Archaea may be the oldest life form. Some peculiar life habits of archaea and the related potential biotechnology development prospects have attracted many people's attention for a long time.Archaea is often found living in various extremesnatural environmentSuch as the high pressure heat overflow at the bottom of the oceanHot spring, saline lakes, etc.
The cell morphology of archaea is spherical, rod-shapedSpiral、EarlobeShaped, discoid, irregular, and multiform, some are thin and flat, some have precise square angles and vertical edges to form a rectangular geometry, some exist as a single cell, some are filamentous oraggregate。Its diameter is generally 0.1~15μm，Filamentous bodyThe length is 200 μ m.
Cellular structure
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The cell structure of archaea is different from that of bacteria, such as archaea cellsadventitiaIt's different from bacteria.
The cell membrane of archaea containslipidUnlike bacteria, the lipid of bacteria isglycerolFatty acid ester, while the lipid of archaea is non saponifiable glycerol diethyl etherphospholipidandGlycolipidsDerivatives of.There are two kinds of cell membranes of archaea: bilayer membrane and monolayer membrane.
Metabolic process
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There are many specialcoenzyme。There are 5 groups of archaea, so their metabolism is diverse.There are heterotrophicAutotrophic typeAnd incompletephotosynthesis3 types.
Archaea[1]
Most archaea are strictly anaerobic, facultative anaerobic, and specialized aerobic.
Reproductive mode
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Archaea utilizationDichotomy, splitting and buddingAsexual reproduction；Archaea will not carry outMeiosisTherefore, a species of archaea with the same gene may have different morphology.The cell division of archaea iscell cycleControlled;After the chromosomes of the cell are duplicated and separated, the cell begins to split in two.Although onlyVulcanized leaf fungusOfCopy cycleIt has been clarified, but such cell cycles are generally similar to those in bacteria and eukaryotes.Like eukaryotes, archaea chromosomes can be located at multiple sites（Copy Start）Start withDNA PolymeraseCopy.However, the proteins used by archaea to control cell division (such as FtsZ, which produces a gradually shrinking "Z ring" to help cytoplasm division) and the membrane part to separate two daughter cells are similar to bacterial dichotomy.
But unlike bacteria and eukaryotes,Not foundWith archaeaSporogenesis。Some halophilic bacteria can undergo phenotypic transformation and grow into different forms.These forms include possession that can preventosmotic pressureThickness of shockcell wallThis allows halophilic bacteria to survive in low salinity water.These archaea features are not reproductive structures, but they can help archaea survive in new environments.
Life habits
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Most archaea live in extreme environments, such as high salinity lake water, extremely hot, extremely acid and absolutely anaerobic environments, and some live in extremely cold environments.
According to 16SrRNASequence analysis, archaea can be classified asQuangu(boundary),Archaea(boundary),Archaea(boundary), andNanoarchaea(Boundary).
With the development and application of microbial molecular ecology technology, a large number of unknown archaea, called non thermophilic Crenarchaeota, which are closely related to spring archaea, exist in the mesophilic environment.With the development of technology, many evidences are found that do not support the hypothesis that Mesothermal Springs Archaea evolved from Thermophilic Springs Archaea.thusExpert adviceThese archaea (Zhongwenquan archaea) are divided into a new phylum and become the third main group of archaea Thaumarchaeota (literally:ZiguDoor).
Quangu
Most Quanarchaea are extremely thermophilic and acidophilic, and metabolize sulfur.Sulfur acts as electron acceptor and electron source of inorganic nutrition in anaerobic respiration.Most of them grow in hot water or soil containing sulfur (such as sulfur rich hot springs in Yellowstone National Park in the United States).
This kind of bacteria can only be usedfluorescence in situ hybridizationTechnical tests confirm its existence, and there are still unculturedmicroorganism（VBNC, viable but nonculturable microorganisms）。
Korarchaeum
Nanoarchaea
In 2005, German scientistsarctic oceanThe oldest, simplest and smallest bacterium among the parasitic archaea was found riding on the fireball nanometerArchaea(Nanoarchaeum equitans), onlyEscherichia coli1/160 of (vsSmallpox virusIt has the same size), 5 million bases (the smallestmycoplasma80000 less).
