page_title: Microorganism
section_title: Symbiosis
hierarchical_section_title: Microorganism / Ecology / Symbiosis
context_page_description: A microorganism, or microbe, is a microscopic organism, which may exist in its single-celled form or a colony of cells.
  The possible existence of unseen microbial life was suspected from ancient times, such as in Jain scriptures from 6th century BC India and the 1st century BC book On Agriculture by Marcus Terentius Varro. The scientific study of microorganisms began with their observation under the microscope in the 1670s by Antonie van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s, Robert Koch discovered that microorganisms caused the diseases tuberculosis, cholera, and anthrax.
  Microorganisms include all unicellular organisms and so are extremely diverse. Of the three domains of life identified by Carl Woese, all of the Archaea and Bacteria are microorganisms. These were previously grouped in the two domain system as Prokaryotes, the other being the eukaryotes. The third domain Eukaryota includes all multicellular organisms and many unicellular protists and protozoans. Some protists are related to animals and some to green plants.
context_section_description: A lichen is a symbiosis of a macroscopic fungus with photosynthetic microbial algae or cyanobacteria.
caption: The photosynthetic cyanobacterium Hyella caespitosa (round shapes) with fungal hyphae (translucent threads) in the lichen Pyrenocollema halodytes
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