A decade ago, the scientific community was unaware of the existence of Asgard archaea. It wasn’t until 2015 that researchers discovered genetic fragments in deep-sea sediments that hinted at a previously unknown group of microbes. Using computational tools, scientists reconstructed these fragments to reveal a complete genome, leading to the realization that they had uncovered a new branch of the archaeal domain.
Archaea, like bacteria, are single-celled organisms, but their genetic makeup and cellular processes differ significantly. Subsequent investigations identified and described the organisms behind the gene sequences, leading to the classification of Asgard archaea as a distinct subgroup. Their name, inspired by Norse mythology, references their discovery near Loki’s Castle—a hydrothermal vent along the Mid-Atlantic Ridge between Norway and Svalbard.
Asgard archaea soon became a scientific revelation. They appeared to bridge the gap between archaea and eukaryotes—organisms with complex cells, including plants and animals. This discovery challenged the traditional three-domain system of life (bacteria, archaea, eukaryotes), suggesting that eukaryotes may have evolved from Asgard archaea. Some scientists have even proposed redefining the tree of life to include just two domains: bacteria and archaea (with eukaryotes nested within the latter).
At ETH Zurich, Professor Martin Pilhofer and his team have spent years studying Asgard archaea. In a 2022 study published in Nature, the group examined Lokiarchaeum ossiferum, an Asgard archaeon isolated from Slovenian brackish water sediments by researchers at the University of Vienna. They found that this organism possessed features previously thought to be unique to eukaryotes—specifically, an actin protein called Lokiactin.
Using advanced microscopy, the researchers demonstrated that Lokiactin forms filaments concentrated in the archaeon's tentacle-like extensions. These structures likely serve as a cellular skeleton, supporting the organism’s complex architecture.
In eukaryotes, the cytoskeleton is composed not only of actin filaments but also of microtubules—tubular structures formed by tubulin proteins, which are critical for intracellular transport and chromosome segregation. Until recently, the origins of these microtubules were uncertain.
Now, in a new study published in Cell, the ETH team has identified tubulin-like proteins in Asgard archaea that form microtubule-like structures, although smaller than those found in eukaryotes. These structures were found in only a few Lokiarchaeum cells and appear in limited species within the Asgard group. The reason for this rarity is still unclear, as is the function of these tubulins in archaeal cells.
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While Asgard archaea have yet to show motor proteins that traverse microtubules as in eukaryotes, the researchers observed that the archaeal tubules grow from one end—a characteristic suggestive of transport functionality. Jingwei Xu, co-first author of the study, produced these tubulins in cultured insect cells to analyze their structure.
The interdisciplinary study brought together experts in microbiology, structural biology, cell biology, and biochemistry. Pilhofer credits the collaboration for the project’s success.
The findings raise a fundamental question: Was the cytoskeleton a key innovation that enabled the rise of complex life? Researchers speculate that Asgard archaea may have once enveloped a bacterium using their appendages—an event that could have led to the development of mitochondria, and eventually, the complex eukaryotic cell.
According to Pilhofer, this cytoskeletal framework may have set the stage for such an evolutionary leap. The team now plans to explore the roles of actin and tubulin filaments in Asgard archaea and investigate the surface proteins of these microbes. By creating specific antibodies, they hope to "fish out" Asgard archaea from mixed microbial communities.
Despite the progress, many questions remain about these elusive organisms and their link to eukaryotic origins. For Pilhofer and his colleagues, uncovering the mysteries of Asgard archaea continues to be a captivating pursuit.
Nebitno