Nanoarchaeum
Kigula
This kind of fungus is the latest discovered extreme species.
The concept of "archaea" was introduced in 1977 by Carl Worth andGeorge FoxThe reason is that they are in the 16SrRNAOfSystem occurrenceDifferences between trees and other prokaryotes.These two groups of prokaryotes were initially identified asArchaea(Archaebateria) andEubacteria(Eubacterium) two boundaries orSubrealm。Woese thought that they were two fundamentally different creatures, so he renamed them Archaea and Bacteria, which together with Eukarya formed the biologicalThree domain system。
If we compare the earth's age of about 4.6 billion years to one year, then archaea appeared as early as March 20, and the birth of human beings was only on December 31.They mostly grow inextreme environment, such asHot springHigh voltageSubmarine volcanoMouthsalt lakeEtc.
Interest in archaea did not begin in the 1970s.In fact, on our planet, archaea represents the limit of lifebiosphereRange of.For example, an archaea called Pyrobacterium can grow at a temperature of up to 113 ℃.This is the highest biological growth temperature found so far.In recent years, using molecular biology methods, it has been found that archaea is also widely distributed in various natural environments, and there are archaea living in soil, seawater, and marshes.
Although their life habits are quite different, each group of archaea has the same and differentcytologyandbiochemistryfeatures.For example, archaeacell membraneContaining branchesHydrocarbonChain and D-typeGlycerol phosphate, connected by ether bondlipidWhile bacterial and eukaryotic cell membranes containL-typeGlycerol phosphate, a kind of lipid connected by ester bond.Bacterial cell wallThe main components ofPeptidoglycan, and ancientBacterial cellThe wall contains no peptidoglycan.
Interestingly, although similar to bacteria, archaea chromosome DNA is closedannular, genes are also organized intoOperon(Operons are prokaryotesgene expressionAnd the basic structural unit of regulation,biological activityRelated genes often coordinate the opening and closing of gene expression in the form of operons), butDNA replication, transcription, translation and other aspects, archaea has obvious eukaryotic characteristics: non formyl methylthiocarbamidetRNAAsInitial tRNA，Promoter、transcription factor、DNA Polymerase、RNA polymeraseEquivalency andEukaryoteIs similar to.
How many life forms are there on earth?WhenAristotleWhen establishing biology, he used the rule of dichotomy to divide biology into animals and plants.The birth of microscope made people find bacteria invisible to the naked eye.BacteriaCellular structureThe most fundamental difference between animals and plants is that animalsplant cellThere are nuclei in it,genetic materialDNA is mainly stored here, while bacteria have no nuclei, and DNA is free in the cytoplasm.Because the difference between animals and plants is smaller than that between them and bacteria, E. Chatton proposed in 1937BiosphereNew dichotomy, that is, organisms are divided into nuclearEukaryoteAnd non nuclearprokaryote。Animals and plants belong to eukaryotes, while bacteria belong to prokaryotes.
1859DarwinPublication《Origin of species》Later, biologists began to build on evolutionary relationships rather than phenotypesSimilarityOfclassification systemNamely, the so-called phylogenetic classification system.However, due to the lack ofFossil record, thisclassification method It has not been effectively applied to the classification of prokaryotes for a long time.In the 1970s, withmolecular biologyWoods finally made a significant breakthrough in this regard.
In the long process of evolutionBiological cellInInformation molecule（nucleic acidAnd protein).Many information molecule sequence changes are generated randomly in time,Rate of evolutionRelatively constant, that is, it has clock characteristics.Therefore, the genetic relationship between species can be quantitatively described by the sequence difference of a common gene with clock characteristics or its products (such as proteins).These genes or their products become recordsBiological evolutionMolecular chronograph of process（chronometer）。Obviously, this kind of molecular timekeeper recording the phylogenetic process should be widely distributed in all organisms.Based on this consideration, Wuss chose a small subunitribosomeNucleic acid (SSUrRNA）As a molecular timer.This molecule is inside the cellprotein synthesisMachine - a component of ribosome, and protein synthesis is almost all living thingsLife activitiesAn important aspect of.Therefore, it is appropriate to use SSU rRNA molecules as molecular timers.
