Scientists discovered the "fingerprints" of bizarre viruses hidden in an ancient group of microbes that may have fueled all complex life on Earth, from fungi to plants to humans.
These microbes known as Asgard archaea after the abode of the gods in Norse mythology reside in the frigid sediments deep in the ocean and in boiling hot springs, and existed on Earth prior to the firsteukaryoticcells, which carry theirDNAinside a nucleus.
Some researchers believe viruses may have influenced how life forms first evolved by infecting Asgard archaea, and may even have given rise to some of the first nucleus precursors. But before now, no Asgard-infecting viruses had been discovered.
Scientists have identified a slew of viruses that might infect the ancient archaea in a trio of experiments published Monday (June 27) in the journal Nature Microbiology.
"These are the first investigations investigating Asgard archaeal viruses, and there was nothing known before," said Susanne Erdmann, the group leader of the Max Planck Institute for Marine Microbiology in Bremen, Germany, who was not involved in the studies.
Erdmann told Live Science that this line of research might prove if and how viruses were involved in the formation of eukaryotic cells on Earth in the future.
Scientists discover viruses that secretly rule the world's oceans.
Dusting off viral 'fingerprints'
Scientists searched for evidence of viral infection embedded in the DNA of Asgard archaea in the latest research, which includes brief fragments of viral DNA known as "CRISPRspacers."
Ian Rambo, a former doctoral candidate at the University of Texas at Austin Marine Science Institute and the first author of one of theNature Microbiologystudies, thinks of the famous gene-editing tool that allows scientists to manipulate genetic sequences. However, this gene-editing technique was originally derived from the natural defense mechanisms ofbacteria and archaea.
"CRISPR" is a short acronym that means "clusters of regularly interspaced short palindromic repeats," which refers to a DNA region composed of short, repeated sequences with "spacers" sandwiched between the repeats. Bacteria and archaea remove these spacers from viruses that infect them, thus, the cells retain a viral DNA bank that helps them recognize viruses if they attack again.
"It's an adaptive immune system that remembers these previous infections," Rambo said, who is now a postdoctoral fellow with the USDA's Agricultural Research Service.
Rambo and his colleagues hunted for such DNA spacers in Asgard archaea specimens collected from sediments near hydrothermal vents, about 1.25 miles (2 kilometers) beneath the water's surface in the Guaymas Basin in the Gulf of California.
The researchers matched the spacers they discovered to long stretches of viral DNA extracted from the deep-sea environment.
Are viruses still alive?
"It's quite straightforward to extract viruses from deep-sea sediments, but the challenge is to identify which hosts these viruses infect," said Mart Krupovic, the head of the Institut Pasteur's Archaeal Virology Unit, and a co-author of the othertwostudies. "CRISPR spacer matching is the most convenient, most convincing, and reliable approach to assigning the host."
Rambo's team discovered six viruses that infect two kinds of Asgard archaea, named Lokiarchaeota and Helarchaeota for the Norse god Loki and goddess Hel, respectively. The newfound viruses are named after Norse mythological creatures, including the giant wolf Fenrir and the dragon Nidhogg.
Krupovic and his colleagues examined two viruses that they named Huginn and Muninn, after the two ravens that serve as scouts for the Norse god Odin; these viruses were discovered in an Asgard genome sampled from a hot spring in Yellowstone National Park.
Krupovic and his coauthors discovered viruses in deep-sea sediments from the Shimokita Peninsula, the northeastern cape of the Japanese island of Honshu, as well as two other sites in the Pacific and one in the Indian Ocean.
In these samples, researchers discovered three family-level viruses, named after the three Norns Wyrd, Verdandi, and Skuld, which are supernatural beings that determine the destiny of gods and mortals in Norse mythology.
Related: A strange blob from deep-sea mud may reveal clues to the origins of complex life.
The researchers could interpret the viral DNA to determine what kinds of proteins each gene encodes, and therefore, how the viruses might appear and function.
Krupovic and his colleagues have determined that the viruses named for Norn Verdandi have tails that extend out of their outer shells, or capsids.
The group of researchers from Rambo's laboratory discovered that Nidhogg viruses might be able to hijack key proteins in their host cells, which would help the viruses make new copies of themselves. (Viruses that infect eukaryotic cells hijack their hosts in a similar manner.)
Researchers could only determine the functions of some of the viruses' genes, although the majority of the genes remain unexplored, according to Erdmann. Many additional Asgard-infecting viruses are likely to be discovered as a result of CRISPR.
One strategy to discover these hidden viruses would be to grow Asgard archaea in the lab and isolate any viruses found within their cells, according to Erdmann.
Only one research group has successfully cultured Asgard archaea until now, and it took them 12 years to finish it. That's partially because archaeal cells take weeks to replicate. (By comparison, thebacteriumEscherichia coli, for example, takes about 20 minutes, according to Science News).
CRISPR spacer matching is probably the most efficient way to discover more viruses, until more Asgards are grown in the lab, according to Krupovic. And as more and more viruses are discovered, their role in the evolution of eukaryotes including humans might become more clear, according to Rambo.
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