Asgard Archaea: Vienna Researchers Film the Closest Microbial Relatives of Complex Cells on the Move

1.What Are Archaea, and Why Are Asgards Studied?

Cellular life is divided into three domains: bacteria, archaea and eukaryotes. Archaea are single-celled organisms that, like bacteria, lack a nucleus. Yet their molecular machinery for transcription, replication and translation resembles that of eukaryotes much more closely than that of bacteria, which indicates that archaea are the most closely related prokaryotes to eukaryotes.

The Asgard archaea were discovered in 2015, when scientists analysed environmental DNA from marine sediments in the North Atlantic. What made them stand out were their genomes. They encode proteins that had long been considered hallmarks of eukaryotic cells, among them components of an actin-based cytoskeleton and many small GTPases, to an extent never found in other prokaryotes. This makes them central to research on the origin of complex cells and the evolution of the first eukaryotes. Although current models differ in their details, most agree that the first eukaryotic cell emerged about two billion years ago from a symbiosis between an Asgard archaeon and at least one bacterium, which later evolved into the mitochondria. In this scenario, the archaeon took up the bacterium, which then continued to live inside its archaeal host as a symbiotic partner.

Hundreds of Asgard archaeal species are known today, but most are known only from traces of DNA found in environmental samples. Only a few have been grown in the laboratory so far. They grow slowly, survive only in the absence of oxygen, and usually depend on partner organisms, making them difficult to isolate in pure culture. Obtaining a culture therefore takes years.

2.An Unusual Cell Shape and a Culture From Vienna

The first Asgard culture was reported in 2020 by Hiroyuki Imachi and Masaru Nobu at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) after more than a decade of research. Their organism would only grow alongside a methane-producing partner.

In Vienna, Christa Schleper’s group at the University of Vienna’s Department of Functional and Evolutionary Ecology started from shallow sediment collected off the coast of Slovenia. It took six years to obtain a stable culture, which still contained several other microbial species. In a Nature paper published at the end of 2022, the team described the organism as Candidatus Lokiarchaeum ossiferum, collaborating with Martin Pilhofer’s group at ETH Zurich to obtain images of the species. Imaging revealed that the cells have a distinctive shape: a small central body with long, thin protrusions, no cell wall, and a cytoskeleton built from an actin homologue called Lokiactin, one of the most highly expressed proteins in the organism. Most of what was known about these cells came from DNA sequences and from microscopy of dead, fixed cells. According to Hiroyuki Imachi, cells invest a substantial amount of resources in building and maintaining such protrusions, so they likely serve a biological function. Until now, researchers could only speculate what these protrusions are capable of.

Schleper’s group and first author Philipp Radler took up this question in their new study, “Dynamic protrusions mediate crawling motility in Asgard archaea”, published in Nature on 30 September 2026. They wanted to know whether the protrusions grow and retract in living cells, and whether this allows the cells to change their shape and move. To find out, the team worked with two organisms: the Vienna culture and a more recent one from JAMSTEC, Candidatus Margulisarchaeum peptidophilum, a member of the Hodarchaeales lineage.

3.Actin, Cell Shape and Crawling

In eukaryotic cells, actin forms filaments that are constantly assembled and disassembled, building a dynamic scaffold that reshapes the cell. This turnover allows cells to form protrusions and lets them move across surfaces. More than 150 actin-binding proteins fine-tune the process. This so-called crawling motility is considered a hallmark of eukaryotic cells and requires a dynamic actin cytoskeleton, regulated adhesion and spatially organised signalling pathways. Immune cells use it to move through tissue, as do certain single-celled protists such as amoebae.

One of the proteins of the Asgard archaea studied by Christa Schleper’s group, called Lokiactin, is closely related to eukaryotic actin and fills the protrusions of L. ossiferum. Earlier biochemical studies have also shown that actin regulators from Asgard archaea can bind and modulate eukaryotic actin, hinting at an evolutionarily conserved mechanism. What remained open was whether this machinery is active in living cells. Images of fixed cells could show where the filaments are, but not whether they grow and shrink. Whether the Lokiactin cytoskeleton is dynamic, and whether it drives movement, had never been tested.

