An Asgard archaeon could provide the key to understanding the birth of the eukaryotic cell

An article published in the journal “Nature” reports a study on an archaeon called Candidatus Prometheoarchaeum syntrophicum, part of the proposed phylum Lokiarchaeota. A team of researchers took samples from the bottom of the Pacific Ocean near the Japanese coasts, managing to cultivate these Archaea contained in a special laboratory environment specifically created. Years of studies made it possible to separate various strains and to discover that some have long and branched protrusions, a feature that led the researchers to suggest that in the past a bacterium became entangled in similar protrusions becoming an organelle of what became over time a eukaryotic cell.

The progress of genetic research is leading to great discoveries among microorganisms with very interesting studies also concerning Archaea, organisms made up of small and simple cells. The theory that they’re direct descendants of the first cellular organisms that existed on Earth started having some confirmation with the discovery of Archaea with characteristics found only in eukaryotes. An article published in the journal “Nature” in January 2017 reported the discovery of groups of Archaea with those characteristics, whose names were inspired by Norse mythology.

The proposal to approve the creation of a kingdom called Proteoarchaeota which would include the phyla of Archaea with typical eukaryotic characteristics is now under discussion. This kingdom includes several phyla, including those of the proposed Asgard superphylum, the ones that in recent years are at the center of research on the birth of eukaryotic cells. The archaeon Candidatus Prometheoarchaeum syntrophicum belongs to the phylum Lokiarchaeota, nicknamed Loki, where Candidadum means that initially it wasn’t possible to cultivate it to analyze it. After years of trying, a strain cataloged as MK-D1 was successfully cultivated in an environment created specifically in the laboratory, allowing to study its characteristics.

The researchers found that Prometheoarchaeum syntrophicum only grows in the presence of two other microorganisms: another archaeon called Methanogenium and a bacterium of the genus Halodesulfovibrio. The hydrogen produced by Prometheoarchaeum syntrophicum’s metabolism is used by the other two microbes in a symbiosis.

The image (Courtesy Nature) shows electron microscopic images of the MK-D1 archaeon (a-c), a cryoelectronic tomography of MK-D1 (d), a tomography of the vesicle attached to MK-D1 (e), an ultra-thin section of a cell and vesicle (f), images of MK-D1 cells producing protrusions (g, h), an ultra-thin section of MK-D1 with protrusions (i) and chromatography-mass spectrometry chromatography for lipids extracted from a highly purified culture of MK-D1 (j).

The study of the MK-D1 archaeon produced interesting observations, made under the electron microscope because they grow up to a diameter of about 550 nanometers. These microorganisms have long, branching protrusions that prompted researchers to suggest that a couple of billion years ago a bacterium got caught in the protrusions of an archaeon with similar characteristics. A relationship that perhaps began as a symbiosis became closer because the bacterium ended up being incorporated into the archaeon becoming its organelle and this led to the birth of the eukaryotes.

It’s possible that similar Archaea lived before oxygen was present in quantity in the Earth’s atmosphere thanks to the symbiosis with other microorganisms. When the oxygen increased, it could have started a new symbiosis with bacteria that used oxygen obtaining an evolutionary success which, with the absorption of symbiotes transformed into organelles, led to the eukaryotic cell we know today.

It’s impossible to know how different the Archaea that existed 3 billion years ago were from the Asgard Archaea existing today. This means that scientists can study the possible symbiosis for these Archaea without any certainty that it’s similar to past ones. However, these are important steps to understand the history of the Archaea and the possible stages of the birth of ecukaryotic cells.

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