Giant viruses discovered on Greenland ice


Part of the Greenland ice sheet with ice blackened by algae.

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Photo credit: Shunan Feng

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

But the animals are not the only creatures that are brought back to life by the spring sun. Algae resting on the ice also begin to bloom in spring and blacken large areas of ice.

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

But researchers may have found a way to control the growth of snow algae – and thus perhaps prevent the ice from melting in the long term. Postdoctoral researcher Laura Perini from the Institute of Environmental Sciences at Aarhus University and her colleagues have discovered giant viruses on the ice in addition to the algae.

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

– We don't know much about the viruses, but I think they could be useful in slowing down the melting of ice caused by algal blooms. How specific they are and how efficient they would be, we don't yet know. But through further research we hope to be able to answer some of these questions, she says.

Bigger than bacteria

Viruses are usually much smaller than bacteria. Normal viruses are 20-200 nanometers in size, 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 be up to 2.5 micrometers in size. That is larger than most bacteria.

But giant viruses are not only bigger. Their genome is also much larger than that of normal viruses. Bacteriophages – bacteria that infect viruses – have between 100,000 and 200,000 letters in their genome. Giant viruses have about 2,500,000.

Never before found on the ice

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

But it is the first time that giant viruses have been discovered living on ice and snow surfaces dominated by microalgae, explains Laura Perini.

– We analyzed samples from dark ice, red snow and melt holes (cryoconite). In both dark ice and red snow we found traces of active giant viruses. And this is the first time they have been found on surface ice and snow, which contain a large amount of pigmented microalgae.

Only a few years ago, this part of the earth was considered desolate and lifeless. But today we know that numerous microorganisms live there – including 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, various species of fungi that parasitize them and the giant viruses that we have found that infect them.

– To understand the biological controls on algal blooms, we need to study these last three groups.

Didn't see it with the naked eye

Although the viruses are huge, they cannot be seen with the naked eye. Laura Perini has not even seen them with a light microscope. But she hopes to be able to do so in the future.

– We discovered the viruses by analyzing all the DNA in the samples we took. By searching this huge dataset for certain marker genes, we found sequences that bear 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, they also extracted all of 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 serve as blueprints for the proteins the virus needs. When 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 were transcribed. This means that the viruses are alive and active on the ice, she says.

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 the virus.

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

Normal viruses cannot do this. Instead, their cells have strands of RNA floating around, just 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 detected in samples, mRNA is broken down much more quickly. mRNA is therefore an important marker for the activity of viruses. In other words, mRNA recipes for certain proteins show that the viruses are alive and well.

I'm not sure how they work exactly

Since 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 and transcribe DNA.

But why this is so and what exactly they use it for is not known.

– We cannot say exactly which hosts the giant viruses infect. Some of them may infect protists, while others attack the snow algae. We simply cannot say for sure yet, says Laura Perini.

She is working hard to find out more about the giant viruses and will publish further research results shortly.

– We are continuing to study the giant viruses to learn more about their interactions and their exact role in the ecosystem. Later this year, we will publish another scientific study with more information about giant viruses infecting a cultured microalgae thriving on the surface ice of the Greenland ice sheet, she concludes.

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