Domain archaea

1. Overview of Archaea

• Single-celled prokaryotic microorganisms.

• Form one of the three domains of life proposed by Carl Woese in 1977 (alongside Bacteria and Eukarya).

• Lack a membrane-bound nucleus and internal organelles.

2. Key Characteristics

• Cell Wall Composition:

  • Does not contain peptidoglycan (a key feature distinguishing them from Bacteria).

  • Cell walls are typically composed of pseudopeptidoglycan, glycoproteins, or pure polysaccharides.

• Cell Membrane Structure:

  • Possess ether-linked lipids rather than the ester-linked lipids found in Bacteria and Eukarya.

  • Hydrocarbon chains are branched isoprenoids instead of unbranched fatty acid chains.

  • Can form lipid monolayers, providing enhanced stability in extreme conditions.

• Genetic and Molecular Features:

  • Protein synthesis begins with methionine (similar to Eukarya), unlike N-formylmethionine used by Bacteria.

  • RNA polymerases and ribosomal structures closely resemble those of eukaryotes.

3. Major Functional Groups

• Methanogens:

  • Obligate anaerobes that produce methane ($CH_4$) as a metabolic byproduct.

  • Inhabit anoxic environments such as marshes, hydrothermal vents, and gut environments of animals.

• Halophiles:

  • Adapted to environments with high salt concentrations ($> 1.5\,M$ $NaCl$).

  • Utilize specialized light-driven proton pumps such as bacteriorhodopsin to generate ATP.

• Thermophiles & Hyperthermophiles:

  • Flourish at elevated temperatures ($60\,^\circ\text{C}$ to over $100\,^\circ\text{C}$).

  • Possess heat-stable enzymes and highly stable membrane structures.

4. Evolutionary Importance

• Genetically and biochemically represent a bridge between prokaryotic cell architecture and eukaryotic molecular machinery.

• Asgard Archaea are recognized as the closest prokaryotic relatives to Eukaryotes.