The study, conducted by Dr Philipp Radler in the laboratory of Prof. 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 again and use them to actively crawl across surfaces – a behavior previously known only from eukaryotes.
The results not only provide an unprecedented insight into the behavior of Asgard archaea, but may also offer important clues as to how complex life arose. Current models suggest that the first eukaryotes emerged around two 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 renowned JAMSTEC Institute in Japan (co-authors of the current study) and in Christa 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 succeeded in observing these dynamics: They placed Asgard archaea in an oxygen-free environment and filmed the living cells under the 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 using a novel crawling motion. Such a movement had not previously been described in microbes and had only been observed in more complex cells, including human immune cells.
Furthermore, the international team, which beside the Japanese microbiologists also included collaborators from IST Austria and the research groups “Cell Biology” led by Prof. Theresia Stradal and “Molecular Cell Biology” led by Prof. Klemens Rottner at the HZI in Braunschweig (Germany) demonstrated that actin inhibitors suppress these dynamic behaviors. The HZI researchers provided a comprehensive panel of cytochalasans previously characterized in the framework of a DFG-funded project. Collectively, obtained results suggest a central role for an actin-based cytoskeleton, the fundamental cellular machinery that also controls shape changes and motility in human cells.
An exciting evolutionary perspective opens up
The discovery offers an exciting evolutionary perspective: 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 already been relevant in the ancient symbiosis from which mitochondria later emerged. The new findings, in particular oxygen-free live-cell microscopy, now make it possible for the first time to empirically test models of the origin of complex life.
