A “Missing Link” – Researchers Shed Light on the Origin of Complex Life Forms

Asgard archaeon Illustration
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Archaeon of Asgard Illustration

Cryo-electron tomography provided insight into the cellular structure of a newly cultured Asgardian archaeon shown here. The broad actin cytoskeleton filaments (orange) in the cell bodies and cell protrusions as well as the unique cell envelope (blue) are striking. Contribution: © Margot Riggi, Animation Lab, University of Utah

Researchers at the University of Vienna and ETH Zurich are developing a “missing link” microorganism.

What led to the emergence of complex organisms on Earth? An important unanswered question in biology. Researchers from Christa Schleper’s team, University of Vienna and Martin Pilhofer’s team ETH Zurich took steps to solve it. Scientists managed to breed a special archaeon and characterize it more precisely by microscopic methods.

This member of the Asgard archaea exhibits unique cellular features and may represent an evolutionary “missing link” of more complex life forms such as animals and plants. The research was recently published in the journalism. Nature.

All life forms on earth are divided into three main domains: eukaryotes, bacteria and archaea. Eukaryotes include groups of animals, plants, and fungi. Its cells are usually much larger and at first glance more complex than bacterial and archaeal cells. For example, the genetic material of eukaryotes is packaged in a cell nucleus, and cells also have numerous other compartments. Cell shape and transport within the eukaryotic cell are also based on an extensive cytoskeleton. So how did the evolutionary leap towards such complex eukaryotic cells take place?

Fusion of an Asgard Archaeon with a Bacteria

One of the currently most popular evolutionary theories postulates that eukaryotes (including animals, plants, and fungi) arose from the fusion of an Asgardian archaeon with a bacterium. Credit: © Florian Wollweber, ETH Zurich

Most existing models assume that archaea and bacteria played a central role in the evolution of eukaryotes. A eukaryotic primitive cell is believed to have evolved from a close symbiosis between archaea and bacteria about two billion years ago. In 2015, genomic studies of deep-sea environmental samples discovered the so-called Asgard archaea group, which represents the closest relatives of eukaryotes in the tree of life. The first images of Asgard cells were released in 2020 by a Japanese group from enrichment cultures.

Asgard archaea grown from marine sediments

Christa Schleper’s working group at the University of Vienna has now, for the first time, succeeded in growing a representative of this group at higher concentrations. She comes from marine sediments on the Piran coast of Slovenia, but is also a resident of Vienna, for example, in coastal sediments of the Danube river. Due to its growth to high cell densities, this representative is particularly well studied. “Getting this highly sensitive organism in stable culture in the laboratory was a very difficult and demanding task,” says Thiago Rodrigues-Oliveira, postdoc in the Archaea working group at the University of Vienna, one of the first authors of the study.

rafael ponce

Co-first author Rafael Ponce samples marine sediment in the Seca Channel in Piran, Slovenia. Credit: © Thiago Rodrigues-Oliveira, Univ. Vienna

Asgard archaea have a complex cell shape with an extensive cytoskeleton.

The remarkable success of the Viennese group in breeding a highly enriched representative of Asgard finally allowed the cells to be studied in more detail by microscopy. ETH researchers in Martin Pilhofer’s group used a modern cryo-electron microscope to take pictures of shock-frozen cells. “This method provides a three-dimensional view of the internal cellular structures,” explains Pilhofer.

Scanning Electron Micrograph of a Lokiarchaeum ossiferum

Scanning electron micrograph of a Lokiarchaeum ossiferum cell showing long and complex cell projections. Credit: © Thiago Rodrigues-Oliveira, Univ. Vienna

“Cells consist of round cell bodies with thin, sometimes very long cell extensions. These tentacle-like structures sometimes seem to connect different cell bodies,” says Florian Wollweber, who has been following the cells for months under the microscope. Cells also contain an extensive network of actin filaments thought to be unique to eukaryotic cells. This suggests that extensive cytoskeletal structures arose in archaea before the first eukaryotes appeared, fueling evolutionary theories around this important and spectacular event in the history of life.

Future predictions through new model organism

“Our new organism, Lokiarchaeum ossiferum, has great potential to provide further groundbreaking insights into the early evolution of eukaryotes,” says microbiologist Christa Schleper. “It took six long years to achieve a stable and highly enriched culture, but we can now use that experience to do many biochemical studies and to breed other Asgard archaea.” In addition, scientists can now use new imaging methods developed at ETH, for example, to investigate close interactions between Asgard archaea and their bacterial partners. Fundamental cell biological processes such as cell division may also be studied in the future to shed light on the evolutionary origins of these mechanisms in eukaryotes.

Reference: “The actin cytoskeleton and complex cell architecture in an Asgard archaea”, by Thiago Rodrigues-Oliveira, Florian Wollweber, Rafael I. Ponce-Toledo, Jingwei Xu, Simon K.-MR Rittmann, Andreas Klingl, Martin Pilhofer, and Christa Schleper, 21 December 2022, Nature.
DOI: 10.1038/s41586-022-05550-y

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