How giant viruses engineer extreme polar ecosystems

Thomas M Pitot and Catherine Girard

Viruses play a major role in ecosystems, profoundly influencing microbial community dynamics, matter flow and global biogeochemical cycles. Yet despite their abundance, many long remained invisible because environmental virologists traditionally isolated them by filtering out larger organisms.

This changed in the early 2000s with the accidental discovery of an atypical, microbe-mimicking virus, eventually named Mimivirus bradfordmassiliense. This initiated the discovery of “giant” viruses, called Nucleocytoviricota. Distinguished by their exceptional size, similar to small bacteria, and massive DNA genomes, research now reveals these entities are essential to the resilience of extreme polar environments.

Giant viruses infect a wide variety of microalgae and small zooplankton. Challenging the boundary between the living and non-living, some even carry their own replication machinery to conduct most of their reproductive cycle within the host cell. Today, advanced DNA sequencing and bioinformatics tools have demonstrated their widespread distribution.

In the aquatic or frozen habitats of the North and South Poles, life is dominated by single-celled microorganisms due to the absence of large multicellular predators. Protists and microalgae play central roles here, but they are also the preferred hosts of giant viruses, which sit at top of the food pyramid.

These viruses act as biogeochemical engineers via two key mechanisms: By causing host cell breakdown, they release massive amounts of organic matter, feeding nutrients back into the microbial cycle to support local productivity. And through auxiliary metabolic genes, they modulate host physiology, optimising nutrient acquisition and influencing energy output during infection.

This dominant influence is regulated by virophages. These small viruses replicate by parasitising the “viral factories” created by giant viruses inside host cells, reducing the giant viruses’ ability to infect. This parasite parasitism introduces an additional layer of complexity. Modelling from the Organic Lake in Antarctica shows that virophages reduce microalgal mortality, allowing for more frequent algal blooms by limiting giant virus virulence, thereby stabilising the food web. Certain virophages even integrate into a microbial host’s genome, reactivating during a giant virus attack to function as antiviral defence system.

These interactions make polar environments, like the Last Ice Area, unique reservoirs of viral diversity. Located along the northern coasts of Greenland and the Canadian Arctic Archipelago, this region is expected to retain its multi-year sea ice longest against global warming, serving as a climate refuge for ice-dependent organisms.

Along this margin lies a coastal strip of permanently ice-covered freshwater systems, fjords and bays. These systems have experienced millennia of stable cold conditions and extreme isolation. This region serves as a climate sentinel. Rapid warming threatens the perennial ice cover and stratified water columns that maintain isolation. The breakdown of these physical barriers could trigger rapid ecological restructuring, a loss of unique microbial communities and long-term changes in Arctic ecosystems.