Cryoelectron tomography provided insight into the cytoskeleton of freshly cultured Asgards shown here. Of note are the broad filaments of the actin cytoskeleton (orange) in the cell bodies and protrusions of the cells, as well as the single cell envelope (blue). Credit: © Margot Riggi, The Animation Lab, University of Utah
Researchers from the University of Vienna and ETH Zurich are cultivating the ‘missing link’ microorganism.
What led to the emergence of complex organisms on Earth? This 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 were able to grow a particular archon and characterize 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 generally much larger and more complex at first glance than those 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. The shape of cells and their transport within the eukaryotic cell also depend on an extensive cytoskeleton. But how did the evolutionary progression towards these complex eukaryotic cells happen?
One of the most popular evolutionary theories today posits that eukaryotes (including animals, plants, and fungi) arose from the fusion of Archaeon, Asgard, and bacteria. Image source: Florian Wollweber, ETH Zürich
Most current models assume that archaea and bacteria played central roles in the evolution of eukaryotes. The primitive eukaryotic cell is thought to have evolved from a close symbiosis between archaea and bacteria about 2 billion years ago. In 2015, genetic studies of deep-sea environmental samples discovered a group of so-called Archaea of Asgard, which on the tree of life represent the closest relatives of eukaryotes. The first images of cells from Asgard were released in 2020 from enrichment cultures by a Japanese group.
Asgard archaea cultured 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 the marine sediments of the Piran coast of Slovenia, but is also a Viennese inhabitant, for example in the sediments of the banks of the Danube. Because they grow at high cell densities, this representative can be studied particularly well. “It was very delicate and arduous to get this highly susceptible organism into a stable culture in the laboratory,” says Tiago Rodriguez-Oliveira, 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 representative of Asgard eventually allowed for a more detailed examination of the cells by means of microscopy. ETH researchers from Martin Pilhofer’s group used a state-of-the-art cryoelectron microscope to take images of shock-frozen cells. “This method allows for a 3D view into the inner cellular structures,” says Pilhofer.
Scanning electron micrograph of a cell of Lokiarchaeum ossiferum showing long, complex cell protrusions. Credit: © Thiago Rodrigues-Oliveira, Univ. Vienna
Cells also contain an extensive network of actin filaments that is thought to be unique to eukaryotic cells, says Florian Wollweber, who spent months tracking the cells under a microscope. that developed around this important and amazing event in the history of life.
Future prospects thanks to the new organism
“Our new organism, called Lokiarchaeum ossiferum, has great potential to provide groundbreaking new insights into the early evolution of eukaryotes,” commented microbiologist Christa Schliper. “It took six long years to get a stable, highly enriched culture, but we can now use this experience to carry out many biochemical studies as well as to grow other archaea from Asgard.” In addition, scientists can now use new imaging methods developed at ETH to study the close interactions between Asgard archaea and their bacterial partners, for example. Basic cellular biological processes such as cell division may also be studied in the future 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 canon 1st (December) 2022, nature.
DOI: 10.1038/s41586-022-05550-y