Microorganismorganism that is so small that it is invisible to the naked eye. The term is synonymous by usage to single-celleded organism, even though some unicellular protists are visible to the naked eye, and some colonial species are microscopic.
Microorganisms may be found almost anywhere in the taxonomic structure. Bacteria and archaea are always or almost always microscopic, as are most protists. Even some fungi, a primarily macroscopic taxon, are microorganisms.
Microorganisms are found everywhere in nature, owing to the existence of extremophiles, microorganisms that have adapted to generally hostile environments. Extremophiles may be found in environments such as the poles, deserts, geysers, just beneath the surface of rockss, and the bottom of the deep sea. Some are known to survive prolonged time in vacuum, or to be unusually resistant to radiation.
Microorganisms can be helpful in recycling other organisms' remains and waste products, or when employed in biotechnology, e.g., for brewing and bakery. They can also be harmful as pathogens when, as parasites, causing infections.