Study Notes on Archaeal Cell Structure

Chapter 4: Archaeal Cell Structure

Learning Objectives

• Describe a typical archaeal cell.

• Discuss key differences between bacterial and archaeal cells.

• Determine the type of microbe (bacterium, fungus, etc.) when given a description of a microorganism.

• Draw an archaeal cell envelope and identify the component layers.

• Compare and contrast archaeal and bacterial cell envelopes in terms of their structures, molecular makeup, and functions.

• Compare and contrast nutrient uptake mechanisms observed in bacteria and archaea.

• Compare and contrast the cytoplasm of bacterial and archaeal cells.

• Compare and contrast bacterial and archaeal pili.

• Compare and contrast bacterial and archaeal flagella in terms of their structure and function.

Introduction to Archaea

• Previously, archaea and bacteria were collectively referred to as prokaryotes.

• Currently, archaea and bacteria are recognized as distinct taxa, each with unique characteristics.

• Archaea exhibit significant diversity in terms of morphology, physiology, reproduction, and ecological roles.

• Notable habitats for archaea include environments that are anaerobic (lacking oxygen), hypersaline (high salt concentration), extreme pH levels, and high temperatures environments.

Shared Features

• Common Features with Eukarya:

  • Genes involved in protein replication, transcription, and translation.

• Common Features with Bacteria:

  • Genes for metabolic processes.

• Unique Features of Archaea:

  • Unique structure of rRNA genes.

  • Ability to perform methanogenesis (producing methane as a metabolic byproduct).

Comparative Analysis of Bacterial and Archaeal Cells

• Table 4.1 Comparison of Bacterial and Archaeal Cells:

  • Property:

  • Bacteria:

    • Plasma membrane lipids: Ester-linked phospholipids

    • Cell wall constituents: Peptidoglycan in nearly all; some lack cell walls.

    • Inclusions present: Highly diverse.

    • Ribosome size: 70S

    • Chromosome structure: Circular, double-stranded DNA

    • Plasmids present: Yes, circular and linear dsDNA.

    • Capsules or slime layers: Common.

    • Fimbriae, pili: Common, including archaella.

    • Motility structure: Flagellum

    • Cytoskeleton: Common

    • Extracellular vesicles: Rare.

  • Archaea:

    • Plasma membrane lipids: Glycerol diethers, glycerol tetraethers.

    • Cell wall constituents: Peptidoglycan is always absent; most have an S-layer.

    • Inclusions present: Yes.

    • Ribosome size: 70S

    • Chromosome structure: Circular, double-stranded DNA

    • Plasmids present: Yes, circular dsDNA

    • Capsules or slime layers: Common

    • Fimbriae, pili: Common, including unique structures.

    • Motility structure: Archaellum

    • Cytoskeleton: Multiple homologues of tubulin and actin present.

    • Extracellular vesicles: Yes.

Archaeal Shape, Arrangement, and Size

• Common cell shapes of archaea include cocci (spherical) and rods (bacilli).

• Other shapes can also exist, but no spirochete-like or mycelial forms have been documented.

• Unique branched and flat shapes have been observed among archaeal species.

• Size Variability:

  • Rods: Typically range from 1 to 2 μm wide and 1 to 5 μm long.

  • Cocci: Generally range from 1 to 3 μm in diameter.

Archaeal Cell Morphology and Sizes

• Smallest observed archaeal cell measures approximately 0.2 μm in diameter (Candidatus Nanoclepta minutus).

• Largest known archaeal form can reach 30 mm in length, exhibiting multicellular characteristics and filamentous structures.

Archaeal Cell Envelopes

• Differences from bacterial cell envelopes include distinct molecular makeup and organizational structure.

  • Plasma membranes are composed of unique lipids, which may form a monolayer or bilayer.

  • The S-layer (surface layer) may be the only component external to the plasma membrane.

  • Some archaea lack a cell wall entirely.

  • Slime layers observed in some archaea have roles in mediating cell-cell interactions; however, the composition and regulation of these layers are not well understood.

Archaeal Membrane Lipids

• Archaeal membrane lipids differ from those of Bacteria and Eukarya in two key ways:

  • They feature isoprenoid chains as opposed to fatty acids, which influences membrane permeability and fluidity.

  • Archaeal lipids have hydrophobic chains connected to glycerol via ether linkages, which are more resilient to chemical attack and heat compared to ester linkages found in bacteria and eukarya.

• Archaeal membranes contain polar phospholipids, sulfolipids, glycolipids, and unique lipids.

• Despite variations in membrane lipid structure, the overall design of archaeal membranes parallels that of bacterial and eukaryotic membranes, consisting of two hydrophilic surfaces with a central hydrophobic core.

