Archaea Domain

Extreme Microscopic Organisms

Methanococcoides Archaea
This is a colored transmission electron micrograph (TEM) of a section through the Archaebacterium Methanococcoides burtonii. This psychrophilic (cold-loving) Archaebacterium was discovered in 1992 in Ace Lake, Antarctica, and can survive in temperatures as low as -2.5 degrees Celsius. As a methanogenic bacterium, it is able to form methane from carbon dioxide and hydrogen. DR M.ROHDE, GBF/Science Photo Library/Getty Images

What Are Archaea?

Archaea are a group of microscopic organisms that were discovered in the early 1970s. Like bacteria, they are single-celled prokaryotes. Archaeans were originally thought to be bacteria until DNA analysis showed that they are different organisms. In fact, they are so different that the discovery prompted scientists to come up with a new system for classifying life. There is still much about archaeans that is not known. What we do know is that many are extreme organisms that live and thrive under some of the most extreme conditions, such as extremely hot, acidic, or alkaline environments.

Key Takeaways

  • Originally thought to be bacteria, Archaea are a separate group of microscopic organisms discovered in the 1970s. Archaeans are single-celled prokaryotes.
  • Archaeans are extreme organisms. They can survive and even thrive under some of the most difficult conditions on planet Earth like very hot, extremely acidic, or very alkaline environments.
  • Similar to bacteria, Archaeans have a number of different shapes. Cocci (round), bacilli (rod-shaped), and irregular are some examples.
  • Archaeans possess the typical prokaryotic cell anatomy that includes plasmid DNA, a cell wall, a cell membrane, a cytoplasmic area, and ribosomes. Some archaeans can also have flagella.

Archaea Cells

Archaeans are extremely small microbes that must be viewed under an electron microscope to identify their characteristics. Like bacteria, they come in a variety of shapes including cocci (round), bacilli (rod-shaped), and irregular shapes. Archaeans have a typical prokaryotic cell anatomy: plasmid DNA, cell wall, cell membrane, cytoplasm, and ribosomes. Some archaeans also have long, whip-like protrusions called flagella, which aid in movement.

Archaea Domain

Organisms are now classified into three domains and six kingdoms. The domains include Eukaryota, Eubacteria, and Archaea. Under the archaea domain, there are three main divisions or phyla. They are: Crenarchaeota, Euryarchaeota, and Korarchaeota.


Crenarchaeota consist mostly of hyperthermophiles and thermoacidophiles. Hyperthermophilic microorganisms live in extremely hot or cold environments. Thermoacidophiles are microscopic organisms that live in extremely hot and acidic environments. Their habitats have a pH between 5 and 1. You would find these organisms in hydrothermal vents and hot springs.

Crenarchaeota Species

Examples of Crenarchaeotans include:

  • Sulfolobus acidocaldarius - found near volcanic environments in hot, acidic springs containing sulfur.
  • Pyrolobus fumarii - live in temperatures between 90 and 113 degrees Celsius.


Euryarchaeota organisms consist mostly of extreme halophiles and methanogens. Extreme halophilic organisms live in salty habitats. They need salty environments to survive. You would find these organisms in salt lakes or areas where sea water has evaporated.
Methanogens require oxygen free (anaerobic) conditions in order to survive. They produce methane gas as a byproduct of metabolism. You would find these organisms in environments such as swamps, wetlands, ice lakes, the guts of animals (cow, deer, humans), and in sewage.

Euryarchaeota Species

Examples of Euryarchaeotans include:

  • Halobacterium - include several species of halophilic organisms that are found in salt lakes and high saline ocean environments.
  • Methanococcus - Methanococcus jannaschii was the first genetically sequenced Archaean. This methanogen lives near hydrothermal vents.
  • Methanococcoides burtonii - these psychrophilic (cold-loving) methanogens were discovered in Antarctica and can survive extremely cold temperatures.


Korarchaeota organisms are thought to be very primitive life forms. Little is currently known about the major characteristics of these organisms. We do know that they are thermophilic and have been found in hot springs and obsidian pools.

Archaea Phylogeny

Archaea are interesting organisms in that they have genes that are similar to both bacteria and eukaryotes. Phylogenetically speaking, archaea and bacteria are thought to have developed separately from a common ancestor. Eukaryotes are believed to have branched off from archaeans millions of years later. This suggests that archaeans are more closely related to eukayotes than bacteria.

Interesting Archaeans Facts

While Archaeans are very similar to bacteria, they are also much different. Unlike some types of bacteria, archaeans can not perform photosynthesis. Similarly, they cannot produce spores.

Archaeans are extremophiles. They can live in places where most other life forms cannot. They can be found in extremely high temperature environments as well as extremely low temperature environments.

Archaeans are a natural part of human microbiota. At present, pathogenic archaeans have not been identified. Scientists assume that they do not exist.

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Your Citation
Bailey, Regina. "Archaea Domain." ThoughtCo, Feb. 16, 2021, Bailey, Regina. (2021, February 16). Archaea Domain. Retrieved from Bailey, Regina. "Archaea Domain." ThoughtCo. (accessed March 27, 2023).

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