Science

Has the 'missing link' microorganism been detected? – Digital Journal


Cryo-electron tomography of a newly cultured Asgard archaeon. (© Margot Riggi, The Animation Lab, University of Utah, for press purposes)

Microbiologists are researching the so-called ‘missing link’ microorganism, with the aim of understanding both microbial and more complex life-forms a little better. This is centred on the question: “How did the complex organisms on Earth arise?” This represents one of the big open questions in biology.

University of Vienna scientists are examining an archaeon and are characterizing it precisely to determine the connection with other cells relating to plants and animals. Although they resemble bacteria microscopically, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes. Eukaryotes include animals, plants, and fungi. These cells are larger and more complex than the cells of bacteria and archaea.

The organism is a member of the so-called Asgard archaea (the proposed phylum Asgardarchaeota) and it exhibits unique cellular characteristics to the extent that it may represent an evolutionary ‘missing link’ to more complex life forms.

The Asgardarchaeota is a proposed superphylum consisting of a group of archaea, based on the theory that eukaryotic cells (from which plants and animals are composed) emerged within the Asgard, in a branch containing the Heimdallarchaeota.

The member of the Asgard archaea exhibits unique cellular characteristics and may represent an evolutionary “missing link”. It has been named Lokiarchaeum ossiferum (a further od to Scandinavia mythology) and it was isolated from marine sediments on the coast of Piran, Slovenia, as well as from bank sediments of the Danube in Austria.

It is theorized that archaea and bacteria played a central role in the evolution of eukaryotes. A eukaryotic primordial cell is believed to have evolved from a close symbiosis between archaea and bacteria about two billion years ago.

Members of the so-called “Asgard archaea” represent the closest relatives of eukaryotes and scientific research as focused on finding potential species that can explain the evolutionary bridge.

The main challenge was with cultivating the organism. This required a highly enriched environment. Once this was completed the researchers could undertake a detailed examination of the cells by cryo-electron microscopy to take pictures of shock-frozen cells. This technique provided a three-dimensional insight into the internal cellular structures.

Of particular interest wee the round cell bodies contained thin, very long cell extensions. These tentacle-like structures sometimes can connect different cell bodies with each other. The cells also contain an extensive network of actin filaments. These were previously thought to be unique to eukaryotic cells.

Hence the images suggest that extensive cytoskeletal structures arose in archaea before the appearance of the first eukaryotes. This supports evolutionary theories around eukaryotic cells evolving from archaeal cells.

The research has been published in the journal Nature, titled “Actin cytoskeleton and complex cell architecture in an Asgard archaeon.”



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