- Crenarchaeota
- Euryarchaeota
- Korarchaeota
- Nanoarchaeota
- Thaumarchaeota

Major Phylum of Archaea

The cell membrane of organisms in the domain Archaea differs from that of bacteria and eukaryotes in three key ways:
1. The membrane phospholipids contain ether linkages, rather than ester linkages.
2. The lipid chains are branched isoprene chains, rather than straight-chained fatty acids.
3. The membrane can exist as a lipid monolayer instead of a lipid bilayer.

Archaeal Plasma Membrane Structure

Archaeal cell wall structure differs from bacterial cell walls primarily because archaea do not contain peptidoglycan. Instead, they often contain a similar polymer called pseudopeptidoglycan (or pseudomurein). Alternatively, some archaea possess a layer of glycoproteins or polysaccharides that serves as the cell wall, and a few archaeal species appear to lack cell walls entirely.

Archaeal Cell Walls vs. Bacterial Cell Walls

Eukaryotic cells contain multiple, linear chromosomes that are haploid or diploid and are wrapped around histone proteins. In contrast, prokaryotes typically possess a single, circular chromosome that is haploid, with bacteria lacking histones and archaea containing histones.

Comparison of Eukaryotic and Prokaryotic Chromosomes

The genomes of organisms in the domain Archaea are typically larger and more complex than those of bacteria, which presents difficulties in their taxonomic classification.

Archaeal Genomes

Methanogenesis is a unique metabolic process in which methane is produced. It is performed exclusively by certain members of the domain Archaea and is absent in both bacteria and eukaryotes.

Methanogenesis

Although archaea inhabit human and animal microbiomes—such as certain methanogens in the human gastrointestinal tract—they are not known to cause infectious diseases in humans, animals, plants, bacteria, or other archaea. Despite ongoing speculation, scientists have yet to find definitive evidence explaining why no members of the domain Archaea appear to be pathogenic.

Archaea in Human Microbiota and Disease

Fossil evidence indicates that some members of the domain Archaea are among the oldest organisms to have existed on Earth.

Fossil Evidence of Archaea

Archaea is a domain of unicellular prokaryotes that exhibit a diversity comparable to bacteria and are found across all habitats. While some archaea are mesophiles that inhabit moderate environments, many are extremophiles that thrive in harsh conditions such as extreme temperature or salinity. Structurally, they differ from bacteria in their cell wall and membrane composition, and their genomes are typically larger and more complex. Unlike bacteria and eukaryotes, some archaea are capable of performing methanogenesis.

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A prokaryotic cell is a major category of cell that lacks a nucleus surrounded by a complex nuclear membrane. Instead, its genetic material is generally a single, circular chromosome located in a nucleoid region. While prokaryotic cells generally lack complex membrane-bound organelles, they often contain inclusions that compartmentalize their cytoplasm. Microorganisms composed of these cells are classified within the domains Archaea and Bacteria.

Prokaryotic Cell

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Bacteria are prokaryotic microorganisms found in nearly every habitat on Earth, including within and on the human body. As prokaryotes, their genetic material (DNA) is not housed within a true nucleus. Most bacterial cells feature cell walls containing peptidoglycan. While many bacteria are harmless or beneficial, some act as pathogens and cause diseases in humans and other animals.

Bacteria

Photosynthesis is the process of using the sun's light as energy and converting it to chemical energy to fix gaseous carbon dioxide and create sugars such as glucose. The first organisms with the capability to photosynthesize stemmed from prokaryotes 3.8 billion years ago. What is required for photosynthesis include a light source, carbon, electron donor, and pigments to harvest energy.

Photosynthesis

Bacteria is a very diverse domain, characterized by their prokaryotic cells and lack of ether linkages, with membranes made of phospholipid bilayers.

Bacteria (domain)

Archaea

The cell envelope is a collective term for the structural layers that enclose the cytoplasm and internal components of a cell. In prokaryotic organisms, the precise composition of the cell envelope varies depending on the species and cell type, but it universally includes a plasma membrane and, in the vast majority of cases, a surrounding cell wall.

Cell Envelope

Prokaryotes are found virtually everywhere on Earth, demonstrating extreme adaptability. They reside in diverse and harsh environments, including boiling hot springs, the Antarctic ice shield, and deep-sea regions under intense hydrostatic pressure.

Ubiquity of Prokaryotes

Prokaryotes are highly abundant in the atmosphere and can be found even at high altitudes in the air. The atmosphere can contain up to 2,000 distinct types of bacteria, showing a level of bacterial diversity comparable to that found in soil.

Abundance and Diversity of Airborne Bacteria

Prokaryotes typically reside in structured communities where they interact both among themselves and with larger organisms, such as humans, that they utilize as hosts.

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