An article published in the journal “Nature” reports a study on archaea that belong to the proposed superphylum called Asgard and in particular on their characteristics that make them a possible evolutionary missing link with eukaryotes. A team of researchers formed by the collaboration of the working groups of Christa Schleper of the Austrian University of Vienna and Martin Pilhofer of ETH Zurich was successful in cultivating a species belonging to this group of microorganisms to study them in the laboratory. This allowed conducting thorough examinations of their cellular structures such as the extensive cytoskeleton.
Advances in genetic techniques allowed to greatly expand our knowledge of microorganisms with continuous discoveries of new branches of the tree of microscopic life. This also allows investigating the relationships between the various domains of life and the possible origin of eukaryotic organisms, which include animals, plants, and fungi.
One possibility regarding the origin of eukaryotes is given by a symbiosis between an archaeon and a bacterium which, a couple of billion years ago, became so close that two very different organisms became a single organism. The discovery of a group of archaea for which a new superphylum called Asgard was proposed could bring evidence for such an origin, illustrated in the bottom image (Courtesy Florian Wollweber, ETH Zurich. All rights reserved). The interest in the Asgard archaea has been considerable since the announcement of their discovery also due to some of their characteristics which previously were found in eukaryotic cells only.
To conduct in-depth research on the Asgard archaea, it’s important to have them available in the laboratory, preferably in high concentrations. The Vienna group led by Christa Schleper had success with a species discovered in marine sediments off the coast of Piran, Slovenia, that also lives in sediments in the Danube River. These are anaerobic organisms that live in specific conditions, so it’s necessary to replicate them correctly in the laboratory.
The success achieved in the cultivation of the proposed species Candidatus Lokiarchaeum ossiferum (Top image courtesy Thiago Rodrigues-Oliveira, University of Vienna. All rights reserved) enabled the thorough examinations desired by the Vienna and Zurich teams to be conducted. Microscopic examinations made it possible to study the details of this archaeon, obtaining confirmation of some similarities with eukaryotic cells.
A very interesting discovery was that of an extensive network of filaments that make up the so-called cytoskeleton. This type of structure isn’t exclusive to eukaryotes but in Lokiarchaeum ossiferum it has characteristics that were thought to be present only among eukaryotes. The fact that an Asgard archaeon has an extended cytoskeleton suggests that this type of structure already evolved in archaea before the birth of the first eukaryotic cell.
This study offers new confirmation of links between archaea and eukaryotes and ways to cultivate and investigate an Asgard archaea species that may also be adapted to other similar species. Nearly six years after the announcement of the discovery of this group of archaea and the first proposal for the creation of a superphylum, much work remains to be done to study them. The study of Lokiarchaeum ossiferum took six years of work, and this shows its complexity but slowly this and other research groups are shedding light on the origin of eukaryotes.