Microorganism

A micro-organism or microbe is an organism which is microscopic, which means that they are so small that people cannot see them with the naked eye. The study of microorganisms is called microbiology.

Micro-organisms include bacteria, fungi, archaea, protists and viruses, and are among the earliest known life forms. The first of these four types of micro-organisms may either be free-living or parasitic. Viruses can only be parasitic, since they always reproduce inside other living things.^[1][2][3]

Most micro-organisms are unicellular organisms with only one cell, but there are unicellular protists that are visible to the human eye, and some multicellular species are microscopic.

Micro-organisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the earth's crust. Such habitats are lived in by extremophiles.

Micro-organisms are critical to nutrient recycling in ecosystems, because they act as decomposers. Because some micro-organisms can also take nitrogen out of the air, they are an important part of the nitrogen cycle. Pathogenic, or harmful, microbes can invade other organisms and cause disease.

1. ↑ Rybicki E.P. 1990. The classification of organisms at the edge of life, or problems with virus systematics. S African J Sci. 86: 182–6. {{cite journal}}: Missing or empty |title= (help)CS1 maint: numeric names: authors list (link)
2. ↑ Lwoff A. 1957. The concept of virus (1957). "The concept of virus". J. Gen. Microbiol. 17 (2): 239–53. doi:10.1099/00221287-17-2-239. PMID 13481308.{{cite journal}}: CS1 maint: numeric names: authors list (link)
3. ↑ Forterre P. 2010. Defining life: the virus viewpoint. Orig Life Evol Biosph. 40(2):151-60. [1]