How did the complex living things on earth come into being? This is one of the great unanswered questions in biology. A collaboration between the research groups of Christa Schleper at the University of Vienna and Martin Pilhofer at ETH Zurich has brought the answer one step closer. The researchers succeeded in cultivating a special archaeon and characterizing it more precisely using microscopic methods. This representative of the Asgard archaea exhibits unique cellular features and could be an evolutionary "missing link" to the more complex life forms such as animals and plants. The study is currently published in the journal "Nature".
Living organisms on Earth are divided into three major domains: Eukaryotes, Bacteria, and Archaea. Eukaryotes include the groups of animals, plants and fungi. Their cells are generally much larger and, at first glance, more complex than cells of bacteria and archaea. For example, the genetic material of eukaryotes is packaged in a nucleus, and the cells also have a variety of other compartments. The cell shape and transport within the eukaryotic cell are also based on an extended cytoskeleton. But how did the evolutionary leap to such complex eukaryotic cells occur?
Most current models assume that archaea and bacteria played a central role in the evolution of eukaryotes. It is assumed that about two billion years ago, a primordial eukaryotic cell arose from a close symbiosis between archaea and bacteria. In 2015, genome studies of environmental samples from the deep sea revealed the group of so-called "Asgard archaea," which are the closest relatives of eukaryotes in the phylogenetic tree. First images of Asgard cells were already published in 2020 from enrichment cultures of a Japanese group.
Asgard archaea cultivated from marine sediments
The research group of Christa Schleper at the University of Vienna has now succeeded for the first time in cultivating a representative of this group in higher enrichment. It originates from marine sediments on the coast of Piran, Slovenia, but is also a resident of Vienna, for example in the bank sediment of the Danube. Because of its growth to large cell densities, this representative can be studied particularly well. "It took a lot of fine-tuning to get this extremely sensitive organism in a stable culture in the lab," reports Thiago Rodrigues-Oliveira, postdoc in the Archaea Working Group at the University of Vienna and one of the study’s first authors.
Asgard archaea have a complex cell shape with extensive cytoskeleton
The remarkable success of the Viennese researchers with the cultivation of a highly enriched Asgard representative finally allowed the in-depth examination of the cells by microscopy. The researchers in Martin Pilhofer’s group used a state-of-the-art cryo-electron microscope to capture images of snap-frozen cells. "This method provides a three-dimensional view of internal cell structures," Pilhofer explains. "The cells consist of round cell bodies with thin, sometimes very long cell processes. These tentacle-like structures sometimes even appear to connect different cell bodies," says Florian Wollweber, who spent months tracking down the cells in the microscope. The cells also contain an extensive network of actin filaments, previously known only from eukaryotic cells. This indicates that extensive cytoskeletal structures arose in archaea before the appearance of the first eukaryotes and spurs evolutionary theories surrounding this important and spectacular process in evolution.
Future insights from the new model organism.
Original publication:
Rodrigues-Oliveira T, Wollweber F, Ponce-Toledo RI, Xu J, Rittmann SKMR, Klingl A, Pilhofer M, Schleper C. 2022. actin cytoskeleton and complex cell architecture in an Asgard archaeon. Nature
DOI 10.1038/s41586’022 -05550-y
Illustrations:
Figure 1: Cryo-electron tomography provided insight into the cellular structure of a newly cultured Asgard archaeon illustrated here. Note the extensive actin cytoskeletal filaments (orange) in the cell bodies and cell processes, as well as the uniquely constructed cell envelope (blue). (© Margot Riggi, The Animation Lab, University of Utah)
Figure 2: Scanning electron micrograph of a cell of
Figure 3: One of the currently most popular evolutionary theories assumes that eukaryotes (including animals, plants and fungi) arose through a fusion of an Asgard archaeon with a bacterium. (© Florian Wollweber, ETH Zurich)
Figure 4: Co-first author Rafael Ponce sampling marine sediment at the Seca Channel in Piran, Slovenia. (© Thiago Rodrigues-Oliveira, Univ. Vienna)
