A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, in contrast to a multicellular organism that consists of multiple cells. Historically simple single celled organisms have sometimes been referred to as monads.
Prokaryotes, most protists, and some kinds of fungi are unicellular. Although some of these organisms live in colonies, they are still unicellular. These organisms live together, and each cell in the colony is the same. However, each cell must carry out all life processes in order for that cell to survive. In contrast, even the simplest multicellular organisms have specialized cells that depend on each other in order to survive.
Some organisms are partially uni- and multicellular, like Dictyostelium discoideum. Other can be unicellular and multinucleate, like Myxogastria and Plasmodium.
‘Candidatus Magnetoglobus multicellularis’, related to Deltaproteobacteria, is a multicellular prokaryote. It's not a unicellular, nor a colony.
Most unicellular organisms are of microscopic size and are thus classified as microorganisms. However, some unicellular protists and bacteria are macroscopic and visible to the naked eye. Examples include:
- Xenophyophores, protozoans of the phylum Foraminifera, are the largest examples known, with Syringammina fragilissima achieving a diameter of up to 20 cm.
- Nummulite, foraminiferans
- Valonia ventricosa, algae of the class Chlorophyceae, can achieve a diameter of 1 to 4 cm.
- Acetabularia, algae.
- Caulerpa, algae
- Gromia sphaerica, amoeba
- Thiomargarita namibiensis is the largest bacteria, achieving a diameter of up to 0.75 mm.
- Epulopiscium fishelsoni, a bacteria.
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