The kingdom Fungi is composed of diverse eukaryotic and heterotrophic organisms, including filamentous molds and yeasts. Almost all fungi have cell walls composed of rigid, covalently linked polymers, including chitin and glucan, and many fungi produce spores. Most fungi are aerobic, although some are obligate anaerobes and a few are facultative anaerobes. Most are also mesophilic (grow at temperatures between 18 and 25 °C), but some are thermophilic (grow at temperatures from > 20 up to ~50 °C) and some are psychrophilic (grow at temperatures between 0 and 5 °C, up to a maximum of about 16 to 20 °C).
Generally, fungi are saprophytic, secreting extracellular enzymes and absorbing nutrients from dead or decaying organic matter. Some fungi are parasitic; however, relatively few species are overt pathogens and some fungi live in a symbiotic relationship with plants or other microorganisms (e.g., lichens and mycorrhizal fungi).