* Bacteria: These are prokaryotic organisms, meaning they lack a nucleus and other membrane-bound organelles.
* Archaea: These are also prokaryotes, but they are genetically and biochemically distinct from bacteria.
* Protozoa: These are eukaryotic organisms, meaning they have a nucleus and other membrane-bound organelles. Examples include amoeba, paramecium, and Euglena.
* Algae: Some algae are unicellular, such as diatoms and dinoflagellates.
* Yeasts: These are single-celled fungi.
It's important to note that some organisms, like fungi and algae, can be multicellular as well.
