Unicellular organisms are organisms with a single cell. All characteristics of life like reproduction, locomotion etc is carried out by this single cell. Majority are microscopic.
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1. All bacteria or prokaryotes are unicellularBacteria are prokaryotic organisms without a true nucleus. The genetic material DNA is not surrounded by a membrane as in eukaryotes. Membrane bound cell organelles are also absent.
Reproduction is primarily by binary fission.
Eg: Escherichia coli (Intestinal Bacteria), Lactococcus lactis in milk, Streptococcus pyogenes, Halobacteria
   2. All protists: Protists are eukaryotic organisms which are unicellular or unicellular-colonial and which form no tissues. They are diverse group of organisms that are not animals, plants or fungus Eg: Amoeba, Euglena, Paramecium, Plasmodium
   3. Unicellular algae: Algae are chlorophyll bearing thallophytes with autotrophic mode of nutrition. The thallus ranges from a single cell to very elaborate multicellular parenchymatous structure.
Some examples of unicellular algae are Chlamydomonas, Chlorella etc
   4. Unicellular fungus: Saccharomyces cerevisiae (Brewer's yeast)
Fungi are eukaryotic, non-chlorophyllated thallophytes with heterotrophic mode of nutrition. Majority are multicellular. Saccharomyces cerevisiae (Brewer's yeast) is an example of unicellular fungus.
Learn more: Unicellular vs Multicellular Organisms