In biology, the term Fungi (Latin plural for fungus) refers to a taxon or group of eukaryotic organisms including molds, yeasts and mushroom-producing organisms. They are classified in a different kingdom than plants, animals and protists. They differ from plants in that they are heterotrophic; and from animals in that they have cell walls, like plants, composed of chitin, rather than cellulose. Fungi in terms of phylogeny are more closely related to animals than to plants, however in antiquity it was thought that fungi were related to plants.
It has been discovered that organisms that looked like fungi in reality were not, and that organisms that did not look like fungi in reality were, if we call “fungi” all organisms derived from the one that ancestrally acquired the capacity to form a chitin cell wall. Because of this, although this taxon is well delimited from the evolutionary point of view, the phylogenetic relationships of the less known groups are still being studied, and its list of subtaxons has changed a lot over time with respect to very derived or very basal groups.
Fungi are found in very diverse habitats: they can be pyrophilic (Pholiota carbonaria) or coprophilic (Psilocybe coprophila). According to their ecology, they can be classified into four groups: saprophytes, lichenics, mycorrhizogens and parasites. Saprophytic fungi can be specific substrate: Marasmius buxi or non-specific: pure Mycena. The symbionts can be: lichenized basidiolichen fungi: Omphalina ericetorum and ascolichen: Cladonia coccifera and mycorrhizal fungi: specific: Lactarius torminosus (only mycorrhiza with birch) and non-specific: Hebeloma mesophaeum. In most cases, their representatives are not very conspicuous due to their tiny size; they usually live in soils and together with decomposing materials and as symbionts of plants, animals or other fungi. When they bear fruit, however, they produce striking sporocarps (mushrooms are an example of this). They perform an external digestion of their food, secreting enzymes, which are then absorbed by the dissolved molecules resulting from the digestion. This form of feeding is called osmotrophy, which is similar to that given in plants, but, unlike those, the nutrients they take are organic. Fungi are the primary decomposers of dead matter of plants and animals in many ecosystems, and as such they have a very relevant ecological role in biogeochemical cycles.
Fungi have a great economic importance: yeasts are responsible for the fermentation of beer and bread, and the harvesting and cultivation of mushrooms such as truffles. Since 1940 they have been used to industrially produce antibiotics, as well as enzymes (especially proteases). Some species are biocontrol agents for pests. Others produce mycotoxins, bioactive compounds (such as alkaloids) that are toxic to humans and other animals. Fungal diseases affect humans, other animals and plants; in the latter, they affect food safety and crop yields.
Fungi occur in two main forms: filamentous fungi (formerly called “molds”) and yeast fungi. The body of a filamentous fungus has two portions, one reproductive and one vegetative. The vegetative part, which is haploid and generally uncolored, is composed of filaments called hyphae (usually microscopic); a set of hyphae forms the mycelium (usually visible). Often the hyphae are divided by partitions called septa.
Yeast fungi – or simply yeasts – are always unicellular, almost spherical in shape. There is no distinction between vegetative and reproductive bodies.
Within the scheme of the five kingdoms of Wittaker and Margulis, fungi belong partly to the protista kingdom (amoebic fungi and fungi with zoospores) and to the Fungi kingdom (the rest). In the scheme of eight kingdoms of Cavalier-Smith they belong partly to the Protozoa kingdom (the amoeboid mushrooms), to the Chromista kingdom (the Pseudofungi) and to the Fungi kingdom all the rest. The diversity of taxa included in the group is little studied; it is estimated that there are about 1.5 million species, of which only 5% have been classified. During the 18th and 19th centuries, Charles Linnaeus, Christiaan Hendrik Persoon, and Elias Magnus Fries classified fungi according to their morphology or physiology. Currently, molecular biology techniques have allowed the establishment of a molecular taxonomy based on deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) sequences that divide the kingdom into a number of divisions.
The specialty of biology that deals with fungi is called mycology, where the suffix -mycota is used for the divisions and -mycetes for the classes.