Definition, Examples, Vs Bacteria, Characteristics & Class

Definition:  What are Archaea?

Archaea are unicellular organisms that make up the third domain of organisms on earth. As such, they are different from the other two domains that include Bacteria and Eukaryota.

Like bacteria, however, archaea are prokaryotes that share certain characteristics with bacteria (this is one of the reasons archaea were previously thought to be a type of bacteria).

Due to their ability to survive extreme conditions, they can be found in a variety of environments ranging from lakes and soil to the Dead Sea and the deepest parts of the ocean (ocean floor).

Some examples include:

  • Aeropyrum pernix
  • Thermosphaera aggregans
  • Ignisphaera aggregans
  • Sulfolobus tokodaii
  • Metallosphaera sedula
  • Staphylothermus marinus
  • Thermoproteus tenax


Based on various molecular and genetic compositions, the domain Archaea is subdivided into five (5) phyla that include:

  • Euryarchaeota
  • Crenarchaeota
  • Korarchaeota
  • Thaumarchaeota
  • Nanoarchaeota

Phylum Euryarchaeota

The phylum Euryarchaeota is one of the best-studied phyla of the domain (Archaea). Consisting of more than 70 genera, members of the phylum are extremely physiologically diverse with the ability to survive some of the most extreme environments around the world.

The following are characteristics of different groups of the phylum Euryarchaeota:

Euryarchaeotae is composed of mesophilic, thermophilic and psychrotolerant species spread across eight (8) classes. 

Class Archaeoglobi - The class Archaeoglobi is made up of a single order (Archaeoglobales) and family (Archaeoglobaceae). Archaeoglobaceae is further divided into three genera that include Ferroglobus, Geoglobus, and Archaeoglobus. Like class Thermococci, members of Archaeoglobi are spherical in appearance and thus may be described as having a coccoid morphology.

Some of the main characteristics of this group include:

  • They are found in hydrothermal vents (in deep-sea)
  • Anaerobic
  • They grow well in marine salinity
  • The three genera in the phylum contain species that are mixotrophic (use different sources of energy) and lithoautotrophic (obtain energy from reduced compounds) in nature
  • Optimum temperature range for members of this phylum is between 83 and 88 degrees Celsius (maximum temperature is about 95 degrees Celsius)

Class Protoarchaea - Also known as class Thermococci, Protoarchaea is also made up of a single order (Thermococcales) and family (Thermococcaceae). The family Thermococcaceae is further divided into the genera Pyrococcus, Thermococcus, and Paleococcus.

With the exception of a few species, a majority of species in these genera have the following characteristics:

  • Obligate anaerobes
  • Hyperthermophilic (thrive in extremely hot environments)
  • Organotrophic
  • Grow well in neutral pH and marine salinity
  • Coccoid in morphology
  • Capable of movement
  • Found in hydrothermal vents
  • Some species are capable of fermentation
  • Are all obligate organo-heterotrophs (they can obtain energy/nutrition from organic compounds or from other organisms)

Class Thermoplasmata - Class Thermoplasmata is made up one order (Thermoplasmatales) and three families.

Members of this class have the following characteristics:

  • Extreme acidophiles (thrive under acidic conditions)
  • Aerobic in nature (they need air for growth)
  • Spherical/coccoid morphology
  • Majority of species are moderate thermophiles (they grow under moderate temperatures- below 40°C)
  • Some species (e.g. members of family Ferroplasmaceae) can oxidize iron compounds to obtain energy
  • They are incapable of fermentation

Class Halobacteria - Consisting of about 30 genera, the Class Halobacteria is made up of organisms that are highly halophilic in nature. As such, they are able to thrive well in environments with high salt concentrations (30 to 36 percent sodium chloride). For this reason, they can be found in such extreme environments as the Dead Sea.

Some of the other important characteristics of the members of this class include:

  • Aerobic organisms (need air for growth)
  • Many of the species also need magnesium salts for their development
  • Most of the species are pleomorphic in morphology which is caused by the high minerals in their environments
  • A majority of the species are Gram-negative with only a few being Gram-variable
  • Mesophilic
  • Chemo-organotrophs - They oxidize the chemical bonds of complex organic compounds (peptone etc) to obtain energy
  • A majority of the species are found in such salty environments as soda lakes and marine springs containing sulfur and sulfide among others


Methanogens encompass four classes of the Phylum Euryarchaeota that are characterized by their ability to produce methane. These include Methanotherma, Methanobacteria, Methanopyri, and Methanomicrobia.

