What Are Archaea and How Do They Differ from Bacteria?

Updated

Archaea are a distinct domain of single-celled microorganisms, separate from bacteria and eukaryotes. They differ from bacteria in cell wall structure, membrane lipids, and ribosomal RNA, and they include groups such as methanogens, halophiles, and thermophiles. Use this explainer if you need a working understanding of where archaea fit in the tree of life and why they are not just "extreme bacteria."

The three-domain framework

Life is commonly divided into three domains: Bacteria, Archaea, and Eukarya (eukaryotes). Archaea were once classified as bacteria, but molecular evidence—especially comparisons of ribosomal RNA and other conserved genes—showed they form their own lineage. In many phylogenies, archaea and eukaryotes share a more recent common ancestor with each other than either does with bacteria.

Key differences from bacteria

Feature Archaea Bacteria
Cell wall Often lacks peptidoglycan; some have S-layer or pseudopeptidoglycan Typically contains peptidoglycan
Membrane lipids Ether-linked, often branched isoprenoid chains Ester-linked, usually unbranched fatty acids
Ribosomal RNA Distinct archaeal signature sequences Distinct bacterial signature sequences
DNA packaging Histone-like proteins in many groups Different nucleoid-associated proteins

These differences are not cosmetic. The ether-linked membranes of many archaea are more stable at high temperatures and in extreme salt or acid, which helps explain why some archaea thrive where bacteria cannot.

Major groups and where they live

  • Methanogens produce methane in anaerobic environments such as wetlands, rice paddies, and the guts of ruminants. They are not confined to extremes.
  • Halophiles require high salt concentrations and can dominate salt lakes and solar salterns.
  • Thermophiles and hyperthermophiles live in hot springs, hydrothermal vents, and other high-temperature settings.
  • Ammonia-oxidizing archaea are abundant in soils and oceans and play a role in the nitrogen cycle.

The idea that archaea are only extremophiles is a misconception. Archaea are widespread in ordinary soils, oceans, and even the human microbiome.

Why archaea matter

Archaea drive biogeochemical cycles, including methane production and nitrogen oxidation. They also sit at the center of a leading theory for the origin of eukaryotes: the idea that an archaeal host cell engulfed a bacterial symbiont that became the mitochondrion. This makes archaea relevant not only to microbiology but to questions about how complex life arose.

Common misconceptions

  • "Archaea are a type of bacteria." No—they are a separate domain.
  • "Archaea only live in extreme places." Many are common in moderate environments.
  • "Archaea are always harmful." Most are not pathogens; no archaeal pathogens of humans are well established.

If you need archaea images

PhyloPic is an open database of free silhouette images of animals, plants, and other life forms, available for reuse under Creative Commons licenses. Its popular taxonomic groups include bacteria, viruses, plants, fungi, and animals, and the site notes 13,038 silhouette images in the database. Anyone may upload silhouettes, and software engineers may access the data using the API. If you need a silhouette for an educational diagram or research figure, check the license on the specific image before reuse.