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 organells and a nuclei like plant and animals. 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 sexual and asexual. But in contrast to plants, the cell walls are made of chitin and glucans. Mostly, 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 in the 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.

Figure illustrating the difference between septated and unseptated fungi hyphae and apical growth
Hyphae with (A) and without (B) septs
hyphae septae
Septated hypae with pores in the septs

YouTube video of apical hypae growth

Yeast Saccharomyces cerevisiae; by Mogana Das Murtey and Patchamuthu Ramasamy (licensed under CC BY-SA 3.0)

Dense mycelia can be seen by the naked eye, e.g. mould on bread, cheese or jam. But most often 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 fix atmospheric nitrogen either, as it stands in some few – outdated – textbooks.

In contrast to animals, fungi digest their food externally before absorption into the cell as dissolved organic compounds (se 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 shall reproduce sexually. Sporocarps are, with very few exceptions, only produced by fungi belonging to either phylum Ascomycota (sac fungi) or Basidiomycota (club fungi); ascocarps or basidiocarps, respectively. Most sporocarps are short lived mushrooms and not necessarily produced every year depending on variation of yearly weather conditions 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.

Mycelium in substrate with fruiting body
Example of a basidium of Basidomycota (left) and and ascus of Ascomycota (right)

The sexual spores are produced on the sporocarps. The spores, ascospores, of the Ascomycota are inside a sac formed sporangium, the ascus (plural: asci), usually eight spores. They are normally released when the turgor inside the ripening ascus rise, and the spores are forced out of an opening on the top. The Basidiomycota produce the basidiospores sitting on small stalks on a basidium (plural: basidia), usually four spores. They are shed by a special snap mechanism, still not exactly understood. Some Basidiomycota have basidiocarps with inside spore production where the spores are passively realised, e.g. the puffballs.

Fungi may also produce asexual spores by mitosis, so called mitospores or conidia. Normally they are not produced on the sporocarps, but directly on the mycelium, often from specialised structures. Mitospores are common in many groups of Ascomycota, e.g. moulds as Penicillium, Aspergillus and Cladosporium. They are not so frequent in Basidiomycota.

The fungi can have very different sizes. Some parasiting fungi can live inside a plant cell or on a pollen grain and can only be seen in microscope with great magnification. On the other hand, in North America there are giants which mycelium can cover several square kilometres and weighing 600 tons (more than three blue whales!). In the Arctic Agaricus aristocratus can produce fairy rings 50 meters in diameter. All sporocarps in a ring belong to the same individual where the mycelium is growing radially from the primary germination point. The ring is widening every year as the mycelium grow.

Fungi cultivated from Lonyearbyen, Svalbard soil
Agaricus aristocratus
Illustration of a fairy ring

Photos © Pernille Bronken Eidesen