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 ProteobakterienRickettsialesCyanobakterienFirmicutesEuryarchaeotaCrenarchaeotaLokiarchaeotaDiaphoretickesGlaucophytaPlantaeExcavataAmorphaeaFungiAnimaliaArchaeaBakterienEukaryoten
Abb. 2. Zeitskala in Milliarden Jahren vor heute

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Origin of eukaryotes

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The three-domains tree and the Eocyte hypothesis.[1]
 
Phylogenetic tree showing the relationship between the eukaryotes and other forms of life.[2] Eukaryotes are colored red, archaea green and bacteria blue.

The origin of the eukaryotic cell is considered a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6–2.1 billion years ago. Some acritarchs are known from at least 1.65 billion years ago, and the possible alga Grypania has been found as far back as 2.1 billion years ago.[3]

Organized living structures have been found in the black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, dated at 2.1 billion years old. Eukaryotic life could have evolved at that time.[4] Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of a red alga, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.[5]

Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present in these rocks dated at 2.7 billion years old.[6][7]

Relationship to Archaea

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Eukaryotes are more closely related to Archaea than Bacteria, at least in terms of nuclear DNA and genetic machinery, and one controversial idea is to place them with Archaea in the clade Neomura. However, in other respects, such as membrane composition, eukaryotes are similar to Bacteria. Three main explanations for this have been proposed:

  • Eukaryotes resulted from the complete fusion of two or more cells, wherein the cytoplasm formed from a eubacterium, and the nucleus from an archaeon,[8] from a virus,[9][10] or from a pre-cell.[11][12]
  • Eukaryotes developed from Archaea, and acquired their eubacterial characteristics from the proto-mitochondrion.
  • Eukaryotes and Archaea developed separately from a modified eubacterium.

The chronocyte hypothesis for the origin of the eukaryotic cell[13] postulates that a primitive eukaryotic cell was formed by the endosymbiosis of both archaea and bacteria by a third type of cell, termed a chronocyte.

Endomembrane system and mitochondria

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 ProteobakterienRickettsialesCyanobakterienFirmicutesEuryarchaeotaCrenarchaeotaLokiarchaeotaDiaphoretickesGlaucophytaPlantaeExcavataAmorphaeaFungiAnimaliaArchaeaBakterienEukaryoten
Zelle 3

The origins of the endomembrane system and mitochondria are also unclear.[14] The phagotrophic hypothesis proposes that eukaryotic-type membranes lacking a cell wall originated first, with the development of endocytosis, whereas mitochondria were acquired by ingestion as endosymbionts.[15] The syntrophic hypothesis proposes that the proto-eukaryote relied on the proto-mitochondrion for food, and so ultimately grew to surround it. Here the membranes originated after the engulfment of the mitochondrion, in part thanks to mitochondrial genes (the hydrogen hypothesis is one particular version).[16]

In a study using genomes to construct supertrees, Pisani et al. (2007) suggest that, along with evidence that there was never a mitochondrion-less eukaryote, eukaryotes evolved from a syntrophy between an archaea closely related to Thermoplasmatales and an α-proteobacterium, likely a symbiosis driven by sulfur or hydrogen. The mitochondrion and its genome is a remnant of the α-proteobacterial endosymbiont.[17]

Hypotheses for the origin of eukaryotes

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Different hypotheses have been proposed as to how eukaryotic cells came into existence. These hypotheses can be classified into two distinct classes – autogenous models and chimeric models.

Autogenous models

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Vorlage:Plain image with caption

Autogenous models propose that a proto-eukaryotic cell containing a nucleus existed first, and later acquired mitochondria.[18] According to this model, a large prokaryote developed invaginations in its plasma membrane in order to obtain enough surface area to service its cytoplasmic volume. As the invaginations differentiated in function, some became separate compartments—giving rise to the endomembrane system, including the endoplasmic reticulum, golgi apparatus, nuclear membrane, and single membrane structures such as lysosomes.[19] Mitochondria are proposed to come from the endosymbiosis of an aerobic proteobacterium, and it's assumed that all the eukaryotic lineages that did not acquire mitochondria went extinct.[20] Chloroplasts came about from another endosymbiotic event involving cyanobacteria. Since all eukaryotes have mitochondria, but not all have chloroplasts, mitochondria are thought to have come first. This is the serial endosymbiosis theory.

Some models propose that the origins of double layered organelles, such as mitochondria and chloroplasts, in the proto-eukaryotic cell is due to the compartmentalization of DNA vesicles that were formed from the secondary invaginations or more detailed infoldings of cellular membrane.Vorlage:Citation needed

Chimeric models

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Chimeric models claim that two prokaryotic cells existed initially – an archaeon and a bacterium. These cells underwent a merging process, either by a physical fusion or by endosymbiosis, thereby leading to the formation of a eukaryotic cell. Within these chimeric models, some studies further claim that mitochondria originated from a bacterial ancestor while others emphasize the role of endosymbiotic processes behind the origin of mitochondria.

