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Every spring, when the sun rises in the Arctic after months of darkness, life returns. Polar bears emerge from their winter lairs, arctic terns rise back from their long journey south, and musk oxen wade north.

But animals are not the only life reawakened by the spring sun. Algae dormant on the ice begin to bloom in the spring, blackening large areas of the ice.

When the ice turns black, its ability to reflect the sun decreases and this accelerates the melting of the ice. Increased melting worsens global warming.

But researchers may have found a way to control the growth of snow algae—and perhaps, in the long run, reduce some of the ice melting. Living on the ice alongside the 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 Microbiome.

She suspects that viruses feed on snow algae and may work as a natural control mechanism on algal blooms.

“We don’t know much about viruses, but I think they could be useful as a way to alleviate ice melting caused by algal blooms.” We don’t yet know how specific they are and how effective it would be. by investigating them further, we hope to answer some of these questions,” she says.

Bigger than bacteria

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

However, this is not the case with giant viruses. Giant viruses grow up to 2.5 micrometers. This is larger than most bacteria.

But giant viruses aren’t just bigger in size. Their genome is much larger than ordinary viruses. Bacteriophages – the bacteria that infect viruses – have between 100,000 and 200,000 letters in their genome. There are about 2,500,000 giant viruses.

It has never been found on ice before

Giant viruses were first discovered in 1981 when researchers found them in the ocean. These viruses specialize in infecting green algae in the sea. Later, giant viruses were found in soil on land and even in humans.

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

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

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

“There’s a whole ecosystem around the algae. In addition to bacteria, filamentous fungi and yeasts, there are protists that eat the algae, various types of fungi that parasitize them, and the giant viruses that we discovered that infect them. To understand the biological controls acting on algal blooms, we need to study the latter three groups,” Perini continues.

Although viruses are gigantic, they cannot be seen with the naked eye. Perini hasn’t even seen them under a light microscope yet. But she hopes to do so in the future.

“The way we found the viruses was by analyzing all the DNA in the samples we took. By sifting through this huge data set, looking for specific marker genes, we found sequences that have a high similarity to known giant viruses,” she explains.

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

When the DNA sequences that make up genes are activated, they are transcribed into single-stranded pieces called mRNA. These parts work as recipes to build the proteins the virus needs. If there are, 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 they are transcribed. This means that the viruses live and are active on the ice,” says 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 make proteins, the chemical compounds that do most of the work in a virus.

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

Normal viruses cannot do this. Instead, they have strands of RNA that float around the cell, waiting to be activated when a virus infects an organism and hijacks its cellular production facilities.

Giant viruses can do this on their own, which makes them very different from normal viruses.

While DNA from dead viruses can be found in samples, mRNA degrades much faster. Therefore, mRNA is an important marker of viral activity. In other words, the mRNA-recipes of certain proteins indicate that the viruses are alive and working.

I’m not sure exactly how they work

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

But why this is so and what exactly they use it for is unknown.

“Which hosts the giant viruses infect, we can’t exactly link.” Some of these may infect protists, while others attack snow algae. We just can’t be sure yet,” says Perini.

She is working hard to discover more about the giant viruses and more research will be coming soon.

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