A fungus ( ) is a eukaryotic organism that is a member of the kingdom Fungi ( ). The fungi are heterotrophic organisms possessing a chitinous cell wall. The majority of species grow as multicellular filaments called hyphae forming a mycelium; some fungal species also grow as single cells. Sexual and asexual reproduction of the fungi is commonly via spores, often produced on specialized structures or in fruiting bodies. Some species have lost the ability to form specialized reproductive structures, and propagate solely by vegetative growth. Yeasts, molds, and mushrooms are examples of fungi. The fungi are a monophyletic group that is phylogenetically clearly distinct from the morphologically similar slime molds (myxomycetes) and water molds (oomycetes). The fungi are more closely related to animals than plants, yet the discipline of biology devoted to the study of fungi, known as mycology, often falls under a branch of botany.

Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in all ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or molds. Many fungal species have long been used as a direct source of food, such as mushrooms and truffles and in fermentation of various food products, such as wine, beer, and soy sauce. More recently, fungi are being used as sources for antibiotics used in medicine and various enzymes, such as cellulases, pectinases, and proteases, important for industrial use or as active ingredients of detergents. Many fungi produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans. Some fungi are used recreationally or in traditional ceremonies as a source of psychotropic compounds. Several species of the fungi are significant pathogens of humans and other animals, and losses due to diseases of crops (e.g., rice blast disease) or food spoilage caused by fungi can have a large impact on human food supply and local economies.

Etymology and definition

The English word fungus is directly adopted from the Latin fungus, meaning "mushroom", used in Horace and Pliny. This in turn is derived from the Greek word sphongos/σφογγος ("sponge"), referring to the macroscopic structures and morphology of some mushrooms and molds and also used in other languages (e.g., the German Schwamm ("sponge") or Schwammerl for some types of mushroom).

Diversity

Fungi have a worldwide distribution, and grow in a wide range of habitats, including deserts. Most fungi grow in terrestrial environments, but several species occur only in aquatic habitats. Fungi along with bacteria are the primary decomposers of organic matter in most if not all terrestrial ecosystems worldwide. Based on observations of the ratio of the number of fungal species to the number of plant species in some environments, the fungal kingdom has been estimated to contain about 1.5 million species. Around 70,000 fungal species have been formally described by taxonomists, but the true dimension of fungal diversity is still unknown. Most fungi grow as thread-like filaments called hyphae, which form a mycelium, while others grow as single cells. Until recently many fungal species were described based mainly on morphological characteristics, such as the size and shape of spores or fruiting structures, and biological species concepts; the application of molecular tools, such as DNA sequencing, to study fungal diversity has greatly enhanced the resolution and added robustness to estimates of diversity within various taxonomic groups.