Based on the process of mutualistic symbiosis, the hypotheses can be categorized as – the serial endosymbiotic theory (SET),[21][22][23] the hydrogen hypothesis (mostly a process of symbiosis where hydrogen transfer takes place among different species),[24] and the syntrophy hypothesis.[25][26]

According to serial endosymbiotic theory (championed by Dr. Lynn Margulis), a union between a motile anaerobic bacterium (like Spirochaeta) and a thermoacidophilic crenarchaeon (like Thermoplasma which is sulfidogenic in nature) gave rise to the present day eukaryotes. This union established a motile organism capable of living in the already existing acidic and sulfurous waters. Oxygen is known to cause toxicity to organisms that lack the required metabolic machinery. Thus, the archaeon provided the bacterium with a highly beneficial reduced environment (sulfur and sulfate were reduced to sulfide). In microaerophilic conditions, oxygen was reduced to water thereby creating a mutual benefit platform. The bacterium on the other hand, contributed the necessary fermentation products and electron acceptors along with its motility feature to the archaeon thereby gaining a swimming motility for the organism. From a consortium of bacterial and archaeal DNA originated the nuclear genome of eukaryotic cells. Spirochetes gave rise to the motile features of eukaryotic cells. Endosymbiotic unifications of the ancestors of alpha-proteobacteria and cyanobacteria, led to the origin of mitochondria and plastids respectively. For example, Thiodendron has been known to have originated via an ectosymbiotic process based on a similar syntrophy of sulfur existing between the two types of bacteria – Desulphobacter and Spirochaeta. However, such an association based on motile symbiosis have never been observed practically. Also there is no evidence of archaeans and spirochetes adapting to intense acid-based environments.[27]

In the hydrogen hypothesis, the symbiotic linkage of an anaerobic and autotrophic methanogenic archaeon (host) with an alpha-proteobacterium (the symbiont) gave rise to the eukaryotes. The host utilized hydrogen (H2) and carbon dioxide (Vorlage:CO2) to produce methane while the symbiont, capable of aerobic respiration, expelled H2 and Vorlage:CO2 as byproducts of anaerobic fermentation process. The host's methanogenic environment worked as a sink for H2, which resulted in heightened bacterial fermentation. Endosymbiotic gene transfer (EGT) acted as a catalyst for the host to acquire the symbionts' carbohydrate metabolism and turn heterotrophic in nature. Subsequently, the host's methane forming capability was lost. Thus, the origins of the heterotrophic organelle (symbiont) are identical to the origins of the eukaryotic lineage. In this hypothesis, the presence of H2 represents the selective force that forged eukaryotes out of prokaryotes.

The syntrophy hypothesis was developed in contrast to the hydrogen hypothesis and proposes the existence of two symbiotic events. According to this theory, eukaryogenesis (i.e. origin of eukaryotic cells) occurred based on metabolic symbiosis (syntrophy) between a methanogenic archaeon and a delta-proteobacterium. This syntrophic symbiosis was initially facilitated by H2 transfer between different species under anaerobic environments. In earlier stages, an alpha-proteobacterium became a member of this integration, and later developed into the mitochondrion. Gene transfer from a delta-proteobacterium to an archaeon led to the methanogenic archaeon developing into a nucleus. The archaeon constituted the genetic apparatus while the delta-proteobacterium contributed towards the cytoplasmic features. This theory incorporates two selective forces that were needed to be considered during the time of nucleus evolution – (a) presence of metabolic partitioning in order to avoid the harmful effects of the co-existence of anabolic and catabolic cellular pathways, and (b) prevention of abnormal biosynthesis of proteins that occur due to a vast spread of introns in the archaeal genes after acquiring the mitochondrion and the loss of methanogenesis.

Thus, the origin of eukaryotes by endosymbiotic processes has been broadly recognized and accepted so far. Mitochondria and plastids have been known to originate from a bacterial ancestor during parallel adaptation to anaerobiosis. However, there still remains a greater need in assessing the question of how much eukaryotic complexity is being originated via an implementation of these symbiogenetic theories.

Forterre 2015[28]

Archaea, Bacteria and Eukarya, since they originated by cell division from LUCA. (??)

The editors of research topic on “archaeal cell envelopes and surface structures” gave me the challenging task of drawing an updated version of the universal tree of life.

These observations suggest that thermal adaptation from LUCA to the ancestors of Archaea and Bacteria took place from cold to hot and not the other way around.

The tree is rooted between Bacteria and Arkarya, a new name proposed for the clade grouping Archaea and Eukarya.

a detailed tree of the domain Archaea, proposing the sub-phylum neo-Euryarchaeota for the monophyletic group of euryarchaeota containing DNA gyrase.


    Kasie Raymann, Céline Brochier-Armanet, and Simonetta Gribaldo 

The two-domain tree of life is linked to a new root for the Archaea PNAS 2015 112 (21) 6670-6675; published ahead of print May 11, 2015, doi:10.1073/pnas.1420858112

Methanogene der Klasse I und III

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Taxonomie

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2016 [30]

e--
Donor
e--
Acceptor
End-
produkt
Typ Beispiel
CO O2 CO2 CO verwertende Bakterien Carboxydothermus hydrogenoformans
Fe2+ O2 Fe3+ Eisenoxidierende Mikroorganismen [31] Sulfolobus acidocaldarius, Acidithiobacillus ferrooxidans
H2 O2 H2O Knallgasbakterien[32][33] Cupriavidus metallidurans, Aquifex aeolicus
H2 CO2 CH4 Methanbildner Archaea
H2 SO42− H2S H2 nutzende Desulfurikanten[32] Desulfobacteraceae
HPO32− SO42− HPO42− + H2S δ-Proteobacteria Desulfotignum phosphitoxidans[34]
NH3 O2 NO2- Ammoniakoxidierer[35] Nitrosomonas
NH3 NO2- N2 Anammox-Bakterien[36] Planctomycetes
NO2- O2 NO3- Nitritoxidierer[37] Nitrobacter
S0 O2 SO42− Schwefeloxidierende Bakterien Chemotrophe Rhodobacteraceae
Thiotrichales und Acidithiobacillus thiooxidans
S0 NO3- SO42− Schwefeloxidierende Bakterien[38] Thiobacillus denitrificans
S2− O2 S0 chemotrophe Schwefelpurpurbakterien Halothiobacillaceae
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