Definition of technical terms Chinese name: archaea English name: archaea Other names: archaea definition 1: Prokaryotes that often live in extreme environments such as hot springs, anoxic lake bottoms and salt lakes. It has some unique biochemical properties, such as membrane lipids linked by ether bonds instead of ester bonds. There are many similarities with eubacteria in energy production and metabolism, while replication, transcription and translation are closer to eukaryotes. Eukaryotes and eukaryotes may have a common ancestor, which is not commensurate with the ancestors of eubacteria. Discipline: biochemistry and molecular biology (first-class discipline); General introduction (two disciplines) Definition 2: The oldest biota today is a living fossil that lived in the era of primitive atmospheric hypoxia. As a unicellular organism, there is no real nucleus, the chromosome contains histone, and the composition of RNA polymerase is more complicated than that of bacteria. Methionine, as the initial amino acid of protein synthesis in translation, has no peptidoglycan in the cell wall, which is different from eukaryotes, and its ribosomal protein is similar to eukaryotes. Many species live in extremely harsh environments. Together with eukaryotes and prokaryotes, it constitutes three evolutionary lineages of modern organisms. Subject: Cell Biology (first-class subject); General Introduction (two disciplines) This content was approved and published by the National Committee for the Examination and Approval of Scientific and Technical Terminology. In deep-sea craters, terrestrial hot springs, saline-alkali lakes and other places where life can not survive, there are some strange microorganisms that are little known-archaea. Archaea, also known as archaea or protobacteria, are some bacteria that grow in extremely special environments. In the past, archaea was classified as a prokaryote according to its internal structure without nuclear membrane, circular DNA structure and life style similar to that of prokaryote in cell productivity, cell division and metabolism. Chinese name: archaea

Distribution: Deep-sea craters, terrestrial hot springs and saline-alkali lakes.

This catalogue defines the living environment and its morphological evolution and classification. Representing the history of archaea, thermophilic bacteria and halophilic bacteria, archaea, bacteria and eukaryotes are mainly different from true bacteria. Representing the history of archaea, thermophilic bacteria and halophilic bacteria, the definitions, morphological evolution and classification of archaea, bacteria and eukaryotes are mainly different from those of eubacteria.

definition

Archaea (also known as archaea, archaea, archaea's structural nuclear cells or archaea) is a very special kind of bacteria, most of which live in extreme ecological environment. It has some characteristics of prokaryotes, such as nuclear-free membrane and inner membrane system; It also has the characteristics of eukaryotes, such as the synthesis of protein from methionine, the insensitivity of ribosomes to chloramphenicol, the similarity between RNA polymerase and eukaryotic cells, and the combination of DNA with introns and histones. In addition, it also has different characteristics from prokaryotic cells and eukaryotic cells, such as: the lipid in the cell membrane is unsaponifiable; Cell walls contain no peptidoglycan, some are mainly protein, some contain heteropolysaccharide, and some are similar to peptidoglycan, but none contain muramic acid, D- amino acid and diaminopimelic acid.

Living environment and form

Many archaea live in extreme environments. Some people live in extremely high temperatures (usually above 100℃), such as geysers or black chimneys on the seabed. Others live in a very cold environment or in water with high salt, strong acid or strong alkalinity. However, some archaea are neutral and can be found in swamps, wastewater and soil. Many archaea that produce methane live in the digestive tract of animals, such as ruminants, termites or humans. Archaea is usually harmless to other organisms. The diameter of a single archaea cell of unknown pathogenic archaea is between 0. 1 ~ 15 microns, and some species form cell clusters or fibers, which can reach 200 microns in length. They can have various shapes, such as spheres, rods, spirals, leaves or squares. They have many metabolic types. It is worth noting that halobacter can make ATP by using light energy, although archaea can't realize photosynthesis by using electronic chain conduction like other organisms that use light energy.

Edit this paragraph evolution and classification

From the phylogenetic tree of RNA, archaea can be divided into two types, Archaea and Archaea. In addition, these two unidentified species are composed of some environmental samples and the exotic species Nanoarchaeum equitans discovered by Karl Stetter in 2002. Woese believes that bacteria, archaea and eukaryotes all represent the descendants of a distant ancestor with a simple genetic mechanism. This assumption is embodied in the name archaea (archaea in Greek). Then he officially called these three branches three fields, and each field consisted of several fields. This classification has become very popular, but the idea of distant creatures itself has not been generally accepted. Some biologists believe that archaea and eukaryotes originated from specialized bacteria. The relationship between archaea and eukaryotes is still an important issue. In addition to the similarity mentioned above, many other genetic trees also combine the two. In some trees, eukaryotes are closer to archaea than archaea, but biofilm chemistry has the opposite conclusion. However, genes similar to archaea have been found in some bacteria, such as Huanghua, which complicates these relationships. Some people think that eukaryotes originated from the fusion of archaea and bacteria, which became nucleus and cytoplasm respectively. This explains many genetic similarities, but it is difficult to explain the cell structure. At present, 22 archaea genomes have been sequenced, and another 15 genomes are being sequenced.

