Résumé:
Hunting for Our Microbial Ancestors—The Asgard Archaea
Why Study Asgard Archaea?
Asgard archaea are a group of microbes discovered less than a decade ago, named after Norse gods (Loki, Thor, Odin, Heimdall) because their discovery revolutionized our understanding of how complex life evolved. These microbes are our closest known relatives among the archaea, and they carry genes usually found only in eukaryotes—organisms with complex cells, such as animals, plants, and fungi. It suggests that Asgard archaea might be the living descendants of the ancient microbes that gave rise to all complex life on Earth.
What Do We Know So Far?
Only two Asgard species have been grown in the lab, respectively by a Japanese and an Austrian team: ‘Candidatus Prometheoarchaeum syntrophicum’ lives in partnership with other microbes, forms long cellular extensions, and may have played a key role in the origin of eukaryotes (Imashi et al., 2020). ‘Candidatus Lokiarchaeum ossiferum’: Thrives at 20°C, has a larger genome, and shows complex internal structures, possibly thanks to a primitive cytoskeleton (Rodrigues-Oliveira et al. 2023). These findings support the idea that eukaryotes may have emerged from a fusion between an Asgard archaeon and a bacterium over 2 billion years ago.
Our Project: Cultivating Heimdallarchaeota, Our Closest Microbial Relatives
Microbiologists at the MIO (MEB team) lab have successfully enriched cultures of Heimdallarchaeota—the Asgard group most closely related to eukaryotes—from hydrothermally heated sediments. These microbes live in extreme conditions (45–55°C, acidic, oxygen-free). Scientists used in situ microbial culture devices to enrich Asgard archaea. Back in the lab, they could maintain the growth of these microbes under controlled conditions. Yet Heimdall archaea represent a tiny fraction of the microbial cultures. Still, their cells could be visualized by specific microscopy techniques (FISH-confocal microscopy) at the BIAM lab, thanks to collaboration with researchers of the BEAMM team.
Our Goals in the Asgardomics project:
1) Obtain the first stable culture of Heimdallarchaeota to study their cell structure and metabolism.
2) Understand how these microbes might have given rise to eukaryotes.
3) Test hypotheses about the evolution of complex life.
To achieve these objectives:
We will use innovative and complementary Methods
- Dedicated culture methods, including novel in situ enrichments and middle-throughput techniques
- Deep sequencing of enrichment cultures (metagenomics) for bioinformatic reconstruction of Asgard archaea genomes, which will help us decipher their metabolism and lifestyle and shed light on the evolutionary relationship with other archaea and eukaryotes.
- Advanced microscopy to visualize cell structures, like the primitive cytoskeleton.
Challenges and Stakes
Challenge: These microbes are a minority in their natural ecosystem, grow very slowly, and are fragile.
Originality: No one has yet succeeded in cultivating Heimdallarchaeota.
Potential: A stable culture could unlock mysteries about the origin of eukaryotes
What’s Next?
If successful, these archaea could become a global research model, attracting international collaborations and opening new avenues for understanding the evolution of early eukaryotic life.