MAIN DIFFERENCES BETWEEN ARCHAEA AND BACTERIA - BIOLOGY - 2025

The main differences between archaea and bacteria are based on molecular-structural and metabolic aspects that we will develop below. The Archaea domain taxonomically groups unicellular microorganisms that have prokaryotic cell morphology (without nuclear membrane, or cytoplasmic organelle membranes), characteristics that resemble bacteria.

However, there are also traits that separate them, since archaea are endowed with very particular adaptation mechanisms that allow them to live in environments with extreme conditions.

Figure 1. Escherichia coli bacteria. Source: NIAID, via Wikimedia Commons

The bacterium domain contains the most abundant forms of bacteria called eubacteria, or true bacteria. These are also unicellular, microscopic, prokaryotic organisms that live in any environment with moderate conditions.

Evolution of the taxonomy of these groups

In the 4th century BC, living things were classified into only two groups: animals and plants. Van Leeuwenhoek, in the seventeenth century, using a microscope that he had built himself, was able to observe microorganisms that until then had been invisible and described protozoa and bacteria under the name of "animáculos".

In the 18th century, "microscopic animals" were incorporated into the systematic classifications of Carlos Linneo. In the mid-nineteenth century, a new kingdom groups bacteria: Haeckel postulated a systematic based on three kingdoms; kingdom Plantae, kingdom Animalia and kingdom Protista, which grouped microorganisms with a nucleus (algae, protozoa and fungi) and organisms without a nucleus (bacteria).

Since this date, several biologists have proposed different classification systems (Chatton in 1937, Copeland in 1956, Whittaker in 1969) and the criteria for classifying microorganisms, initially based on morphological differences and differences in staining (Gram stain), they became based on metabolic and biochemical differences.

In 1990, Carl Woese, applying molecular sequencing techniques in nucleic acids (ribosomal ribonucleic acid, rRNA), discovered that among microorganisms grouped as bacteria, there were very large phylogenetic differences.

This discovery showed that prokaryotes are not a monophyletic group (with a common ancestor) and Woese then suggested three evolutionary domains that he named: Archaea, Bacteria and Eukarya (nucleated cell organisms).

Differential characteristics of Archaea and Bacteria

Archaea and Bacteria organisms have common characteristics in that both are unicellular, free or aggregated. They do not have a defined nucleus or organelles, they have cell size between 1 to 30μm on average.

They present significant differences with respect to the molecular composition of some structures and in the biochemistry of their metabolisms.

Habitat

Bacteria species live in a wide range of habitats: they have colonized brackish and fresh waters, hot and cold environments, swampy lands, marine sediments and rock fissures, and they can also live in atmospheric air .

They can live with other organisms within the digestive tubes of insects, mollusks and mammals, oral cavities, respiratory and urogenital tract of mammals, and blood of vertebrates.

Figure 2. Hot springs, extreme habitats where organisms of the Archaea group inhabit, which usually give them bright colors. Source: CNX OpenStax via wikipedia

Also the microorganisms belonging to Bacteria can be parasites, symbionts or commensals of fish, roots and stems of plants, of mammals; they can be associated with lichen fungi and protozoa. They can also be food contaminants (meat, eggs, milk, seafood, among others).

The species of the Archaea group have adaptation mechanisms that allow their life in environments with extreme conditions; they can live at temperatures below 0 ° C and above 100 ° C (a temperature that bacteria cannot support), in extreme alkaline or acidic pHs and saline concentrations much higher than those of seawater.

Methanogenic organisms (which produce methane, CH ₄) also belong to the Archaea domain.

Plasma membrane

The envelope of prokaryotic cells is generally formed by the cytoplasmic membrane, the cell wall and the capsule.

The plasma membrane of organisms of the Bacteria group does not contain cholesterol or other steroids, but rather linear fatty acids linked to glycerol by ester-type bonds.

The membrane of the members of Archaea can be constituted by a bilayer or by a lipid monolayer, which never contain cholesterol. Membrane phospholipids are made up of long-chain, branched hydrocarbons linked to glycerol by ether-type bonds.

Cellular wall

In organisms of the Bacteria group, the cell wall is made up of peptidoglycans or murein. Archaea organisms possess cell walls that contain pseudopeptidoglycan, glycoproteins or proteins, as adaptations to extreme environmental conditions.

Additionally, they can present an outer layer of proteins and glycoproteins, covering the wall.

Ribosomal ribonucleic acid (rRNA)

RRNA is a nucleic acid that participates in protein synthesis –production of the proteins that the cell requires to fulfill its functions and for its development-, directing the intermediate steps of this process.

The nucleotide sequences in ribosomal ribonucleic acids are different in Archaea and Bacteria organisms. This fact was discovered by Carl Woese in his 1990 studies, which resulted in the separation of these organisms into two different groups.

Endospore production

Some members of the Bacteria group can produce survival structures called endospores. When environmental conditions are very adverse, endospores can maintain their viability for years, with practically zero metabolism.

These spores are extremely resistant to heat, acids, radiation and various chemical agents. In the Archaea group, no species that form endospores have been reported .

Movement

Some bacteria have flagella that provide mobility; spirochetes have an axial filament by means of which they can move in liquid, viscous media such as mud and humus.

Some purple and green bacteria, cyanobacteria, and Archaea possess gas vesicles that allow them to move by floating. The known Archaea species do not have appendages such as flagella or filaments.

Figure 3. Río Tinto, an extreme environment in Huelva, Spain where the Arqueas of the Metallosphaera and Sulfolobus genera develop. Source: Riotinto2006, from Wikimedia Commons

Photosynthesis

Within the Bacteria domain, there are species of cyanobacteria that can carry out oxygenic photosynthesis (which produces oxygen), since they have chlorophyll and phycobilins as accessory pigments, compounds that capture sunlight.

This group also contains organisms that carry out anoxygenic photosynthesis (which does not produce oxygen) through bacteriochlorophylls that absorb sunlight, such as: red or purple sulfur and red non-sulfur bacteria, green sulfur and green non-sulfur bacteria.

In the Archaea domain, photosynthetic species have not been reported, but the genus Halobacterium, of extreme halophytes, is capable of producing adenosine triphosphate (ATP), with the use of sunlight without chlorophyll. They have the retinal purple pigment, which binds to membrane proteins and forms a complex called bacteriorhodopsin.

The bacteriorhodopsin complex absorbs energy from sunlight and when released it can pump H ⁺ ions to the outside of the cell and promote the phosphorylation of ADP (adenosine diphosphate) to ATP (adenosine triphosphate), from which the microorganism obtains energy.

References

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