The domain Archaea, once grouped with bacteria under the name Archaebacteria, represents a distinct and ancient branch on the tree of life. Although archaea are prokaryotes, meaning they lack a membrane-bound nucleus, their cellular architecture exhibits fundamental differences when compared to the domain Bacteria. Analyzing the structure of an archaeon reveals a unique composition of its cell membrane, cell wall, and internal components. A closer inspection of these structures demonstrates why Archaea were designated as their own domain, alongside Bacteria and Eukarya.
Defining the Domain: Prokaryotic Form and Diversity
Archaea are unicellular organisms that share the basic prokaryotic structural plan with bacteria, which involves the absence of internal membrane-bound organelles. They are microscopic, generally measuring a few micrometers in size, similar to many bacteria. Like other prokaryotes, archaeal cells are defined by a cell membrane, a surrounding cell wall or protective layer, and internal cytoplasm containing genetic material.
These organisms display a variety of common shapes, including cocci (spherical), bacilli (rod-shaped), and spirilli (spiral) forms. This structural diversity is a reflection of the wide range of environments these organisms inhabit, from soil and ocean water to extreme conditions like hot springs and highly saline lakes.
The Unique Cell Membrane
The cell membrane of an archaeon is perhaps its most distinguishing structural feature, differing significantly from the membranes of both bacteria and eukaryotes. The lipids in the archaeal membrane utilize ether linkages to connect the hydrocarbon chains to the glycerol head group, a contrast to the ester linkages used by all other life forms. This ether bond is chemically more stable and imparts increased resistance to high temperatures and harsh chemical conditions, a necessity for many archaeal extremophiles.
The hydrocarbon chains themselves are composed of repeating isoprenoid units, which are often branched, rather than the straight-chain fatty acids found in bacterial lipids. This unique chemical composition contributes to the overall stability and rigidity of the membrane. In some Archaea, particularly those living in hyper-hot environments, the lipids form a single, fused monolayer instead of the typical bilayer structure. This monolayer is formed by diglycerol tetraether lipids that span the entire width of the membrane, creating a more rigid and impermeable barrier that helps the cell survive extreme heat.
The Outer Protective Layer
Outside the cell membrane, the archaeal cell is protected by a layer that is structurally diverse but universally lacks the characteristic peptidoglycan found in bacterial cell walls. The absence of this molecule was a primary factor in distinguishing Archaea as a separate domain of life. Many archaea instead use a proteinaceous layer known as the S-layer (Surface layer) as their sole cell wall.
The S-layer is a lattice-like structure composed of interlocking protein or glycoprotein subunits that self-assemble into a highly organized, two-dimensional crystalline array. This structure provides mechanical strength and acts as a selective sieve, protecting the cell from large molecules and particles. In some methanogenic archaea, a structurally distinct polymer called pseudopeptidoglycan, or pseudomurein, is present. This molecule resembles bacterial peptidoglycan in function but substitutes N-acetylmuramic acid with N-acetyltalosaminuronic acid and uses \(\beta\)-1,3 glycosidic bonds instead of \(\beta\)-1,4 linkages, making it resistant to the enzyme lysozyme.
Internal Machinery and Genetic Material
Within the protective cell envelope is the cytoplasm. Like all prokaryotes, archaea lack a membrane-enclosed nucleus, and their genetic material is organized into a nucleoid region. The DNA is typically a single, circular chromosome, though some species may contain additional smaller circular DNA molecules called plasmids.
The ribosomes, which are the cell’s protein synthesis factories, are similar in overall size to bacterial ribosomes, designated as 70S. However, their composition reveals a closer structural relationship to eukaryotes. Specifically, the ribosomal RNA sequences and many of the associated ribosomal proteins share greater similarity with eukaryotic counterparts than with bacterial ones. This structural blend highlights the unique evolutionary position of Archaea, which possess the simple organization of a prokaryote but with a molecular machinery that hints at a shared ancestry with the complex Eukaryotes.