Discovering viruses that infect our closest prokaryotic relatives

Published on
September 12, 2022

In recent years, biologists have discovered an astounding diversity of all kinds of microorganisms: bacteria, archaea, eukaryotes and viruses. Using this information, multiple scientific teams have tried to delineate our evolutionary past. By studying our microbial relatives, we can better understand our own origins and the complex ecological communities of which they are part. This could also bring unexpected benefits in the form of new molecular tools or helpful chemical compounds.

Unveiling our microbial ancestry

Eukaryotic cells, such as ours, evolved through the merging of at least two separate lineages: a bacterium and an archaeon. In a study published in 2015, a team led by Thijs Ettema analyzed marine sediments from the Arctic Mid-Ocean Ridge, near a field of active hydrothermal vents called Loki’s castle. It obtained genome sequences from a new archaeal lineage, the Lokiarchaea, which displayed a close relationship with eukaryotes in evolutionary analyses1.

These were the founding members of an archaeal group named Asgard archaea, which also included Thorarchaea, Heimdallarchaea and Odinarchaea2. Despite their tremendous interest as our microbial relatives, Asgard archaea have not been known for long, and much of their ecology, physiology and general lifestyle is still unknown.

A complete genome from Asgard archaea

Despite the sufficient presence of genomes from Asgard archaea in public databases, most are not complete. At the laboratory of Thijs Ettema, PhD student Eva F. Cáceres and Post-doc Daniel Tamarit joined forces, together with other researchers. They improved the genome sequence of Ca. Odinarchaeum. The obtained complete genome opened the door to new types of studies, to better understand our Asgard archaeal relatives.

Asgard archaeal viruses

Through bioinformatic searches, these researchers found foreign elements, some of which were clear Ca. Odinarchaeum viruses. Viruses are abundant in the environment, and have a big impact on the physiology and evolution of their hosts. Finding viruses that infect Asgard archaea brings new knowledge on our closest prokaryotic relatives. Yet, it is possible that the current searches do not recover a full picture of the diversity of viruses infecting these organisms. Future analyses can bring information on whether the Asgard archaeal virosphere remains typically archaeal or is populated by other viruses. Viruses that bridge the boundaries between archaea and eukaryotes. With that, step by step, the ecological networks of Asgard archaea are being revealed.

On the research

This research3 is accompanied by two other studies4,5, published in the same issue of Nature Microbiology, which complement each other and together describe the diverse menagerie of viruses infecting Asgard archaea.