The Asgard archaea are believed to be the closest relatives of the eukaryotes. In their genomes, numerous eukaryotic signature proteins (ESPs) have led to theories about how eukaryotic cells evolved. Although never proven, ESPs may play a role in the development of intricate cytoskeletons and complicated cellular structures.

A collaboration between the working groups of Christa Schleper at the University of Vienna and Martin Pilhofer at ETH Zurich shed light on the origins of Earth’s complex organisms. Scientists have successfully cultivated a special archaeon and more accurately characterized it using microscopic methods. This member of the Asgard archaea shows distinct cellular features and could serve as an evolutionary “missing link” to more complex living forms such as mammals and plants.

Most current theories assume that archaea and bacteria were crucial in the development of eukaryotes. It is believed that a close relationship between archaea and bacteria led to the evolution of the first eukaryotic primordial cell about two billion years ago. In 2015, the so-called “Asgard archaea”, which represent the closest ancestors of eukaryotes in the tree of life, were found through genomic analyzes of deep-sea environmental samples. A Japanese study revealed the first pictures of Asgard cells in 2020 using enrichment cultures.

Scanning electron micrograph
Scanning electron micrograph of a Lokiarchaeum ossiferum cell showing the long and complex cell protrusions. (© Thiago Rodrigues-Oliveira, Univ. Wien)

Scientists have now succeeded for the first time in growing a representative of this group in higher concentrations. For example, it originates from Vienna, in the Danube riparian sediments, and it originates from marine sediments on the coast of Piran, Slovenia. The ability to grow to high cell densities allows for particularly good study of this sample.

Thiago Rodrigues-Oliveira, a postdoc in the Archaea Working Group at the University of Vienna and one of the study’s first authors, said: “It was very difficult and labor intensive to get this highly sensitive organism in a stable culture in the laboratory.”

Pilhofer said, “Our remarkable success in culturing a highly enriched Asgard representative finally enabled a more detailed examination of the cells by microscopy. We used a modern cryo-electron microscope to take pictures of cells frozen in shock.”

One of the currently most popular theories of evolution assumes that eukaryotes
One of the currently most popular theories of evolution assumes that eukaryotes (including animals, plants, and fungi) arose from the fusion of an Asgard archaeon with a bacterium. (© Florian Wollweber, ETH Zurich)

“This method allows a three-dimensional understanding of the internal cellular structures.”

Florian Wollweber, who spent months tracking the cells under the microscope, said: “The cells consist of round cell bodies with thin, sometimes very long cell elongations. These tentacle-like structures sometimes seem to connect different cell bodies.”

“The cells also contain an extensive network of actin filaments that are believed to be unique to eukaryotic cells. This suggests that extensive cytoskeletal structures in archaea arose before the appearance of the first eukaryotes and fueled evolutionary theories surrounding this important and spectacular event in the history of the to live.”

Microbiologist Christa Schleper said: “Our new organism, called ‘Lokiarchaeum ossiferum’, has great potential to provide further groundbreaking insights into the early evolution of eukaryotes. It has taken six years to obtain a stable and highly enriched culture. Yet we can use this experience now use it to conduct many biochemical studies and also cultivate other Asgard archaea.”

Magazine reference:

  1. Rodrigues-Oliveira, T., Wollweber, F., Ponce-Toledo, RI et al. Actin cytoskeleton and complex cell architecture in an Asgard archaeon. Nature (2022). DOI: 10.1038/s41586-022-05550-y