Life on Earth consists of three basic domains: Archaea, Bacteria, and Eukarya. A domain is the highest taxonomic level of classification. This level of classification is present above the kingdoms in the taxonomic hierarchy.

In this article, we will discuss Archae and Bacteria, their differences, and the organisms present in these domains.

Domain Archaea

The word Archaea is derived from ‘Archaios,’ which means ‘ancient’ and ‘primitive.’ This indicates that the domain Archaea consists of the most primitive life forms that existed on the Earth.

Structure of archaea with a labeled diagram
Image: Structure of Archaea

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The domain Archaea contains the simplest organisms that inhabited the Earth when life started here.


These organisms lived in the most hostile environments, such as saline water (containing high salt concentrations), hot springs (environment with boiling water temperatures), and hydrothermal vents. 

Archaea are considered extremophiles because they can tolerate and even grow in hostile environments where no other form of life could exist. 


The metabolism of Archaea was also adapted to harsh environments. They could perform methanogenesis. Methanogenesis is the form of respiration in which methane is produced as a by-product of respiration. 

This ability is present in only some bacteria and archaea. This is also evidence that these organisms are ancient and performed methanogenesis because there was no free oxygen when life started on Earth.


Archaea reproduced by asexual methods such as budding, binary fission, and fragmentation. The sexual reproduction methods, i.e., mitosis and meiosis, did not exist in these organisms.


Archaea have great diversity so are divided into many phyla. But taxonomists agree that there are only five major phyla in the Archaea. These phyla include

  1. Crenarchaeota
  2. Euryarchaeota
  3. Korarchaeota
  4. Nanoarchaeota
  5. Thaumarchaeota

Among these five phyla, most of the well-cultivated and cultured organisms are present in phylum Crenarchaeota and phylum Euryarchaeota.

Domain Bacteria

Bacteria are a diverse group of single-celled organisms that are part of almost all ecosystems present on the planet. They have a great diversity in shapes, size, and pathogenicity, ranging from friendly bacteria E. coli to the most pathogenic ones such as Clostridium tetani.

Bacteria cell with labeled parts diagram
Image: Bacteria Cell with labelled parts

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Bacteria are present in almost all parts of the planets based on their characteristics. They can be present in glaciers (psychrophiles) or tropics(mesophiles), or hot-water springs and volcanos(thermophiles). 

Some bacteria can also survive hostile conditions such as high salt or sugar concentrations and acidic or basic pH.


Bacteria have diversity in the way they metabolize nutrients. They can be photosynthetic or chemosynthetic bacteria. They can be fermenting bacteria or aerobically respiring bacteria.

Bacteria can also be differentiated based on the need for oxygen for respiration, i.e., they may be aerobic, facultative aerobes, microaerophilic or anaerobic bacteria. All these adaptations make bacteria survive in every type of environment. 


Bacterial growth is usually determined by its reproduction. Bacteria reproduce by asexual methods, just like archaea. Some bacteria also possess the capability of conjugation. Conjugation is a method that produces genetic variability among bacteria. 

Just like archaea, bacteria CANNOT reproduce sexually.


Bacteria are a diverse group. They can be classified into different groups. The most common basis of classification is Gram Staining. Other methods of bacterial classification are also present, e.g., Endospore formation, Lactose fermentation, etc.

Similarities between Bacteria and Archaea

Bacteria and Archaea, although they belong to two different domains but have some very similar characteristics. Some of them are listed as follows:

  1. Both of them do not have any membrane-bound organelles.
  2. Both contain gas vesicles; gas vesicles are vacuole-like structures that provide buoyancy to the cell.
  3. Both contain ribosomes; the size of ribosomes of bacteria and archaea are the same, i.e., the 70S.
  4. Both contain only one type of DNA polymerase enzyme; DNA polymerase is involved in DNA replication.

