diverse group that includes common yeasts used for baking and brewing, various macrofungi such as the cup-fungi, and the fungi involved in lichen symbioses.

Basidiomycota is another diverse group which includes a large variety of macrofungi and many plant pathogens like the rusts and smuts. The majority of mushroom-forming fungi are Basidiomycetes, and can be separated from Ascomycetes based on their reproductive cells and DNA.

Some Mushroom (Macrofungi) Photos

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Crown Coral Mushroom (Clavicorona pyxidata)

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Crown Coral Mushroom (Clavicorona pyxidata), Coral Fungi Family (Clavariaceae). is a medium to large, dingy yellowish, coral-like mass with many branches. As it matures it often becomes yellowish brown to grayish brown. When it fruits in late spring to early summer or fall you will find it solitary or in groups or clumps on decaying stumps and logs of deciduous trees, particularly on aspens, cottonwoods and willows. Photo by Linea-Gillman.

The Ecology of Fungi

Fungi play essential ecological roles across a large majority of terrestrial ecosystems. The macrofungi generally fall into three ecological categories as saprobes, parasites, or mutualists, but these categories can sometimes blur.

Saprobic, or saprotrophic, fungi are defined as deriving their nutrition from decaying biological matter, such as dead wood, feces, or even animal bones and feathers like in the genus Onygena. These organisms secrete unique extracellular enzymes that chemically degrade biological compounds into consumable nutrients. They are the main organisms responsible for degrading plant matter such as wood, and their role in nutrient cycling is irreplaceable.

Parasitic fungi derive some or all of their nutrition at the expense of living hosts, and are known to parasitize plants, animals, and other fungi. They differ from other biological parasites in the way that their parasitism often kills the host, and they may continue to saprobically derive nutrition from the dead tissues. After infection, these fungi are even known to alter host behavior. Infected insects are often observed exhibiting unique behaviors that aid in the reproductive success of these parasites.

– Gary Olds

Read More About Mutualistic Mycorrhizal Fungi

Lastly, are the mutualistic mycorrhizal fungi, which form mycorrhizae with a vast majority of land plants. The word mycorrhiza literally means “fungus-root,” and is a symbiotic relationship between fungal mycelia and plant roots where nutrients, water, and various biochemicals are exchanged between organisms. This relationship has evolved since plants first colonized land, and is preserved in some of the earliest fossils of plant roots.

Mycorrhizal fungi are generally characterized as either arbuscular-mycorrhizal (AM) or ectomycorrhizal (EcM), and differ by the way the fungus colonizes the plant root tissues. The fungi receive photosynthates from the plants, and exchange water, soluble minerals, and nutrients, and have even been shown to transport biochemicals from plants of one species to plants of another species through their mycelium. Macrofungi in this group are ectomycorrhizal and form relationships in about 10% of plant families, mainly with woody plants such as pines, birch, maples, and willows.

Practical Uses of Fungi

Just like plants, bacteria, and animals, fungi can be manipulated and applied for a variety of practical purposes. Mushrooms have been cultivated for food by humans for thousands of years and continue to represent huge markets for wild and grown crops. Because of their association with specific plants, ectomycorrhizal fungi are often wild-harvested, while saprobic varieties are much more easily grown on substrates such as wood chips or hay. Edible macrofungi, such as truffles, represent some of the most valuable food products and support massive global markets. Additionally, many different fungi are involved in the fermentation of various foods and beverages.

Bread uses Saccharomyces yeasts, soy sauce and miso are fermented using the mold Aspergillus, and certain cheeses employ Penicillium mold for their production. Fermented beverages such as beer, wine, liquor, and kombucha also involve certain fungi that convert sugars into alcohols and other products. Fungi also produce countless unique metabolites medically employed as antibiotics, antifungals, immunosuppressants, psychotropics, and other drugs. In industrial settings, fungi are used in fermentation to produce massive quantities of important chemicals like citric acid and ethanol.