Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso


  • José Luis SanzEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_992-2



Etymologically, microorganisms are small organisms whose size is too small to be observed by the naked eye. The resolution of the human eye is 0.2 mm, and this establishes 1 mm as an artificial limit for the maximum size of microorganisms. Taxonomically, “microorganism” lacks a precise meaning, and the term includes prokaryots (bacteria and archaea) and eukaryots (protists, microscopic algae, and fungi), which are all unicellular or lacking true tissues. Exceptionally, virus should be included in the definition of “microorganisms,” even though they lack cellular arrangement. Microbiology is the area of biological sciences that studies microorganisms. Prokaryots are the main focus of study in Microbiology. All are unicellular and lack a nucleus (pro-karion); their response to oxygen is very diverse; they can be auto- or heterotrophic and they employ the entire range of energy-obtaining systems found in living beings: anaerobic or aerobic respiration, fermentation...


Bioorganic Chemistry Entire Range Maximum Size Biological Science Aerobic Respiration 
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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Departamento de Biología MolecularUniversidad Autónoma de MadridMadridSpain