Apart from the ability to produce methane, all members of this group are obligate anaerobes that use carbon dioxide to accept electrons. As such, they cannot tolerate the presence of oxygen.

Some of the other characteristics of Methanogens include:

  • Can be found in the digestive system of ruminant animals (e.g. goats and domestic yak) - In the rumen of these animals, Methanogens are involved in the fermentation process
  • Are found in the deep-sea and saline environments (e.g. members of Class Methanopyri and Methanothermea)
  • Vary from cocci and curved rods to long and short in morphology
  • Grow well in neutral environments (or slightly alkaline environments)

Phylum Crenarchaeota

Apart from the Phylum Euryarchaeotae, Phylum Crenarchaeota is the other group of organisms that has received a lot of attention over the years.

Although this phylum contains fewer genera compared to the former, it consists of a great diversity of organisms that can be found in various types of environments. For instance, whereas some of the species can be found in the soil, others can be found in high-temperature environments (thermophiles).

Compared to the phylum Euryarchaeotae, a single class (Crenarchaeota) of phylum Crenarchaeota has been identified so far.

Crenarchaeota is further divided into five orders that include:

Order Acidolobales - Members of this order are acidophiles that use sulfur during respiration (as the electron acceptor). They are spherical in shape (cocci) and include members of two major families namely, Acidilobaceae and Caldispheraceae.

Order Desulfurococcales - This order is divided into family Pyrodictiaceae and family Desulfurococcaceae. Whereas some are cocci in their morphology (the majority of Desulfurococcaceae), others are rod-shaped. 

Some of the other characteristics of Order Desulfurococcales include:

  • Some are obligate anaerobes in nature (Desulfurococcaceae)
  • Slightly acidophilic or neutrophilic organisms that can be found in some marine environments (Desulfurococcaceae)
  • Some members are fermenters
  • Some members of the family Pyrodictiaceae are thermophiles and can survive extremely high temperatures
  • G+C content varies from one group to another

Order Sulfolobales - The Order Sulfolobales consists of a single family (Sulfolobaceae) which is in turn made up of hyperthermophilic and acidophilic organisms spread across six genera.

Characteristics of the order Sulfolobaceae vary from one group of organisms to another. For instance, whereas the genus Sulfurisphera is made up of an organism that is strictly organotrophic facultative anaerobe, members of Acidianus and Sulfolobus among others have been shown to use lithoautotrophic and organotrophic metabolism.

Order Thermoproteales - This order is composed of two families (Thermofilaceae and Thermoproteaceae) that have the following characteristics:

  • Neutrophiles or slightly acidophilic organisms that can be found in high-temperature environments (hyperthermophiles)
  • Rod-shaped
  • Some are fermenters (Thermoproteus)
  • Some can grow on compounds containing hydrogen and thiosulfates etc
  • Organo-heterotrophs

Order Fervidicoccales - This order consists of a single family and species that can be found in hot springs.

The other three phyla of Archaea are not fully understood and no valid representatives have been agreed on.

The following are some of the characteristics of the three groups:

Korarchaeota - This phylum was discovered in both marine and terrestrial hot environments thus suggesting that members of the phylum are hyperthermophilic. In order to determine the diversity and abundance of the group in nature, studies have been conducted in various environments and in a number of countries.

Through these studies, it became evident that members of the phylum grow in environments that range between 70 to 97 degrees Celsius in temperature, and 2.5 and 6.5 in pH. Currently, very few organisms have been identified as belonging to the phylum. One example of this is the Candidatus Korarchaeum cryptofilum that was isolated from a culture containing sediments from Obsidian Pool. 

Based on studies on the organism, the following characteristics were identified:

  • Capable of degrading peptides through fermentation
  • Cannot synthesize such co-factors as vitamins

Nanoarchaeota - Like the phylum Korarchaeota, only one member (Nanoarchaeum equitans) of phylum Nanoarchaeota has currently been identified. 

N. equitans has been shown to grow attached (in a symbiotic relationship) to the surface of various Ignicoccus species and has the following characteristics:

  • Hyperthermophilic organisms
  • Can be found on hot rocks, marine thermal vent environments and springs
  • Are strict anaerobes
  • Contain divergent 16S rDNA sequence
  • Lack genes responsible for the synthesis of lipid, amino acids, and nucleotide
  • Grow well in high temperatures (about 90 degrees Celsius) and pH range of between 2.5 and 3.0

Thaumarchaeota - As compared to Korarchaeota and Nanoarchaeota, Thaumarchaeota is better understood with the group making up about five (5%) percent of all prokaryotes in soil systems. They can also be found in hot springs and marine waters and consist of ammonia-oxidizing organisms. 

