A fungus is a member of the kingdom Fungi. Fungi feed through a filamentous structure, and periodically produce spores. A number of independent types of organisms have been referred to as protists - including amoeboid slime moulds, the oomycetes that are related to diatoms and brown algae, and the true fungi that are related (or, according to some, include) chytrids. The true 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 (mushrooms). 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 more closely related to animals than plants, even though the discipline devoted to the study of fungi, known as mycology, often falls under botany. The closest relatives of the fungi, as defined here to include only species with mycelia, are the chytrids, a group of unicellular organisms that can swim using flagella. 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.