The five main groups of single-celled organisms are protozoa, archaea, bacteria, unicellular fungi and unicellular algae. While single-celled organisms can be either eukaryotic or prokaryotic, most are prokaryotes. The two domains into which prokaryotic cells are divided are archaea and bacteria.
Even though protozoa are single-celled organisms, they are biologically classified as eukaryotes. Eukaryotes are organisms with cells that have nuclei. In spite of their simplicity, protozoa have similarities to animals, such as mobility. Unicellular algae and fungi, which can be quite large in spite of having simple anatomy, are also eukaryotes.
Until the late 1970s, Microbe World states that scientists thought archaea were a type of bacteria. However, even though they look alike and share many similarities, the genetic makeup of these two microorganisms is different. Both archaea and bacteria are examples of prokaryotic single-celled organisms. Archaea and bacteria are two of three major domains most biologists use to group living organisms.
Archaea have been discovered thriving in extreme conditions, such as hot springs and hot underwater vents, as well as highly acidic environments. Bacteria can also be found in extreme environments as well as nearly everywhere else, making them one of the most diverse groups on the planet.