Historic specimens highlight the key role viruses play in Arctic ecosystems

As we continue to manage the ongoing pandemic, the role that viruses play has never been more relevant. They are present in and attack pretty much every single living organism, from the biggest whale to the smallest bacteria.

This includes a type of blue-green algae known as cyanobacteria. These are microscopic organisms found in environments across the world and play a vital role as the basis of many food chains.

By studying the genomes of Nostoc cyanobacteria that live in the extreme polar regions, scientists are hoping to understand not only how they manage to survive, but also the role that viruses play in the ecology of these environments.

Dr Anne D. Jungblut is a microbial researcher at the Museum, who has been involved in a recent study looking into these organisms and published in Environmental Microbiology.

Along with her colleagues at Université Laval, Quebec City, Canada, Anne has peered into the DNA of Arctic cyanobacteria looking for evidence of viruses left behind in their genomes.

The team also looked for compounds known as secondary metabolites which help the microorganisms survive the extreme conditions, and are of great interest in the development of new medicinal drugs.

'This is the first time that anyone has looked in detail at this viral interaction in cyanobacteria,' explains Anne. 'It highlights how viruses are an integral part of the microbial ecology and the ecology of polar ecosystems. 

'One of our analyses also found a high number of secondary metabolites. These are of interest because they are often bioactive, and some people screen for them for use in developing new medicinal drugs.'

The study was also able to make use of historic cyanobacteria samples collected during the British Arctic Expedition, which took place between 1875 and 1876. Despite being collected over 100 years ago, the team were able to extract DNA and then compare it to modern samples to see if there had been any notable changes in the organisms over time.

Micro ecosystems in the polar regions

In the far north the polar environment is so extreme it is difficult for life to exist. But it still thrives, just not in a way most of us imagine.

The extremely limited sunlight and freezing temperatures in many habitats mean that plants cannot form the basis of the food chains. Instead, cyanobacteria take this role.

'They are blue-green algae and were the first organisms to produce oxygen on earth,' explains Anne. 'They are really important for the polar regions because they are able to live in extreme environments and at the limits of life in the polar regions, where most other photosynthetic organisms can't survive.

'Because of this they are the primary producers, meaning they are the basis of the food webs. Kind of like the trees of the ice shelves.'

The cyanobacteria use what little light is available to produce sugars and absorb nitrogen out of the air to produce proteins. When they die, their remains form the food for a whole host of other microscopic organisms, such as bacteria, fungi, amoeba, protists and ciliates. These in turn are then eaten by slightly larger - but still microscopic - creatures such as tardigrades, or water bears.

In some places this is where the food web stops and is known as a 'truncated microbial food web'. But in other locations, the tiny organisms are then eaten by animals such as miniature shrimp and these by fish, which will then go on to feed bigger fish, birds and mammals.

But an often-overlooked aspect of these food webs are the viruses. These go around infecting and killing the cyanobacteria, helping to provide food for the organisms in the next stage of the food chain.

'The viruses are part of the cycling of nutrients, but they also have an effect on the ecology and evolution of these cyanobacteria,' explains Anne. 'So if any one species of cyanobacteria is really abundant then the more likely it will be attacked by the virus and numbers will be brought down.

'The viruses can also transfer little parts of genes when they inject their own DNA. This means they can actually transfer genes that are useful and give the host organism an evolutionary advantage.'

Searching for evidence in DNA

Modern samples of cyanobacteria were compared with samples collected in 1876.

These were picked up by the British Arctic Expedition, which set sail from Portsmouth in 1875 in a bid to reach the North Pole. Led by Captain George Strong Nares, the expedition travelled up the east coast of the Canadian Arctic islands, where it got trapped in the ice. 

'It was one of those failed expeditions where they got scurvy and were stuck north of Ellesmere Island,' says Anne. 'They were trapped for the winter, and so made lots of scientific observations and collected biological samples.'

Over a hundred years later Anne and her colleagues were able to extract the DNA from some of these samples.

By looking into what is effectively the immune system of the cyanobacteria, they were able to see that the viruses that were attacking the microorganisms back in the 1876 were completely different to those which are targeting the modern ones.

'This is the first time that anyone has looked in detail at this viral interaction,' explains Anne. 'We originally had hoped to find the same viruses, but we didn't.

'That is not too surprising though, because viruses have a high turnover and they change a lot, as we see with the emergence of new COVID-19 variants.'

They were also able to show that polar and temperate cyanobacteria had genes that help them to respond to extreme conditions.

While there were no obvious genetic traits that were specific to the polar Nostoc cyanobacteria, they did have higher concentrations of some secondary metabolites. These could be of interest in the future to researchers who are developing new drugs.