"Microbes" spreads across all of the three deepest divisions, or domains, of life on our planet: Bacteria, Archaea, and Eukarya, and is a term that includes all single-celled organisms: autotrophic and heterotrophic prokaryotes (bacteria and cyanobacteria) and autotrophic and heterotrophic eukaryotes (algae and phagotrophic protists), as well as viruses.
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Sherr E., Sherr B.; 2000; Marine Microbes an Overview; in Kirchman D.L. (ed); Microbial Ecology of the Oceans; Willey-Liss Inc.; New York USA;