For this study, Christa Schleper collaborated with Michael Sixt at the Institute of Science and Technology Austria (ISTA) in Klosterneuburg. His group studies how cells, in particular immune cells, migrate through tissue.

4.What the Team Observed, and the Method That Made It Possible

Asgard archaea only survive without oxygen and grow slowly. To observe them alive, the team developed an oxygen-free microscopy set-up in which living cells could be followed for several hours. As no genetic tools exist for these organisms, Lokiactin could not be labelled with a fluorescent protein. The team therefore used a dye originally developed for eukaryotic actin to make the filaments visible.

The recordings show highly dynamic cells. Although L. ossiferum divides only about once a week, the cells changed their shape within minutes. Their protrusions grew and retracted continuously, and many of them branched. The total length of all protrusions of a cell, however, stayed constant. This points to a controlled redistribution of the available membrane rather than the production of new membrane.

The central finding is that about half of the attached cells moved across the glass. They extended protrusions, attached them to the surface with their tips and pulled the cell body after. At around 1.6 µm per minute, this movement is much slower than that of bacteria and archaea that swim with flagella or archaella, but it lies within the range of crawling eukaryotic cells. It also differs from the only form of surface motility previously described in archaea, in which cells pull themselves along with external filaments called pili. In Asgard archaea, movement seemingly depends on the cell’s internal cytoskeleton.

To test whether Lokiactin drives these dynamics, the team treated the cells with swinholide A, a marine toxin that binds actin and cuts its filaments in eukaryotic cells. It took effect at concentrations similar to those used on eukaryotic cells. Treated cells stopped forming new protrusions and could no longer move as before. To check whether this behaviour is specific to L. ossiferum, the team repeated the experiments with M. peptidophilum from the Hodarchaeales lineage. Its cells behaved in the same way, only faster: the protrusions grew about three times as fast, and the cells moved about twice as fast. Swinholide A again stopped both. As two different Asgard lineages show the same dynamics, the authors conclude that they are conserved within the group. This crawling motility is a unique type of prokaryotic movement, resembling the amoeboid crawling of immune cells and certain protists.

5.What the Study Shows, and What Comes Next

The authors point out that the experiments were performed with two present-day organisms, different from the archaeal ancestor of eukaryotes from 2 billion years ago, when the first eukaryotic cell most likely evolved. However, because Lokiactin and its regulators occur in virtually all Asgard genomes, the authors conclude that it is likely that archaea used an actin cytoskeleton for motility early in Asgard evolution, before eukaryotes emerged. Several models of eukaryogenesis (i.e. the evolution of eukaryotes) involve the archaeal host eventually enclosing its bacterial partner, and according to the authors, flexible membranes and protrusions could have facilitated the early steps of symbiont uptake. Work from JAMSTEC points in a similar direction: in a study available as a preprint, the Japanese team observed that the protrusions of their Hodarchaeales strains attach to methane-producing partner cells, which suggests that protrusions may help Asgard archaea hold on to their partners.

The long-term observation of living and moving Asgard archaea under anaerobic conditions is another step in understanding the evolutionary history and similarities between archaea and eukaryotes. One of the main obstacles is the lack of genetic tools that allow scientists to manipulate and delete genes of Asgard archaea, which has so far limited the study of Lokiactin to dyes and inhibitors.

A second project from Schleper’s group aims to address this limitation. In a preprint published in February 2026, the team reports the first cultivated virus of an Asgard archaeon, which infects a new strain of L. ossiferum. These cultures offer significant potential for developing genetic tools for Asgard archaea through transfection experiments with viral DNA.

If successful, this approach would allow researchers to switch individual genes, such as the one encoding Lokiactin and its regulators, on or off, couple it to fluorescent proteins and directly investigate the function of these proteins in living Asgard cells.