What is Microorganism?
[[Image:E coli at 10000x, original.jpg|thumb|250px|right|A cluster of ''Escherichia coli'' [[Bacterium|bacteria]] magnified 10,000 times.]] 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]], [[protist]]s and [[virus]]es, 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.<ref>{{Cite journal|author=Rybicki E.P. 1990. The classification of organisms at the edge of life, or problems with virus systematics |journal=S African J Sci |volume=86 |pages=182–6 }}</ref><ref name=pmid13481308>{{Cite journal |author=Lwoff A. 1957. The concept of virus |title=The concept of virus|journal=J. Gen. Microbiol.|year=1957|volume=17|issue=2|pages=239–53|doi=10.1099/00221287-17-2-239|doi-access=free |pmid=13481308}}</ref><ref>Forterre P. 2010. Defining life: the virus viewpoint. ''Orig Life Evol Biosph''. '''40'''(2):151-60. [https://www.ncbi.nlm.nih.gov/pubmed/20198436]</ref> Most micro-organisms are [[unicellular organism]]s with only one [[cell]], but there are unicellular [[protist]]s 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 spring]]s on the [[ocean]] floor and deep inside [[Rock (geology)|rocks]] within the [[earth]]'s crust. Such habitats are lived in by [[extremophile]]s. Micro-organisms are critical to nutrient recycling in ecosystems, because they act as [[decomposer]]s. 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. == Fre