After comparing differentProkaryoteAfter the similarity of SSU rRNA sequences of eukaryotes, Wuss found that methane originally thought to be bacteriacocciIt represents a kind ofLife forms。Considering theenvironmentIt may be related to thenatural environmentSimilarly, Woese called this kind of creatureArchaea。Accordingly, in 1977, Wuss proposed that organisms can be divided into three major groups, namely eukaryotesEubacteriaAnd archaea.Based on SSU rRNAAnalysis resultsThe phylogenetic (evolutionary) tree of was then born.
Further research shows that the first bifurcation on the evolutionary tree produced a branch of eubacteria and a branch of archaea/eukaryote, and the bifurcation of archaea and eukaryote occurred later.In other words, archaea is closer to eukaryote than eubacteria.
Therefore, in 1990, Wuss proposed three domainstaxonomySaid that the living things are divided into eukaryotesEubacteriaAnd archaea, the domain is defined as higher than the boundaryClassification unit。In order to highlight the difference between archaea and true bacteria, Wuss changed the name of archaea to archaea.Eubacteria is called bacteria.Three domain theoryArchaea obtained the same taxonomic status as eukaryotes and bacteria.
Since its birth, Worth's theory of the three domains has been recognized by some people, especiallymicrobiologyPeople outside the field objected.Opponents insist that Prokaryote andEukaryoteThe distinction between the two is the most fundamental and evolutionary classification rule in the biological world;Compared with eukaryotes with rich and diverse phenotypes, the difference between archaea and bacteria is far from large enough to change the dichotomy.But inMethanococcus jannaschii In the past 20 years before the completion of genome sequencing, phylogenetic studies using multiple molecular timers have repeatedly proved that archaea is a unique speciesLife forms。
Three domain theory
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Despite the above understanding of archaea, people were surprised when they first faced the whole genome sequence of Methanococcus jennificus.There are 1738 genes in Methanococcus Jens, 56% of which have never been seen before!By contrast, the sequencingHaemophilus influenzaeThe unknown genes in the genomes of Haemophilus influenzae and Mycoplasma genitalium only account for about 20%.So people finally realized at the genome level that archaea is a brand newLife forms。
More interestingly, the genes with more or less known functions, which account for 44% of the total genome of Methanococcus zhansi, seem to outline the evolutionary relationship between archaea and other two types of organisms:cell division, metabolism and other aspects are similar to bacteria, while transcription, translation and replication are similar to eukaryotes.In other words, a strange microorganism living in the heat vent at the bottom of the oceaninformation transferIt has genes similar to human (not bacteria in human digestive tract)!While admiring the wonder of life, many people began to cheerThree domain theoryThe final establishment of.United States《science》The weekly magazine listed the genome sequencing of Methanococcus Jens as one of the major scientific breakthroughs in 1996, claiming that this achievement has almost ended the debate around the three domain theory.
New challenges
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Just when the suspense of archaea seemed to be going away, the new discoveries that followed led people back into confusion.Various complete microorganismsgenome sequenceOne by one, they appear in the database that people can look up with a click of the mousegenomeIn the sequence, archaea accounted for 4.More sensitive methods are used to identify the genes (includingMethanococcus jannaschii Genome), and got a surprising result: only 30% (not more than half of the previously estimated) genes in the genome of Methanococcus jandii encode unknown functions, which is similar to the bacterial genome.ArchaeomycoticMysteryAnd uniqueness are reduced.
yesThree domain theoryWhat is more disadvantageous is that those who can speculate the function ofGene product（protein）Medium, 44% haveBacterial proteinFeatures, only 13% of the imagesEukaryoteProtein.In another archaea, thermophilic alkaliMethanobacterium(Metanobacterium thermophilum).Therefore, from the perspective of genome comparison, the difference between archaea and bacteria is far less than that between archaea and eukaryote, which is not enough to convince opponents of the three domain theory.