Archaeal Lipids and Membranes

• Two major types of archaeal lipids identified:

  • Glycerol diethers: Typically 20 carbons in length. Form a standard bilayer membrane.

  • Glycerol tetraethers: Typically 40 carbons in length. Form a rigid monolayer membrane.

• Archaea utilize similar transport systems for nutrient uptake as observed in bacteria.

Archaeal Cell Envelope Compositions

• Archaeal cell walls perform essential functions similar to bacterial cell walls:

  • Determine cell shape.

  • Provide osmotic protection.

  • Offer mechanical strength and act as a permeability barrier.

• Molecules used to construct archaeal cell walls differ from bacterial structures; the most common envelope configuration is an S-layer made of numerous copies of a single protein.

• Some archaeal cells may possess an additional protein or carbohydrate layer above, below, or in place of the S-layer.

• A few archaeal species exhibit a double membrane structure.

• A critical distinguishing characteristic is the absence of peptidoglycan in archaeal cell walls.

Archaeal S-Layer Structure

• The S-layer can reach a thickness of 70 nm and is anchored to the plasma membrane.

• Viewed laterally, the S-layer resembles a protein canopy, while seen from above, it appears as a geometric pattern.

• S-layer proteins are often decorated with carbohydrates to enhance stability.

Archaeal Extracellular Vesicles and Nanotubes

• Extracellular vesicles consist of plasma membrane and surrounding cell wall material, or sometimes just the S-layer.

• Contents inside these vesicles can include cytoplasmic materials, proteins, and nucleic acids.

• They are regarded as important for gene transfer within thermophilic archaea, providing protection for DNA against denaturation at high temperatures.

• Reference: Gill S, Catchpole R, Forterre P. "Extracellular membrane vesicles in the three domains of life and beyond." FEMS Microbiol Rev. 2019;43(3):273-303. doi:10.1093/femsre/fuy042.

Archaeal Cells and Nutrient Uptake

• Archaeal cells utilize various nutrient uptake mechanisms similar to those seen in bacteria, including:

  • Passive diffusion and facilitated diffusion.

  • Active transport mechanisms (primary and secondary).

• Notably, some archaea utilize the phosphoenolpyruvate:sugar phosphotransferase system (PTS) for group translocation.

Archaeal versus Bacterial Cytoplasm

• The cytoplasm of archaea is remarkably similar to that of bacteria, characterized by an absence of membrane-enclosed organelles.

• Archaeal cytoplasm may contain inclusion bodies like gas vacuoles.

• Common components include:

  • Ribosomes

  • Nucleoid region

  • Plasmids

• Some structural components may differ between bacterial and archaeal cells.

Ribosomes

• Archaeal ribosomes share the same size as bacterial ribosomes, both measuring 70S, composed of a 50S and 30S subunit.

• However, the composition of archaeal ribosomes is distinct from bacterial ribosomes:

  • The rRNA molecule is similar in size to those in bacteria but differs in nucleotide sequence.

  • The protein composition is different, with archaeal ribosomes possessing a higher number of r-proteins.

  • The unique composition renders archaeal ribosomes unaffected by antibiotics that are effective against bacterial ribosomes.

Nucleoid

• The nucleoid is an irregularly shaped region within the cytoplasm containing the circular chromosome and nucleoid-associated proteins (NAPs).

• This region is generally not membrane-bound, with a few exceptions in specific archaea.

• Evidence of polyploidy (multiple chromosome copies) exists within some archaea.

• Supercoiling and nucleoid-associated proteins contribute to chromosome folding and condensation.

• Histones are involved in organizing the chromosome into nucleosomes, a feature that is similar to eukaryotic systems.

Archaeal External Structures: Pili

• Archaeal pili are primarily constructed from pilin proteins, synthesized in the cytoplasm, and anchored to a protein complex present in the plasma membrane.

• There are two types of archaeal pili:

  • Cannulae:

  • Hollow, tubular structures found on the surface of thermophilic archaea.

  • Observed to keep daughter cells connected post-cell division.

  • Hami:

  • Structural characteristics resemble grappling hooks.

  • Function as adhesion structures for cells, particularly in biofilm communities.

Archaella and Motility

• Archaeal flagella (archaella) are thinner than their bacterial counterparts and may comprise multiple protein types.

• Unlike bacterial flagella, archaeal flagella filament structures are not hollow.

• The motility mechanism is powered by ATP hydrolysis rather than relying on proton motive force.

• Archaeal cells can move in both forward and backward directions, differing from the bacterial movement pattern (runs and tumbles).

• Archaeal swimming motility can achieve exceptionally high speeds.