Crenarchaeota is a highly diverse class of aquatic Archaea that are thought to be the most abundant microorganisms in the oceans. Although they vary greatly in morphology and growth requirements, the majority of Crenarchaeota are hyperthermophiles, with some species, such as those in the genus Pyrolobus, capable of growing at temperatures up to $${}113$$ °C.

Crenarchaeota

Euryarchaeota is a phylum within the domain Archaea that includes diverse classes of halophilic and methanogenic organisms, such as Halobacteria and various methanogens.

Euryarchaeota

- Crenarchaeota
- Euryarchaeota
- Korarchaeota
- Nanoarchaeota
- Thaumarchaeota

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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.

Archaea

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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.

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