The “missing link” – researchers shed light on the origin of complex life forms

Researchers at the University of Vienna and ETH Zurich are cultivating a ‘missing link’ microorganism.

What led to the emergence of complex organisms on Earth? It is an important unanswered question in biology. Researchers from Christa Schleper’s team at the University of Vienna and Martin Pilhofer’s team at ETH Zurich have taken a step towards solving it. The scientists succeeded in cultivating a particular trace and characterizing it more precisely using microscopic methods.

This member of the Asgard archaea exhibits unique cellular characteristics and may represent an evolutionary “missing link” to more complex life forms such as animals and plants. The study was recently published in the journal nature.

All forms of life on Earth are divided into three main domains: eukaryotes, bacteria and archaea. Eukaryotes include groups of animals, plants, and fungi. Their cells are usually much larger and more complex at first glance than the cells of bacteria and archaea. For example, the genetic material of eukaryotes is encapsulated in the cell nucleus, and cells also contain a large number of other parts. Cell shape and transport within the eukaryotic cell also depend on an extensive cytoskeleton. But how did the evolutionary leap for such complex eukaryotic cells happen?

One of the most popular evolutionary theories currently posits that eukaryotes (including animals, plants, and fungi) arose from the fusion of Archon, Asgard, and bacteria. Image source: Florian Wollweber, ETH Zürich

Most current models assume that archaea and bacteria played a major role in the evolution of eukaryotes. The prokaryotic eukaryotic cell is thought to have evolved from a close symbiosis between archaea and bacteria about 2 billion years ago. In 2015, genomic studies of deep-sea environmental samples discovered a group of so-called Asgard archaea, which on the tree of life represent the closest relatives of eukaryotes. The first images of Asgard cells from enrichment cultures were published in 2020 by a Japanese group.

Asgard archaea cultivated from marine sediments

Christa Schleper’s work group at the University of Vienna has now succeeded for the first time in creating a representative of this group in even higher concentrations. It comes from marine sediments on the coast of Piran, Slovenia, but is also native to Vienna, for example in Danube bank sediments. As they grow to a high cell density, this representative can be studied particularly well. “It has been very difficult and arduous to obtain this highly susceptible organism in a stable culture in the laboratory,” says Tiago Rodriguez-Oliviera, a postdoctoral researcher in the Archaea work group at the University of Vienna and one of the first authors of the study.

First co-author Rafael Ponce sampled marine sediments 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 Vienna group’s remarkable success in cultivating a highly enriched Asgard representative finally allowed for a more detailed examination of the cells by means of microscopy. ETH researchers in Martin Pilhofer’s group used a state-of-the-art cryoelectron microscope to take images of shock-frozen cells. “This method allows a 3D view into the inner cellular structures,” explains Pilhofer.

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

Cells consist of round cell bodies with thin and sometimes very long cell extensions. “It seems that sometimes these tentacle-like structures connect different cell bodies to each other,” says Florian Wollweber, who has spent months tracking the cells under a microscope. The cells also contain an extensive network of actin filaments that is thought to be unique to eukaryotic cells. This indicates that extensive cytoskeletal structures arose in archaea before the appearance of the first eukaryotes and informs evolutionary theories about this important and fascinating event in the history of life.

Future insights through the new model organism

“Our new organism, called Lokiarchaeum ossiferum, has great potential to provide further groundbreaking insights into the early evolution of eukaryotes,” commented microbiologist Krista Schlipper. “It took six long years to get a stable, highly enriched culture, but we can now use this experiment to carry out many biochemical studies and to grow other Asgard archaea as well.” In addition, scientists can now use the new imaging methods developed at ETH to investigate, for example, the close interactions between Asgard archaea and their bacterial partners. Basic cell biological processes such as cell division can also be studied in the future in order to shed light on the evolutionary origin of these mechanisms in eukaryotes.

Reference: “Actin cytoskeleton and complex cytoarchitecture in Archon Asgard” by Tiago Rodriguez-Oliviera, Florian Wollweber, Rafael I. Ponce Toledo, Jingwei Zhou, Simon K.-MR Reitmann, Andreas Klingel, Martin Bielhofer and Krista Schlieber, 21 Dec. 2022, Available here. nature.
DOI: 10.1038/s41586-022-05550-y