Differences between Bacteria and Archaea

DefinitionArchaea are the primitive and basic organisms that existed as the earliest forms of life on Earth. They have distinct features, so they have to be classified from other forms of organisms.Bacteria are prokaryotic, single-celled, diverse organisms that existed as Earth’s earliest form of life.
HabitatArchaea are extremophiles; they can tolerate high salt concentrations and high temperatures.Bacteria have lesser tolerance to high temperature and salt concentration. Bacteria can not grow in an environment with low pH.
Classification Archaea can be divided into further classes based on their characteristics. Some examples include methanogens, thermophiles, mesophiles, etc.Bacteria can be divided based on their characteristics and molecular data analysis. Bacteria can be Gram-positive or negative, Sporulating or non-sporulating. They can also be divided based on fermenting abilities, such as lactose fermenters and non-fermenters.
Sporulation Archaea cannot form spores. They can naturally overcome the conditions that spores are made for.Some bacteria can form spores. Spores help bacteria survive in hostile environments such as high temperature and acidic and basic pH.
Layers in the cell membraneArchaea cell membranes may exist as bilayers or monolayers.Bacterial cell membranes exist as bilayers of lipids.
Cell WallArchaea have various cell walls; unlike bacteria, they lack muramic acid in their cell walls.Bacteria have cell walls made up mainly of peptidoglycan; they contain muramic acid in their cell walls.
D-amino acidsArchaea lack D-amino acids in their cell wallsBacterial cell walls contain D-amino acids in their cell walls.
MethanogenesisArchaea can generate methane as the final product of respiration, so they are considered methanogens.Bacteria can not perform methanogenesis, i.e., they can not produce methane as a by-product of respiration.
PhotosynthesisChlorophyll-based photosynthesis is not present in Araceae. They can not produce energy from sunlight.Some bacteria are photosynthetic; they can convert sunlight to chemical energy that can be utilized during respiration.
Nature of Membrane LipidsThe membrane lipids present in archaea are complex and branched; they are isoprene derived fatty-acids.Bacteria contain straight-chain fatty acids in their membranes.
Lipid linkages in membranesThe fatty acids of the Archaeal cell membrane are bound to glycerol by ether linkages.The fatty acids present in the cell membrane of bacteria are bound to glycerol by ester linkages.
Glucose catabolismMetabolic pathways such as Glycolysis and Kreb’s cycle are absent in Archaea.Bacteria use Glycolysis and Kreb’s cycle for glucose metabolism as the main process for glucose oxidation.
Amino acid carried by tRNAThe amino acid carried by transfer RNA is N-formyl methionineThe amino acid carried by transfer RNA is methionine
The amino acid in tRNAAmino acid Thymine is absent in the tRNA of Archaea.Amino acid Thymine is present in the tRNA of bacteria.
PathogenicityArchaea are non-pathogenic.Bacteria can be pathogenic as well as non-pathogenic.
The action of StreptomycinArchaea is resistant to Streptomycin.Bacteria are sensitive to antibiotics such as Streptomycin.
RNA PolymeraseArchaea contain multiple RNA polymerases. RNA polymerase enzymes are involved in the transcription of DNA to RNA.Bacteria have a single RNA polymerase enzyme.
Origin of ReplicationThere are multiple replication origins in the Archaea. It means replication in archaea can occur at more than one point along the DNA length.There is a single replication origin in the bacteria. Replication in bacterial DNA can occur at only one point at a time.

Frequently Asked Questions

Q1. Give five examples of organisms present in Archaea

The five examples of Archeae are Aeropyrum pernix, Thermosphaera aggregans, Ignisphaera aggregans, Sulfolobus tokodaii, and Metallosphaera sedula.

Q2. Why bacteria and archaea are placed in separate domains

The main reason for this classification is the difference in the structure of both groups’ cell walls and cell membranes.

Q3. What is the mode of nutrition of Archaea?

Archaea can be autotrophic or heterotrophic. They are a diverse group.

Q4. Can Archaea move?

Some Archeae are sessile, i.e., they cannot move, but some are motile, i.e., they can move. Archaea move by their flagella.


  2. Prescott’s Microbiology Chapter 20

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