Hydrogen hypothesis
Encyclopedia
The hydrogen hypothesis is a model proposed by William F. Martin
and Miklós Müller in 1998 that describes a possible way in which the mitochondrion
arose as an endosymbiont
within a prokaryote
(an archaea
), giving rise to a symbiotic
association of two cells from which the first eukaryotic cell
could have arisen.
According to the hydrogen hypothesis:
The hypothesis differs from many alternative views within the endosymbiotic theory
framework, which suggest that the first eukaryotic cells
evolved a nucleus but lacked mitochondria, the latter arising as a eukaryote engulfed a primitive bacterium that eventually became the mitochondrion.
The hypothesis attaches evolutionary
significance to hydrogenosome
s and provides a rationale for their common ancestry with mitochondria. Hydrogenosomes are anaerobic mitochondria that produce ATP
by, as a rule, converting pyruvate into hydrogen, carbon dioxide and acetate
. Examples from modern biology are known where methanogens cluster around hydrogenosomes within eukaryotic cells. Most theories within the endosymbiotic theory
framework do not address the common ancestry of mitochondria and hydrogenosomes.
The hypothesis provides a straightforward explanation for the observation that eukaryotes are genetic chimeras
with genes of archaeal and eubacterial ancestry. Furthermore, it would imply that archaea and eukarya split after the modern groups of archaea appeared. Most theories within the endosymbiotic theory
framework predict that some eukaryotes never possessed mitochondria. The hydrogen hypothesis predicts that no primitively mitochondrion-lacking eukaryotes ever existed. In the 10 years following the publication of the hydrogen hypothesis, this specific prediction has been tested many times and found to be in agreement with observation.
William F. Martin
William Martin is an American botanist, currently Head of the Institut für Botanik III, Heinrich Heine-Universität, Düsseldorf....
and Miklós Müller in 1998 that describes a possible way in which the mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
arose as an endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
within a prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
(an archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
), giving rise to a symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
association of two cells from which the first eukaryotic cell
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
could have arisen.
According to the hydrogen hypothesis:
- The host that acquired the mitochondrion was a prokaryote, a hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-dependent archaea, possibly similar in physiology to a modern methanogenic archaea which uses hydrogen and carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
to produce methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
; - The future mitochondrion was a facultatively anaerobic eubacterium which produced hydrogen and carbon dioxide as byproducts of anaerobic respirationAnaerobic respirationAnaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...
; - A symbiotic relationship between the two started, based on the host's hydrogen dependence (anaerobic syntrophySyntrophySyntrophy, Cross-feeding, or Cross feeding is the phenomenon that one species lives off the products of another species.For example house dust mites live off human skin flakes, of which a healthy human being produces about 1 gram per day...
).
The hypothesis differs from many alternative views within the endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
framework, which suggest that the first eukaryotic cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
evolved a nucleus but lacked mitochondria, the latter arising as a eukaryote engulfed a primitive bacterium that eventually became the mitochondrion.
The hypothesis attaches evolutionary
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
significance to hydrogenosome
Hydrogenosome
A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
s and provides a rationale for their common ancestry with mitochondria. Hydrogenosomes are anaerobic mitochondria that produce ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
by, as a rule, converting pyruvate into hydrogen, carbon dioxide and acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
. Examples from modern biology are known where methanogens cluster around hydrogenosomes within eukaryotic cells. Most theories within the endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
framework do not address the common ancestry of mitochondria and hydrogenosomes.
The hypothesis provides a straightforward explanation for the observation that eukaryotes are genetic chimeras
Chimera (genetics)
A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...
with genes of archaeal and eubacterial ancestry. Furthermore, it would imply that archaea and eukarya split after the modern groups of archaea appeared. Most theories within the endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
framework predict that some eukaryotes never possessed mitochondria. The hydrogen hypothesis predicts that no primitively mitochondrion-lacking eukaryotes ever existed. In the 10 years following the publication of the hydrogen hypothesis, this specific prediction has been tested many times and found to be in agreement with observation.