Giant viruses discovered on Greenland ice sheet could reduce ice melt

Every spring, when the sun rises in the Arctic after months of darkness, life returns. Polar bears emerge from their winter dens, the Arctic tern returns from its long journey south, and musk oxen wade north.

But animals aren’t the only ones awakened by the spring sun. Dormant algae on the ice begin to bloom in spring, blackening large areas of ice.

When ice darkens, its ability to reflect sunlight decreases, which accelerates its melting. Increased melting exacerbates global warming.

But researchers may have found a way to control snow algae growth and perhaps, in the long term, reduce some of the ice melt. Living on the ice alongside algae, postdoctoral researcher Laura Perini from the Department of Environmental Sciences at Aarhus University and her colleagues discovered giant viruses. Their findings are published in the journal Microbiota.

She suspects the viruses feed on snow algae and could function as a natural control mechanism for algal blooms.

“We don’t know much about these viruses, but I think they could be useful in mitigating ice melt caused by algae blooms. We don’t yet know how specific they are and what their potential would be. “By exploring them further, we hope to answer some of these questions,” she says.

Bigger than bacteria

Viruses are normally much smaller than bacteria. Ordinary viruses are 20 to 200 nanometers in size, while a typical bacteria is 2 to 3 micrometers. In other words, a normal virus is about 1,000 times smaller than a bacteria.

This is not the case for giant viruses, however. Giant viruses reach a size of 2.5 micrometers. It’s bigger than most bacteria.

But giant viruses aren’t just bigger. Their genome is much larger than that of ordinary viruses. Bacteriophages, bacteria that infect viruses, have between 100,000 and 200,000 letters in their genome. Giant viruses number around 2,500,000.

Never found on ice before

Giant viruses were first discovered in 1981, when researchers discovered them in the ocean. These viruses have specialized in infecting marine green algae. Later, giant viruses were discovered in Earth’s soil and even in humans.

But this is the first time that giant viruses have been discovered living on the surface of ice and snow dominated by microalgae, says Perini.

“We analyzed samples of black ice, red snow and melt holes (cryoconite). In both black ice and red snow we found signatures of active giant viruses. And this is the first time that “They are found on surface ice and snow containing a high abundance of pigmented microalgae,” she says.

A few years ago, everyone thought that this part of the world was barren and devoid of life. But we now know that several microorganisms live there, including giant viruses.

“There is a whole ecosystem around algae. Besides bacteria, filamentous fungi and yeast, there are protists that eat the algae, different species of fungi that parasitize them and the giant viruses we found that infect them “In order to understand the biological controls that act on algal blooms, we need to study these last three groups,” Perini continues.

Even though viruses are giant, they are not visible to the naked eye. Perini hasn’t even seen them under a light microscope yet. But she hopes to do so in the future.

“We discovered the viruses by analyzing all the DNA in the samples we took. By sifting through this enormous dataset for specific marker genes, we found sequences that show high similarity to the giant viruses known,” she explains.

To ensure that the viral DNA did not come from long-dead microorganisms, but from living, active viruses, the team also extracted all of the mRNA from the sample.

When the DNA sequences that form genes are activated, they are transcribed into single-stranded pieces called mRNA. These elements work as recipes for building the proteins the virus needs. If they are present, the virus is alive.

“In the total mRNA sequenced from the samples, we found the same markers as in the total DNA, so we know that they have been transcribed. This means that the viruses are alive and active on ice,” explains Perini.

DNA and RNA in viruses

At the center of giant viruses is a cluster of DNA. This DNA contains all the genetic information or recipes needed to create proteins, the chemical compounds that do most of the work of the virus.

But to use these recipes, the virus must transcribe them from double-stranded DNA into single-stranded mRNA.

Normal viruses can’t do that. Instead, they contain strands of RNA floating around the cell, waiting to be activated when the virus infects an organism and hijacks its cellular production facilities.

Giant viruses can do this themselves, which makes them very different from normal viruses.

While DNA from dead viruses can be found in samples, mRNA is broken down much more quickly. mRNA is therefore an important marker of viral activity. In other words, the mRNA recipes for certain proteins show that viruses are indeed alive.

I don’t know exactly how they work

Because giant viruses are a relatively new discovery, not much is known about them. Unlike most other viruses, they have many active genes that allow them to repair, replicate, transcribe and translate DNA.

But it is not clear why and why they use it exactly.

“We can’t make an exact link to the hosts infected by the giant viruses. Some of them can infect protists while others attack snow algae. We just can’t be sure yet,” explains Perini.

She is working hard to learn more about giant viruses and more research will be published soon.

“We continue to study giant viruses to learn more about their interactions and what exactly their role is in the ecosystem. Later this year, we will publish another scientific study with more information about giant viruses infecting cultivated microalgae thriving on the surface of the ice of the Greenland ice sheet,” she concludes.