Giant viruses discovered on the Greenland ice sheet could reduce ice melting

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

But animals aren’t the only life reawakened by the spring sun. Algae that lies dormant on the ice begins 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. Increasing melting worsens global warming.

But researchers may have found a way to control snow algae growth — and perhaps prevent some of the ice from melting in the long run. Postdoc Laura Perini from the Department of Environmental Sciences at Aarhus University and her colleagues have found giant viruses on the ice next to the algae. Their findings are published in the journal Microbiome.

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

“We don’t know much about the viruses, but I think they could be useful as a way to reduce ice melt caused by algae blooms. We don’t know yet how specific they are and how efficient it would be. By investigating them further, we hope to be able to answer some of those questions,” she says.

Bigger than bacteria

Viruses are normally much smaller than bacteria. Regular viruses are 20-200 nanometers in size, while a typical bacterium is 2-3 micrometers in size. In other words: a normal virus is about 1000 times smaller than a bacterium.

However, that is not the case with giant viruses. Giant viruses grow to a size of 2.5 micrometers. That’s bigger than most bacteria.

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

Never found on the 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 it is the first time that giant viruses have been found living on the surface ice and snow, dominated by microalgae, Perini explains.

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

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

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

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

“The way we discovered 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 show high similarity to known giant viruses,” she explains out.

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 sequences of DNA that form genes are activated, they are transcribed into single-stranded pieces called mRNA. These pieces work like 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 they are transcribed. It means the viruses are alive and active on the ice,” says Perini.

DNA and RNA in viruses

At the center of the giant viruses is a cluster of DNA. That DNA contains all the genetic information or recipes needed to make proteins – the chemical compounds that do most of the work in the virus.

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

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

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

Where DNA from dead viruses can be found in samples, mRNA is broken down much faster. mRNA is therefore an important marker of viral activity. In other words, mRNA recipes of certain proteins show that the viruses are alive and well.

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 that is and what exactly they use it for is not known.

“We can’t exactly link which host the giant viruses infect. Some of them may infect protists, while others attack snow algae. We just don’t know for sure yet,” says Perini.

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

“We continue to study the 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 some more information about giant viruses infecting cultivated microalgae that thrive on the surface ice from the Greenland ice sheet,” she concludes.