What is more difficult to understand is that different genes in the same organism are used toPhylogenyLearning orientation often leads to different results.Recently, the genome of a bacterium (Aquifex aeolicus) that can grow at a temperature close to the boiling pointSequencingDone.Phylogenetic studies on several genes of the bacterium showed that ifcell divisionThe regulated protein FtsY acts as a molecular timerEvolutionary treeThe one located on the branch of bacteriaSoil bacterium——Bacillus subtilisclose;If in a way of participationTryptophanThe synthetic enzyme shall prevail, and the bacterium shall belong to archaea;When comparing the synthesis of this bacterium with that of other organismsCytidine triphosphate（DNABasics ofStructural unit(1), it was found that archaea no longer formed an independent group.It seems that different genes are telling different evolutionary stories.Then, archaea can be unique and unifiedLife formsIs it?
Eukaryoticbeer yeastgenome sequenceAfter measurement,Three domain theoryHas encountered a greater crisis.yeastnucleusAmong the genes, there aregenetic relationshipThey are twice as related to archaea.Some people also commented on the 34Protein familyAfter analysis, 17 families were found to originate from bacteria, and only 8 showed the relationship between archaea and eukaryotes.
If the Woese evolutionary tree is correct and the divergence between archaea and eukaryotes in the evolutionary process is later than that between archaea and bacteria, how can we explain these results?
According to the popular research of cell evolutionsymbiosisHypothesis,Eukaryotic cellOrganelle（mitochondrion、chloroplast）The production of the bacteria originated from the symbiotic relationship established between bacteria and proeukaryotes in the early evolution.In this relationship, eukaryotic cells provide stableMicroenvironment，Endosymbiont(Bacteria) provide energy, and over time, the endosymbiont evolves intoOrganelle。Some bacterial genes in eukaryotic nuclei may come from mitochondria, and these few genes usually code for retransmissionmitochondrionProtein molecules.However, many bacteria derivedNuclear geneCode those incytoplasm, rather than the functional protein in mitochondria.So where do these genes come from?obviously,Endosymbiosis hypothesisNot enough to save the Woese evolutionary tree.
But, WorthEvolutionary treeIt will not fall down easily, and there are still many hypotheses supporting it.Recently, someone proposed a new version of“Horizontal gene transfer”Hypothesis.According to this hypothesis, the heterozygous composition of the genome is generated between different lineages during evolutionGene transferCaused by.One kind of organism can acquire the genes of another kind of organism that may have a distant genetic relationship by means of swallowing.Wuss speculated that,Ancestral organismBefore the evolution of bacteria, archaea and eukaryotesMutual exchangeIn the "commune" of genes, organisms from this "prehistoric commune" may have obtained different genetic heritages.All this makes it difficult for the evolutionary treeBranchesClear.However, Worth believes that based on SSUrRNAThe evolutionary tree ofLife formsIt exists.
Keep arguing
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The publication of the genome sequence of Methanococcus Jens three years ago seemed to indicate the end of a debate that lasted more than 20 years about how many life forms there are on earth.Archaea seems to be recognized as the third form of life.Now, just three years later, even the most optimistic people can't predict that WussEvolutionary treeThe fate of.This debate is still ongoing, although the archaeaClassified statusIt was questioned, but the uniqueness of archaea, a life form, was still affirmed to varying degrees.
Archaea research is warming up all over the world, not only because archaea contains more unknown biological processes and functions than the other two kinds of organisms, as well as clues that help to clarify the laws of biological evolution, but also because archaea has an immeasurable prospect of biotechnology development.The archaea has surprised people again and again. It is certain that this unique group of creatures will continue to show people the endless mystery of life in the coming years.
