A team of scientists has found giant viruses dating back 1.5 billion years in geothermal springs in Yellowstone, in the United States.
These viruses contain very large genomes compared to ordinary viruses, and do not pose any danger to humans, but they may reveal the conditions in which life was formed on Earth.
Rutgers University scientists found that viruses thrive in bacteria, while others belong to archaea (a single-celled organism similar to bacteria), where they require harsh environments to reproduce.
Initially, scientists thought that the giant viruses would not be very ancient, because as the hot springs formed and disappeared, the viruses should form again under higher temperatures in the newly emerging hot springs.
Hot springs are found in dormant volcanoes whose lava heats groundwater, causing less dense steam and hot water to rise from cracks in the ground, thus forming geysers and hot springs.
Yellowstone's hot springs formed at least 15,000 years ago, after the last glaciers in the region melted, allowing hot springs to appear, but bacteria had been thriving for more than a billion years before that.
The results showed that "the links between viruses and hot springs are ancient," as viruses reproduce at temperatures exceeding 93 degrees Celsius, high pressures, or excessive salt concentrations, and scientists believe that they reproduce by infecting red algae in hot springs.
The study analyzed DNA in Lemonade Creek, an acidic hot spring in Yellowstone that reaches temperatures of approximately 43 degrees Celsius.
The team took samples of the thick red algae covering the creek floor, and from nearby soil and the area between the rocks.
The DNA was found to contain sequences from archaea, algae (eukaryotes) and bacteria that host 3,700 potential viruses (about two-thirds of which were giant viruses that do not infect humans).
The team used computer analyzes to narrow the range of officially registered viruses to 25 different species, which they believe use red algae to reproduce.
Scientists believe this connection began 1.5 billion years ago when viruses first evolved by borrowing each other's genes to cope with heat and toxins, such as arsenic found in hot springs.
“This work supports the concept that viruses are present wherever there is cellular life, and that viruses have been present for at least as long as cellular life has existed,” said Mark Young, an emeritus environmental virologist at Montana State University.
The study was published in the journal Science.