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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Archaea | 4/9 | https://en.wikipedia.org/wiki/Archaea | reference | science, encyclopedia | 2026-05-05T07:15:06.056512+00:00 | kb-cron |
The evolutionary relationship between archaea and eukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two. Complicating factors include claims that the relationship between eukaryotes and the archaeal phylum Thermoproteota is closer than the relationship between the Methanobacteriati and the phylum Thermoproteota and the presence of archaea-like genes in certain bacteria, such as Thermotoga maritima, from horizontal gene transfer. The standard hypothesis states that the ancestor of the eukaryotes diverged early from the Archaea, and that eukaryotes arose through symbiogenesis, the fusion of an archaean and a eubacterium, which formed the mitochondria; this hypothesis explains the genetic similarities between the groups. The eocyte hypothesis instead posits that Eukaryota emerged relatively late from the Archaea. A lineage of archaea discovered in 2015, Lokiarchaeum (of the proposed new phylum "Lokiarchaeota"), named for a hydrothermal vent called Loki's Castle in the Arctic Ocean, was found to be the most closely related to eukaryotes known at that time. It has been called a transitional organism between prokaryotes and eukaryotes. Several sister phyla of "Lokiarchaeota" have since been found ("Thorarchaeota", "Odinarchaeota", "Heimdallarchaeota"), all together comprising a newly proposed supergroup "Asgard". Details of the relation of Promethearchaeati / "Asgard" members and eukaryotes are still under consideration, although, in January 2020, scientists reported that Promethearchaeum syntrophicum, a type of Promethearchaeati / "Asgard" archaea, may be a possible link between simple prokaryotic and complex eukaryotic microorganisms about two billion years ago.
== Morphology == Individual archaea range from 0.1 micrometers (μm) to over 15 μm in diameter, and occur in various shapes, commonly as spheres, rods, spirals or plates. Other morphologies in the Thermoproteota include irregularly shaped lobed cells in Sulfolobus, needle-like filaments that are less than half a micrometer in diameter in Thermofilum, and almost perfectly rectangular rods in Thermoproteus and Pyrobaculum. Archaea in the genus Haloquadratum such as Haloquadratum walsbyi are flat, square specimens that live in hypersaline pools. These unusual shapes are probably maintained by both their cell walls and a prokaryotic cytoskeleton. Proteins related to the cytoskeleton components of other organisms exist in archaea, and filaments form within their cells, but in contrast with other organisms, these cellular structures are poorly understood. In Thermoplasma and Ferroplasma the lack of a cell wall means that the cells have irregular shapes, and can resemble amoebae. Some species form aggregates or filaments of cells up to 200 μm long. These organisms can be prominent in biofilms. Notably, aggregates of Thermococcus coalescens cells fuse together in culture, forming single giant cells. Archaea in the genus Pyrodictium produce an elaborate multicell colony involving arrays of long, thin hollow tubes called cannulae that stick out from the cells' surfaces and connect them into a dense bush-like agglomeration. The function of these cannulae is not settled, but they may allow communication or nutrient exchange with neighbors. Multi-species colonies exist, such as the "string-of-pearls" community that was discovered in 2001 in a German swamp. Round whitish colonies of a novel Methanobacteriati species are spaced along thin filaments that can range up to 15 centimetres (5.9 in) long; these filaments are made of a particular bacteria species.
== Structure, composition development, and operation == Archaea and bacteria have generally similar cell structure, but cell composition and organization set the archaea apart. Like bacteria, archaea lack interior membranes and organelles. Like bacteria, the cell membranes of archaea are usually bounded by a cell wall and they swim using one or more flagella. Structurally, archaea are most similar to gram-positive bacteria. Most have a single plasma membrane and cell wall, and lack a periplasmic space; the exception to this general rule is Ignicoccus, which possess a particularly large periplasm that contains membrane-bound vesicles and is enclosed by an outer membrane.
=== Cell wall and archaella ===
Most archaea (but not Thermoplasma and Ferroplasma) possess a cell wall. In most archaea, the wall is assembled from surface-layer proteins, which form an S-layer. An S-layer is a rigid array of protein molecules that cover the outside of the cell (like chain mail). This layer provides both chemical and physical protection, and can prevent macromolecules from contacting the cell membrane. Unlike bacteria, archaea lack peptidoglycan in their cell walls. Methanobacteriales do have cell walls containing pseudopeptidoglycan, which resembles eubacterial peptidoglycan in morphology, function, and physical structure, but pseudopeptidoglycan is distinct in chemical structure; it lacks D-amino acids and N-acetylmuramic acid, substituting the latter with N-Acetyltalosaminuronic acid. Archaeal flagella are known as archaella, that operate like bacterial flagella – their long stalks are driven by rotatory motors at the base. These motors are powered by a proton gradient across the membrane, but archaella are notably different in composition and development. The two types of flagella evolved from different ancestors. The bacterial flagellum shares a common ancestor with the type III secretion system, while archaeal flagella appear to have evolved from bacterial type IV pili. In contrast with the bacterial flagellum, which is hollow and assembled by subunits moving up the central pore to the tip of the flagella, archaeal flagella are synthesized by adding subunits at the base.
=== Membranes ===
Archaeal membranes are made of molecules that are distinctly different from those in all other life forms, showing that archaea are related only distantly to bacteria and eukaryotes. In all organisms, cell membranes are made of molecules known as phospholipids. These molecules possess both a polar part that dissolves in water (the phosphate "head"), and a "greasy" non-polar part that does not (the lipid tail). These dissimilar parts are connected by a glycerol moiety. In water, phospholipids cluster, with the heads facing the water and the tails facing away from it. The major structure in cell membranes is a double layer of these phospholipids, which is called a lipid bilayer. The phospholipids of archaea are unusual in four ways: