Yeast is any of a group of generally single-celled fungi that are heterotrophic (obtain their nutrients by absorbing organic compounds from their environment), possess chitin in their cell walls and reproduce primarily through budding (in which a new cell forms off an existing cell).
Yeasts differ greatly in their structure, reproduction and ecological roles from molds and mushrooms, which are also members of the fungi kingdom. Molds are multicellular and are characterized by long, filamentous structures that allows them to grow and spread over surfaces. They can reproduce both sexually and asexually, the former typically through the production of spores. Mushrooms are the fruiting bodies of certain fungi and also are multicellular and form filamentous structures. Their reproduction is primarily sexual using spores, although, some can also reproduce asexually through fragmentation or budding.
Yeasts inhabit a wide range of environments, with some species adapted to polar regions, deserts, hot springs and hydrothermal vents, or acidic or saline environments. They exist in various soil types, are found on the surfaces of plants and fruits, as well as on the skins and excretions of animals.
Yeasts have essential functions in various ecological processes, including decomposition and nutrient cycling. They are also extensively used in industries such as baking and brewing because of their ability to ferment sugars. Their application is also being explored for various biotechnological uses, including bioremediation and biofuel production.
Yeasts play a major role in soil enrichment. One way is by making soluble essential nutrients such as phosphorus and potassium, which are often locked in insoluble forms in the soil. Another is by enhancing the microbial activity of other fungi, which form symbiotic relationships with plant roots, thereby increasing nutrient uptake, especially in nutrient-poor soils. Also, yeasts play a crucial role in breaking down organic matter, thus both enriching the soil and also improving its structure and its water retention capacity. Moreover, some yeast species can contribute to soil bioremediation because of their ability to assimilate heavy metals and degrade toxic organic compounds from synthetic pesticides and other pollutants.
The number of species of yeast is unknown, with estimates ranging from upwards of 2,000 to as high as 200,000. As is the case with other types of organisms, it appears that some could be facing extinction. This threat is largely due to climate change and habitat destruction, and it is especially severe in tropical forests, where yeast species are particularly numerous.