New viruses have been discovered that are related to both large viruses and herpesviruses.

A significant group of DNA viruses that are prevalent from the equator to the poles has been identified thanks to metagenomic data on marine plankton from the Tara Oceans: mirusviruses. These infections play a part in managing tiny fish by contaminating an extensive number of single-cell life forms at some random time.

They have a surprising genomic composition and are extremely complex. Herpes viruses share the key genes necessary to form their viral particles, which are a characteristic of these DNA viruses. The latter are present in animals on a regular basis (half of the human population is infected with the herpes virus), but they are absent from other forms of life, which raises the question of their evolutionary origin.

The discovery of mirusviruses suggests that marine single-cell organisms were once infected by herpes viruses. There were additional surprises in mirusvirus evolution. Despite the obvious connection to herpes viruses, the majority of mirusvirus genes, including those involved in viral genome replication, are similar to those of giant viruses, a completely different group of viruses whose amazing properties pique the interest of researchers. This transformative “chimerism” in mirusviruses is special and could give data about the development of DNA infections.

“Tara Oceans has completely changed our understanding of plankton ecology. Our research shows that this extraordinary voyage also solves key evolutionary concerns. Mirusviruses still have a lot to learn and understand. They have yet to be cultivated, there are no photos of their viral particle, and we have yet to examine them outside of the oceans.”

Morgan Gaïa, a virus evolution expert at the CEA and first author.

“In 2019, our examination group noticed an uncommon developmental sign in the enormous amounts of sequencing information given by the Tara Seas project. By following this sign, we found and, afterward, portrayed a significant DNA infection bunch: mirusviruses. Tom Delmont, a CNRS expert in microbial ecology and the study’s final author, explains, “The publication of this discovery in Nature marks the beginning of a new adventure and a gateway for the scientific community to detect and study mirusviruses in any number of ecosystems.”

“Our understanding of plankton ecology has changed as a result of the Tara Oceans. Our review demonstrates that this inconceivable campaign likewise responds to basic developmental inquiries. Concerning mirusviruses, there is still a great deal to be learned. Morgan Gaa, a CEA virus evolution specialist and first author, stated, “We have yet to study them in places other than the oceans. No images of their viral particle exist.”

More about the Tara Oceans Expedition, which took place from 2009 to 2013, For nearly four years, the schooner Tara carried out a singular mission in each oceanic basin: to get a global picture of the world’s marine plankton ecosystems. Unnoticed biodiversity is a crucial indicator of our planet’s and its climate system’s health. A total of nearly 35,000 virus, bacteria, algae, and plankton samples have been collected and are currently being analyzed.

The expedition, which is the biggest genetic sequencing project ever done on a marine ecosystem, has revealed most of the microbial genes that were never seen before. This expedition, which was led by the Tara Ocean Foundation with the help of the Tara Oceans consortium and the GO-SEE (Global Oceans Systems Ecology and Evolution) research network, mostly involved teams from CNRS, CEA, and EMBL.

More information: Morgan Gaïa et al, Mirusviruses link herpesviruses to giant viruses, Nature (2023). DOI: 10.1038/s41586-023-05962-4