A fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which are separate from plants, animals, protists, and bacteria. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants and some protists, which contain cellulose, and unlike the cell walls of bacteria. These and other differences show that the fungi form a single group of related organisms, named the Eumycota (true fungi or Eumycetes), that share a common ancestor (is a monophyletic group). This fungal group is distinct from the structurally similar myxomycetes (slime molds) and oomycetes (water molds). The discipline of biology devoted to the study of fungi is known as mycology. (Mycology has often been regarded as a branch of botany, even though it is a separate kingdom in biological taxonomy). Genetic studies have shown that fungi are more closely related to animals than to plants.
Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. They may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They have long been used as a direct source of food, such as mushrooms and truffles, as a leavening agent for bread, and in fermentation of various food products, such as wine, beer, and soy sauce. Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests.
Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, which are toxic to animals including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals. Losses of crops due to fungal diseases (e.g., rice blast disease) or food spoilage can have a large impact on human food supplies and local economies. The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 1.5 million to 5 million species, with about 5% of these having been formally classified. Ever since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christian Hendrik Persoon, and Elias Magnus Fries, fungi have been classified according to their morphology (e.g., characteristics such as spore color or microscopic features) or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits. Phylogenetic studies published in the last decade have helped reshape the classification of Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla. A group of all the fungi present in a particular area or geographic region is known as mycobiota.
HOW MOULDS ARE CLASSIFIED
Strictly speaking, all fungi should be classified according to their method of sexual reproduction. In many cases this is possible, allowing us to recognize several groups of fungi. Fungi that are encountered most often or exclusively in the asexual condition create special problems and are discussed separately as anamorphs.
Chytridiomycota
The Chytridiomycota, or "chytrids" as they are commonly called, are a group of mostly unicellular fungi occurring in a variety of habitats ranging from soil and roots to the rumens of cows and deer. Asexual and sexual spores are produced in sporangia and escape as zoospores (swimming cells). Because of their basically non-hyphal nature and often unusual requirements for growth, the Chytridiomycota are seldom encountered as moulds.
Oomycota
Members of this group all reproduce by oogonia eggs. The hyphae have few or no cross-walls (septa) and thus appear as long, clear tubes. If a hypha is broken, most of the contents run out. Many oomycetes reproduce asexually by zoospores, which are motile and can swim quite rapidly. Because of their motile zoospores, oomycetes commonly require water for reproduction and are often encountered in water or wet soil. An easy way to see oomycetes is to heat a few sesame seeds until they "pop" and then float three or four on the surface of some pond water in a dish. After a few days the oomycetes with form a thick white growth around the seeds. If you watch there carefully you may be able to observe the release of zoospores.