Unicellular organisms

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An amoeba. Amoebas belong to a diverse group of unicellular organisms

Protozoa are living things that consist of only one cell . They are compared to multicellular or multicellular cells . However, some unicellular cells can form cell colonies , but real multicellular cells require differentiation (division of tasks). The division into single- and multi-cell describes a type and not a species taxon and it gives no information about the degree of relationship, way of life or internal structure and metabolic processes . Single-cell organisms therefore do not form a closed family group .

Include single-celled living things

Their size varies between less than 1 µm (e.g. Nanoarchaeum equitans ) and several centimeters (e.g. Acetabularia ).

The first single-cell organisms were discovered in 1674 by Antoni van Leeuwenhoek .


According to older views, all eukaryotes were only divided into the two kingdoms of plants and animals, according to which there were also unicellular plants and animals. For example, the fungi were assigned to the plants as “ chlorophyllose , eukayontic thallophytes ” (storage plants). The bacteria were as Spaltpilze ( schizomycetes provided) to the mushrooms, the amoebae were counted as single-celled animals, some of their representatives to the slime molds , and flagellate flagellates like Euglena alternately to the unicellular algae or unicellular animals. At the end of the 1970s, the American microbiologists Carl Woese and George Fox recognized and described the independence of the archaea (initially called archaebacteria) and their belonging to a separate systematic unit alongside the bacteria (eubacteria) and eukaryotes.

Accordingly, different name endings (such as -zoa, -phyta, -mykota) were used for different groups, while the neutral -bionta is becoming increasingly popular today. Archaea groups identified early still have the ending -bacteria instead of -archaea or -archaeota (for example the so-called halobacteria ).

Macroscopic protozoa

Most protozoa are microscopic in size and are therefore classified as microorganisms . However, some unicellular protists and bacteria are macroscopic and visible to the naked eye. Some examples:


  • Wilfried Westheide , Reinhard M. Rieger (Ed.): Special Zoology. Part 1: Protozoa and invertebrates. 2nd Edition. Spektrum Akademischer Verlag, Heidelberg 2006, ISBN 3-8274-1575-6 .

Individual evidence

  1. Antony van Leewenhoek: Observations, Communicated to the publisher by Mr. Antony van Leewenhoeck in a Dutch Letter of the 9th of Octob. 1676. Here English'd: Concerning little animals by him observed in Rain, Well, Sea and Snow Water; as also in water wherein Pepper had lain infused. In: Philosophical Transactions of the Royal Society of London . Vol. 11, No. 133, 1677, pp. 821-831. doi : 10.1098 / rstl.1677.0003 ( full text )
  2. ^ Emil Müller, Wolfgang Löffler: Mykologie, dtv, Thieme Stuttgart, 2nd edition 1971, p. 1
  3. ^ Carl R. Woese, George E. Fox: Phylogenetic structure of the prokaryotic domain: the primary kingdoms . In: Proceedings of the National Academy of Sciences of the United States of America . 74, No. 11, November 1977, pp. 5088-5090. bibcode : 1977PNAS ... 74.5088W . doi : 10.1073 / pnas.74.11.5088 . PMID 270744 . PMC 432104 (free full text).
  4. The largest Bacterium: Scientist discovers new bacterial life form off the African coast , Max Planck Society, press release (via WebArchive) 2009
  5. Bruce Ing: The myxomycetes of Britain and Ireland: an identification handbook . Richmond Pub. Co, Slough, England 1999, ISBN 0855462515 , p. 4.
  6. Researchers Identify Mysterious Life Forms in the Deep Sea. , Scripps News (via WebArchive)
  7. Elio: Let's Start Out Big , Small Things Considered, January 3, 2013
  8. M. Maldonado, M. A. López-Acosta, C. L. Sitjà, R. Aguilar, S. García, J. Vacelet: A giant foraminifer that converges to the feeding strategy of carnivorous sponges: Spiculosiphon oceana sp. nov. (Foraminifera, Astrorhizida) . In: Zootaxa . 3669, No. 4, 2013, pp. 571-584. doi : 10.11646 / zootaxa.3669.4.9 . PMID 26312358 .
  9. A giant protozoon that imitates a carnivorous sponge has been discovered . Oceana. June 10, 2013. Accessed February 23, 2020.
  10. Becky Bauer: Gazing Balls in the Sea . In: All at Sea , October 2008. Archived from the original on September 17, 2010. Retrieved on February 16, 2020. 
  11. John Wesley Tunnell, Ernesto A. Chavez, Kim Withers: Coral reefs of the southern Gulf of Mexico . Texas A&M University Press, 2007, ISBN 978-1-58544-617-9 , p. 91.
  12. ^ What is the Largest Biological Cell? (with pictures) . Wisegeek.com. February 23, 2014. Accessed February 16, 2020.
  13. ^ A b Anne Helmenstine: What Is the Largest Unicellular Organism? . sciencenotes.org. November 29, 2018. Retrieved January 7, 2020.
  14. Caulerpa taxifolia green alga , on: Sea water dictionary
  15. Blue whale among the bacteria - On the discovery of Thiomargarita namibiensis , on: scinexx of July 14, 2003
  16. Giant bacteria baffles microbiologists , on: scinexx from September 8, 2017
  17. Methanospirillum hungatei , on: Microbe Wiki