Representative archaea

Thermophilic bacteria: can grow in high temperature environment above 90℃. For example, archaea discovered by scientists at Stanford University, the optimum growth temperature is 100℃, and it will be inactivated below 80℃. A group of archaea discovered by the German research team K. Stetter in the seabed of Italy can survive at the high temperature above 1 10℃, and the optimum growth temperature is 98℃, and it will stop growing when it drops to 84℃. J. A. Baross of the United States found that some bacteria isolated from the crater can survive at 250℃. Thermophilic bacteria have a wide range of nutrition, most of which are heterotrophic bacteria, and many of them can oxidize sulfur to obtain energy. Extremely halophilic organisms: Living in high salinity environment, the salinity can reach 25%, such as the Dead Sea and salt lakes. Acidophilic bacteria: They can live in an environment with a pH lower than 1, and are often thermophilic bacteria. They live in acidic hot water in volcanic areas and can oxidize sulfur and secrete sulfuric acid as metabolites. Extremely alkalophilic: most people live in saline-alkali lakes or alkaline lakes and ponds, and the pH value of their living environment can reach above 1 1.5, and the optimum pH value is 8 ~ 10. Metnanogens is a strict anaerobic organism, which can utilize CO2 to oxidize H2, produce methane and release energy at the same time. The energy of CO2+4H2→CH4+2H2O+ is similar to that of the early earth, with high temperature and lack of oxygen. And because of the particularity of archaea in structure and metabolism, they may represent the oldest bacteria. They kept their ancient form and broke with other bacteria a long time ago. Therefore, it is proposed to separate archaea from prokaryotes and become a parallel species with prokaryotes (eubacteria) and eukaryotes.

Thermophilic bacteria

Thermophilic bacteria can only grow well at high temperature. Up to now, more than 50 thermophilic bacteria have been isolated. Among these bacteria, one strain (Phyolobous fumarii II) has the highest reproductive rate at 105℃, even at 1 13℃. Extremely thermophilic methanogens in the deep sea have attracted people's attention because they are located near the roots of the evolutionary tree of life. Studying it deeply may help us understand how the earliest cells in the world survived. Some people think that the limit temperature of thermophilic bacteria may be 150℃. If this temperature is exceeded, no matter what life form, the chemical bonds that maintain the integrity of DNA and other important life macromolecules will inevitably be destroyed. Taq enzyme used in PCR was isolated from aquatic T.aquaticus thermophilic bacteria. Recently, a Pfu polymerase was isolated from Thermococcus furiosus to replace Taq enzyme. Pfu polymerase can play its best role at 100℃.

Halophilic bacteria

It can grow and reproduce in extreme salt environment, especially in natural salt lakes and solar evaporation salt ponds. According to the osmotic potential principle, cells in high salt solution will lose more water and become dehydrated cells. Halophilic bacteria can produce a large number of internal solutes or keep obtaining solutes from the outside to maintain their own survival. For example, Halobacterium halophila concentrates high concentration of potassium chloride in its cytoplasm, and one of the enzymes is only active in high concentration of potassium chloride, thus playing its role. However, the protein in the cytoplasm of Halobacterium, which is in contact with environmental salts, needs high concentration of sodium chloride to play its role.

Edit this paragraph history

The concept of archaea was put forward by carl woese and george fox in 1977, because they are different from other prokaryotes in the phylogenetic tree of 16SrRNA. These two groups of prokaryotes were originally defined as archaea and eubacteria. Woese thinks that they are two fundamentally different organisms, so she renamed them archaea and bacteria, and together with eukaryotes, they formed three domain systems of organisms. In the late 1970s, Voss and others used their original technology to analyze the oligonucleotide spectra of 16S (or 18S) ribosomal ribonucleic acid (rRNA) of more than 200 kinds of bacteria and eukaryotes (including some organelles). As a result, organisms are divided into three groups: eukaryotes, eubacteria and archaea. Archaea includes three different bacteria: methanogenic bacteria, extreme halophilic bacteria and acidophilic thermophilic bacteria. They live in an extremely special ecological environment and have a unique 16S ribosomal RNA oligonucleotide spectrum. Moreover, they are different from eukaryotes and eubacteria at the molecular level or only the same as one of them. For example, extreme halophilic bacteria can do it, but their photosynthetic pigments are not chlorophyll molecules, but rhodopsin similar to rhodopsin on animal retina. At first, it was thought that only two kinds of organisms had cell morphology: prokaryotic cells and eukaryotic cells. Since the discovery of bacteria since ancient times, organisms have been divided into the above three categories, which provides new clues for exploring the origin of life and eukaryotic cells.

Archaea, bacteria and eukaryotes

In terms of cell structure and metabolism, archaea is close to other prokaryotes in many aspects. In the two central processes of molecular biology, gene transcription does not obviously show the characteristics of bacteria, but is very close to eukaryotes. For example, the translation of archaea uses eukaryotic initiation and extension factors, and the translation process needs TATA box-binding protein and TFIIB in eukaryotes. Archaea has some other characteristics. Unlike most bacteria, they have only one cell membrane and no peptidoglycan cell wall. Moreover, the lipids in the cell membranes of most bacteria and eukaryotes are mainly composed of glycerides, while the membrane lipids of archaea are composed of glycerol ethers. These differences may be the adaptation to the ultra-high temperature environment. The composition and formation process of archaea flagella are also different from bacteria. The phylogenetic tree based on rRNA sequence shows three different branches: bacteria, archaea and eukaryotes.

The main difference with eubacteria

1. Morphologically, archaea has cells with flat rectangular geometry, which has never been seen in eubacteria. 2. Metametabolism, archaea has unique coenzyme. For example, methanogenic bacteria contain F420, F430, COM and B factors. 3. Many archaea have introns with or without introns. 4. According to the structure and composition of the membrane, the archaea membrane contains ether instead of ester, in which glycerol is connected with long-chain hydrocarbon isoprene through ether bond instead of fatty acid through ester bond. 5. On the respiratory type, strict anaerobic is the main respiratory type of archaea. 6. In terms of metabolic diversity, archaea is relatively simple and not as diverse as eubacteria. 7. In terms of molecular plasticity, archaea has more changes than eubacteria. 8. In terms of evolutionary speed, archaea is slower than eubacteria, and retains its original features.