What is a fungus?
The fungal kingdom (Fungi) is related to the animal kingdom (Animalia – a graphical illustration of possible relationships within Eukaryota can be found here). They separated approximately 1.5 billion years ago.
Like animals and plants, fungi are eukaryots, which means the cells contain cell-bound organelles and a nucleus. They are heterotrophic, like animals, but in contrast to most of them they exhibit external digestion. Like plants, they have vacuoles and a cell wall and reproduce both sexually and asexually. But in contrast to plants, the cell walls are made of chitin and glucans. Most fungi are multicellular, but there are also single-celled fungi, where yeasts are the most well-known.
A typical fungus is built up of long, thin cells, the hyphae. They are so thin, often under 1/100 mm, that they can only be studied under a microscope. They have apical growth and branch to a dense network, called mycelium – the vegetative part of the fungus. The hyphae are protected by a cell wall of chitin and/or glucan. The mycelium can be septated, as in Ascomycota and Basidiomycota, or unseptated, as in the other fungal phyla. The cytoplasm content can move through pores in the septa from one hyphal compartment to another.
Dense mycelia can be seen by the naked eye, e.g. mould on bread, cheese or jam. Most often, however, the mycelia are hidden in soil or wood. However, difficult to see, it is supposed that a lump of soil of the size of a piece of sugar can contain as much as 10 km fungal hyphae. The thin and branched hyphae have a large surface and are extremely effective to absorb nutrients. Some fungi lack mycelium, e.g. the budding yeast fungi and many chytrids, which consist of single, rounded cells.
The YouTube video to the left explains how a fungi are build. The YouTube video to the left show the growth of a whole mycelium (below).
Like mentioned earlier, fungi are heterotrophic. Heterotrophic means that fungi cannot produce their own food by photo- or chemosynthesis, but rely on organic compounds from plants, animals or other fungi for nutrition. They cannot fixate atmospheric nitrogen either, contrarily to what is stated in some outdated textbooks.
In contrast to animals, fungi digest their food externally before absorption into the cell as dissolved organic compounds (see animation below).
In the cytosol of the hyphae, various enzymes are produced. Those are transported through the hyphal membrane and, together with organic acids, secreted into the surrounding substrate (e.g. wood, soil, living plants). Depending on the type of enzyme, they can break down cellulose, hemicellulose, lignin, proteins or other complicated organic compounds to simpler soluble substances like sugars (mono- or disaccharides), amino acids or oligopeptides. These substances can be transported through the hyphal membrane into the fungal cell (hyphal compartment).
What we usually see – and what customarily are called “fungi” – are the fruit bodies (sporocarps). They are produced when the fungus reproduces sexually. Sporocarps are, with very few exceptions, only produced by fungi belonging to either the phylum Ascomycota (sac fungi) or Basidiomycota (club fungi); ascocarps or basidiocarps, respectively. Most sporocarps are shortlived mushrooms and are not necessarily produced every year, depending on variation of yearly weather conditions such as precipitation and temperature.
In the arctic climate the main season is late summer (late July to mid-August) or early autumn (mid-August to early September). Some wood-inhabiting fungi produce perennial sporocarps, e.g. the bracket fungi, but since they are bound to wood, they are rare in the Arctic.
The sexual spores are produced on the sporocarps. The – usually 8 – spores (ascospores) of the Ascomycota are inside a sac-shaped sporangium, the ascus (plural: asci). They are released when the turgor inside the ripening ascus rises, and the spores are forced out of an opening on the top. The Basidiomycota produce basidiospores sitting on small stalks on a basidium (plural: basidia), usually four spores per basidium. They are shed by a special snap mechanism which is still not exactly understood. Some Basidiomycota have basidiocarps with spores on the inside, which are passively released, e.g. puffballs.
Many fungi can also produce asexual spores by mitosis; so called mitospores or conidia. These are usually not produced on the sporocarps, but directly on the mycelium, often from specialised structures. Mitospores are common in many groups of Ascomycota, e.g. in moulds such as Penicillium, Aspergillus and Cladosporium. They are not so frequent in Basidiomycota.
Fungi can have widely varying sizes. Some parasitic fungi can live inside a plant cell or on a pollen grain and can only be seen under a microscope with great magnification. On the other hand, in North America there are giant fungi whose mycelia can cover several square kilometres and weigh up to 600 tons (more than three blue whales!). In the Arctic, Agaricus aristocratus can produce fairy rings up to 50 m in diameter. All sporocarps in such a ring belong to the same individual, whose mycelium is growing radially from the primary germination point. The ring widens every year as the mycelium grows.
Photos © Pernille Bronken Eidesen