A microorganism or microbe is a microscopic organism that comprises either a single cell (unicellular), cell clusters, or multicellular relatively complex organisms. Microorganisms are very diverse; on Earth they include bacteria, fungi, algae and protozoa; microscopic plants (green algae); and animals such as rotifers and planarians. Some microbiologists also include viruses, but others consider these as nonliving. Most microorganisms are unicellular (single-celled), but this is not universal since some multicellular organisms are microscopic, while some unicellular protists and bacteria, like thiomargarita namibiensis, are macroscopic and visible to the naked eye.
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