Collective term (also microbes) for microscopically small, mostly unicellular organisms. These include algae, bacteria (e.g. lactic acid bacteria), many fungi (e.g. yeasts) and phytoplasmas (cell wall free bacteria). It is controversial whether viruses also belong to this group, because they are predominantly not regarded as living beings and therefore not as microorganisms. Microorganisms can be found in soil, water and air even under extreme environmental conditions such as in hot water springs and in the Arctic.
They mostly live on dead organic matter of plant and animal origin. By converting humus into various nutrients in a plant-available form, they are of great importance for plant growth and metabolism in nature. However, numerous species are also pathogens of infectious diseases in humans, animals and plants. See also under vine enemies.