Once upon a time, there were the Asgard archaea, true living fossils that certainly hold the secrets of eukaryogenesis.
In the middle of the 20th century, the classification of living organisms was dichotomous: on the one hand, there were eukaryotic organisms, formed by cells with a nucleus, and on the other, prokaryotic cells without a nucleus, represented by bacteria and archaeobacteria. However, the end of the 1970s marked a profound break with the prokaryotic world, thanks to the work of Carl Woese based on the comparative analysis of ribosomal RNA. Archaeobacteria, long considered to be special extremophilic bacteria, do not belong to the bacterial world and constitute a third group of living beings. Intrigued by these cells, renamed archaea, scientists have been striving to understand their origin and to establish evolutionary scenarii for living beings by studying their genome.
In 2015, Thijs Ettema’s team from Uppsala University sequenced the genome of a lineage of archaea from underwater hydrothermal springs in the Norwegian Atlantic belonging to the “Loki Castle” site, which refers to the Norse god of discord. In 2017, the same authors described the genome of new archaea related to the Lokiarchaea and to which they also attributed names derived from Norse gods: Thorarchaea, Odinarchaea and Heimdallaea. Interestingly, analysis of the genomes revealed the presence of several genes thought to be specific to eukaryotes, and involved in key functions such as membrane remodeling, cytoskeleton formation and phagocytosis. This finding suggested that the genetic potential to develop certain eukaryotic traits was already present in these particular archaea.
These archaea, found in a wide range of environments such as hot springs or deep oxygen-poor sediments, were grouped together in the Asgard family of archaea, named after the domain of the gods in Norse mythology.
Their characterization could provide valuable information on the origin, two billion years ago, of the lineage to which we belong. However, until now, no one had succeeded in cultivating and observing these intriguing archaea, keeping their biology and the secrets they hold in the dark.
Then, in 2019, after twelve years of hard work, a team of Japanese microbiologists, Hiroyuki Imachi and Masaru Nobu, succeeded in cultivating Asgard archaea for the first time, isolated from sediments in the deep sea of Japan. For five years, a continuous-flow bioreactor system fed with methane allowed these fragile, slowly dividing cells to multiply. This was followed by seven years of progressive enrichment and transfer, resulting in a pure co-culture of the Candidatus Prometheoarchaeum syntrophicum strain (in symbiosis with Methanogenium), which finally provided the opportunity to study its physiology and perform new genetic analyses.
Key findings included a complex morphology with long, thin polymeric tentacles that provide shelter for partner micro-organisms. In addition, analysis of proteomic signatures and gene expression clearly showed a sister phylogenetic relationship between P. syntrophicum and eukaryotic cells.
The Japanese team has thus provided access to a valuable tool for providing new insights into eukaryogenesis, one of the great questions in biology. Although the evolutionary model remains to be elucidated, the culture of P. syntrophicum gives us hope that the same will be done with other archaea and that we will be able to better understand their place in the evolution of life.
The Asgard archaea have not finished surprising us!