Some of the main characteristics of Thaumarchaeota include:

  • A majority of the organisms can fix carbon dioxide under autotrophic conditions
  • They oxidize ammonia aerobically

Archaeal Groups

Apart from phylum divisions of Archaea, the domain is also divided into the following three groups:

Extreme Halophiles

Extreme halophiles include a variety of organisms that thrive in an environment that contain high salt concentrations. For optimal growth, extreme halophiles have been shown to require at least 1.5 mol l-1 of sodium chloride. Therefore, while many may tolerate high salt conditions, a good number have been shown to actually depend on such conditions for growth.

Examples of extreme halophiles include:

  • Haloadaptus
  • Halobaculum
  • Methanohalobium


Methanogens are characterized by the inability to tolerate oxygen as well as the ability to produce methane.

Methane gas is therefore produced under anaerobic conditions and in the absence of such ions as ferric ions and nitrates. In cases where methanogens live in anoxic soils or in environments where other organisms produce oxygen, they produce methane at a high rate in order to bring about anoxic conditions.

Because of their ability to produce methane, they have been used in some industries to produce the gas.

Examples of methanogens include:

  • Methanopyrus kandleri
  • Methanocaldococcus
  • Methanobrevibacter
  • Methanosphera


Hyperthermophiles are also commonly referred to as heat-loving prokaryotes. This is because they are a group of Archaea that are capable of growing in temperatures of above 80 degrees Celsius.

They are commonly found in environments with very high temperatures such as hot acid springs, in geothermal power plants as well as submarine volcanic habitats and areas with heated soil.

Apart from high temperatures, some Hyperthermophiles have also been shown to tolerate extreme acidity in their environments. However, the majority, which is obligate anaerobes, grow well in environments that are either neutral or mildly acidic.

Examples of Hyperthermophiles include:

  • Geoglobus
  • Archaeoglobus
  • Ferroglobus

Main Characteristics of Archaea (Vs. Bacteria)

Because of their diversity, archaeal cells display significant variance in morphology. Whereas some are rod-shaped, like many bacteria, others are spiral, disk shaped or spherical in shape. On the other hand, some have been shown to portray various irregular shapes.

Although Archaea is a distinct domain, it shares a number of characteristics with both Bacteria and Eukaryota. For instance, like bacteria, a majority of archaea have a cell wall that regulates osmosis and maintains the shape of the cell.

However, unlike bacteria, archaea do not have the peptidoglycan. Rather, they contain pseudopeptidoglycan consisting of N-acetyltalosamine uronic acid (NAT) while others have a cell wall made up of proteins or polysaccharides.

Both (bacteria and archaea) are also capable of locomotion in moist or liquid environments. This is made possible by the presence of flagella. Depending on the species, archaea may possess a one or several flagella allowing it to move from one point to another.

Although they both possess flagella that allow for locomotion, the flagella protein and the structure of flagella is different between the two.

The other difference between archaea and bacteria is with regards to their cell membrane. While both have a cell membrane, there is a difference in how various components of the cell membrane are arranged in archaea. For instance, in archaea, the tails of the hydrophobic lipid are attached to the glycerol by ether linkages. This is different from the ester linkage present in Bacteria and Eukaryota.

With both Bacteria and Archaea being prokaryotes, the chromosome region is composed of bodies known as nucleoids. Compared to Eukaryotes, these diffuse mass lack a membrane envelope and thus reside in the cytoplasm as DNA aggregates.

Both Bacteria and Archaea also contain plasmids (small DNA molecules). These extrachromosomal molecules of DNA are typically circular in shape with genes ranging from about 5 to 100.

* Like Eukaryotes, Bacteria and Archaea have also been shown to have a cytoskeleton that regulates cell division.

Take a look at Eukaryotes also as well as Bacteria

Return to understanding the Kingdom Monera

Return to Unicellular Organisms Main Page

Return to Prokaryotes Main Page

Return from Archaea to MicroscopeMaster Home


Elena V. Pikuta. (2014). Overview of Archaea. ResearchGate. 

Jeffrey C. (1994). Cell Structure and Function in the Bacteria and Archaea. Fundamentals of Microbiology - Pommerville 9th Edition. 

Jet McLain. (2004). Archaea. Encyclopedia of Soils in the Environ., Elsevier Ltd., Oxford, UK, pp. 88-94, 2004..

Jose Berenguer. (2011). Thermophile. Encyclopedia of Astrobiology. 


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