Collective term (also microbes) for microscopically small, mostly unicellular living organisms. These include algae, bacteria (e.g. lactic acid bacteria), many fungi (e.g. yeasts) and phytoplasmas (cell wall-free bacteria). It is disputed whether viruses are also included, because these are predominantly not considered living organisms and therefore not microorganisms. Microorganisms are 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 plant-available form, they are of great importance for plant growth and metabolism in nature. However, numerous species are also pathogens of contagious diseases in humans, animals and plants. See also under vine enemies.