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, arctic terns soar from their long journey south, and muskrats wade north.

But animals are not the only life that the spring sun reawakens. Algae that sleep on the ice begin to bloom in the spring, turning large areas of the ice black.

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

But scientists may have found a way to regulate the growth of snow algae — and perhaps limit ice melt in the long run. Postdoctoral researcher Laura Perini from the Department of Environmental Sciences at Aarhus University and her colleagues found the giant viruses living on the ice next to the algae. Their findings are published in a journal The microbiome.

He suspects that the viruses feed on snow algae and could act as a natural control mechanism for algal blooms.

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

Bigger than bacteria

Viruses are usually much smaller than bacteria. Common viruses measure 20-200 nanometers, while a typical bacterium is 2-3 micrometers. In other words, a normal virus is about 1000 times smaller than a bacterium.

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

But giant viruses aren’t just bigger. Their genome is much larger than normal viruses. Bacteriophages – virus-infecting bacteria – have 100,000 to 200,000 letters in their genome. Giant viruses have around 2,500,000.

Never found on ice before

Giant viruses were first discovered in 1981 when researchers found them in the ocean. These viruses specialized 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, which is dominated by microalgae, Perini explains.

“We analyzed samples from dark ice, red snow and melting holes (cryoconite). We found signs of active giant viruses in both dark ice and red snow. And this is the first time they have been found on surface ice and snow containing high amounts of pigmented microalgae.” he says.

A few years ago, everyone thought that this part of the world was barren and lifeless. 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 species of fungi that parasitize them, and giant viruses that we found that infect them. To understand the biological controls on algae blooms , we need to study these last three groups,” Perini continues.

Although viruses are giant, they are not visible to the naked eye. Perini had never even seen them under a light microscope. But he hopes to do so in the future.

“We discovered the viruses by analyzing all the DNA in the samples we collected. By going through this huge data set and looking for specific marker genes, we found sequences that have a high similarity to known giant viruses,” he explains.

To make sure 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 pieces act as recipes for building the proteins the virus needs. If they are present, the virus is alive.

“We found the same markers in the total mRNA sequenced from the samples as in the total DNA, so we know they were transcribed. This means the viruses are alive and active on 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.

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

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

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

While DNA from dead viruses can be found in samples, mRNA is cleaved at a much faster rate. mRNA is therefore an important marker of viral activity. In other words, the mRNA-recipes of certain proteins show that the viruses are alive and functioning.

Not sure 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 why this is so and what exactly they use it for is unknown.

“Which is the host of the giant viruses they infect, we can’t exactly link. Some of them may infect protists, while others attack snow algae. We simply can’t be sure yet,” says Perini.

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

“We are continuing 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 more information about the giant viruses infecting cultured microalgae thriving on the ice surface.” the Greenland ice sheet,” he concludes.