Microbiology - Archaea Characteristics

Archaeal Cell Structures

• Archaea are a domain of single-celled microorganisms, distinct from bacteria and eukaryotes.
• Genetic and genomic analysis show Archaea are distinct from both bacteria and eukaryotes.
• They are extremophiles, thriving in harsh environments like high temperatures, salinity, or acidity.
• Archaea have a microscopic size, typically 1-5 µm.
• They are usually unicellular.
• Their cell structure is prokaryotic, meaning they lack a nucleus and membrane-bound organelles.
• Archaeal cell walls lack peptidoglycan.

Learning Objectives

• Learning objectives for understanding archaeal cell structures include: comparing and contrasting archaea and bacteria in terms of shape, plasma membrane composition, cell wall composition, ribosome size and composition, chromosome structure, the presence or absence of inclusions and plasmids, and motility.

Domain Classification

• Organisms are categorized into three domains: Bacteria, Archaea, and Eukarya.
• Archaea and Bacteria are prokaryotic, lacking membrane-bound organelles and a nucleus.
• Eukarya are eukaryotic, having a nucleus and membrane-bound organelles.

Archaeal Examples

• Methanobrevibacter smithii is a methanogen.
• Halobacterium salinarum is a halophile tolerating high salt concentrations.
• Sulfolobus are acidophiles that thrive in acidic environments.
• Acidocaldarius are thermophiles surviving in high temperatures.

Archaeal Characteristics

• Archaea are extremophiles, capable of surviving in harsh conditions.
• They have a unique cell wall structure, lacking peptidoglycan (making them different from bacterial cell walls).
• Their plasma membranes have a specific lipid composition (ether linkages instead of ester linkages found in bacterial membranes). This makes archaea's membranes more resistant to heat.
• Archaeal ribosomes are 70S, similar to those in bacteria.
• Archaeal chromosomes are circular, similar to bacteria, but their ribosomal proteins can vary. Some ribosomal proteins are observed in archaea, some bacteria and some eukaryotes.

Comparison of Bacteria and Archaea

• Similarities: Both are prokaryotic with 70S ribosomes. Both can contain inclusions and plasmids. Both can be circular and linear chromosomes, though archaea more commonly feature circular DNA.
• Differences: The key difference lies in the cell wall composition, and plasma membrane structure. Bacteria have peptidoglycan, Archaea do not. Bacterial plasma membranes have ester linkages; archaea feature ether linkages. Motility structures are also different, with bacteria having flagella, while archaea have archaella.

Archaeal Cell Envelopes

• Archaea exhibit diverse cell envelopes, including an S-layer, a protein sheath (sometimes, a glycoprotein layer), or a polysaccharide layer.
• Some archaea lack an S-layer, sometimes having an intermembrane compartment structure instead.

Additional Properties

• Genetic transfer occurred in early cell communities
• Archaea exhibit horizontal gene transfer.
• Ribosomal proteins in Archaea can show homology to those of eukaryotes. This supports the idea of a common ancestry for these two groups – and is a key finding supporting the three-domain system.