Microbiologists at the University of Vienna showed that Asgard archaea move in a dynamic manner. A new study in Nature revealed these cells extend long protrusions to crawl across surfaces. This behavior, previously known only in eukaryotes, suggests that complex life might have started much earlier than we once thought.
Asgard archaea, our closest known microbial relatives, play a key role in current models of the origin of more complex organisms, known as eukaryotes (all animals, plants, protists and fungi). However, their biology has so far been poorly understood.
In a new study published in the journal Nature, microbiologists led by the University of Vienna show that these cells move through their environment in a surprisingly dynamic manner. The findings make it possible to test models for the origin of complex life empirically for the first time.
The study, conducted by Philipp Radler in the laboratory of Christa Schleper at the University of Vienna, shows that tiny Asgard archaea cells (whose volume is about a thousand times smaller than that of a human cell) undergo significant changes in shape. They extend long cell protrusions, retract them and use them to crawl across surfaces—a behavior previously known only in eukaryotes.
The results provide insight into the behavior of Asgard archaea and may offer clues to how complex life arose. Current models suggest that the first eukaryotes emerged around 2 billion years ago from the fusion of a bacterium with an ancestor of today's Asgard archaea. Asgard archaea are therefore a crucial building block in the evolution of complex cells.
It was only recently, in 2020 and 2023, that the first two specimens of these organisms were cultivated at the JAMSTEC Institute in Japan (whose researchers are co-authors of the current study) and in Schleper's laboratory at the University of Vienna. Most of our knowledge about Asgard archaea is based on DNA sequencing or electron microscopy images.
These images revealed impressive cell shapes: a round cell body surrounded by numerous delicate projections that can be up to 20 times longer than the cell body. However, these images provided no insight into the dynamic behavior of the cells.
Filmed live under the microscope
Researchers at the University of Vienna have now observed these dynamics: They placed Asgard archaea in an oxygen-free environment and filmed the living cells under a microscope. They used two strains of Asgard archaea: a so-called Lokiarchaeon (cultivated in Vienna) and a Heimdallarchaeon (cultivated in Japan).
Both organisms drastically change their cell shapes every minute and use their thin, dynamic appendages to attach themselves to surfaces and explore them with a novel crawling motion. Such movement had not previously been described in microbes and had only been observed in more complex cells, including human immune cells.
The international team, which also included Japanese microbiologists and collaborators from IST Austria and HZI Braunschweig (Germany), demonstrated that actin inhibitors suppress these dynamic behaviors. This suggests a central role for an actin-based cytoskeleton, the fundamental cellular machinery that also controls shape changes and motility in human cells.
An evolutionary perspective opens up
The discovery suggests that complex cell motility may have much older origins than previously thought. The unusual crawling behavior of Asgard archaea will provide insights into cellular innovations that arose even before the first eukaryotes and may have been relevant in the ancient symbiosis from which mitochondria later emerged.
The new findings, particularly oxygen-free live-cell microscopy, now make it possible to test models of the origin of complex life empirically for the first time.