Archaea versus bacteria: identify the unifying feature that distinguishes the archaeal lineage Which characteristic most reliably distinguishes the archaea from the bacteria across the domain, based on core cell envelope chemistry?

Introduction / Context:
Distinguishing archaea from bacteria is central to modern microbiology. Although both are prokaryotes, fundamental biochemical differences separate these domains. The most consistent and diagnostic distinction lies in the chemistry and chirality of membrane lipids rather than in habitat or ribosome sedimentation alone.

Given Data / Assumptions:

• Archaea and bacteria both lack a nuclear membrane and have 70S ribosomes.
• Extreme habitats are common for many archaea, but not universal.
• Membrane lipid chemistry differs markedly between the two domains.
• Bacterial outer membranes (in Gram-negative bacteria) include lipopolysaccharide; archaea lack LPS.

Concept / Approach:

Archaeal membranes are built from ether-linked isoprenoid chains attached to glycerol-1-phosphate (G1P), often forming branched phytanyl or biphytanyl chains. Bacteria (and eukaryotes) use ester-linked fatty acids attached to glycerol-3-phosphate (G3P). This ether linkage and opposite glycerol stereochemistry are domain-defining. Some archaea also possess S-layer or pseudopeptidoglycan cell walls, not peptidoglycan, further differentiating them from bacteria.

Step-by-Step Solution:

Verification / Alternative check:

Comparative biochemistry and phylogeny consolidate archaea as a separate domain largely on molecular grounds (including rRNA phylogeny) and supported by unique lipid chemistry that provides stability in harsh conditions.

Why Other Options Are Wrong:

• Extreme habitats: Many archaea are mesophiles; environment is not a strict discriminator.
• Absence of nuclear membrane: True for both bacteria and archaea.
• 70S ribosomes: Both domains possess 70S ribosomes, though archaeal ribosomes resemble eukaryotic in some components.
• Lipopolysaccharide outer leaflet: This is a bacterial (Gram-negative) feature, not archaeal.

Common Pitfalls:

• Overgeneralizing archaea as exclusively extremophiles.
• Confusing bacterial peptidoglycan and LPS with archaeal surface layers or pseudopeptidoglycan.

Final Answer:

Presence of ether-linked isoprenoid lipids built on glycerol-1-phosphate in the cytoplasmic membrane