Source: French to English Tester Published on: 2026-04-29
Source: The Conversation – in French– By Thomas M. Pitot, Postdoctoral Researcher, University of Quebec at Chicoutimi (UQAC)
Long ignored due to their size, giant viruses are now redefining the frontiers of microbiology. At the heart of Arctic ecosystems, they are not mere parasites but true conductors of microbial life and planetary elemental cycles.
Giants long unnoticed
Viruses are major players in the functioning of ecosystems, where they profoundly influence the dynamics of microbial communities, matter fluxes, and global biogeochemical cycles. Yet, despite their abundance and ecological importance, a large part of the virosphere has long remained invisible to science.
This gap is largely explained by the historical methodological approaches of environmental virology. The study of viruses requires collecting natural samples and then isolating the viruses present. For this, it is necessary to separate them from other cellular organisms, which are generally 10 to 100 times larger. Traditionally, the method used to isolate viruses is thus based on size: a filter is used to retain larger organisms, in order to keep only the viruses.
This approach, effective for the majority of viruses known at the time, however led to the unintentional exclusion of very large viruses, whose existence was unknown. These organisms thus remained invisible to so-called “traditional” virology.
An unexpected discovery
It is in this context that, at the beginning of the 2000s, an atypical virus was isolated by chance. It was initially mistakenly identified as an obligate intracellular bacterium of the amoeba.Acanthamoeba polyphaga. Due to its resemblance to a microbe, it is namedMimivirusformicrobe-mimickingvirus in English and will be registered with theInternational Committee on Taxonomy of Virusesunder the species nameAcanthamoeba polyphaga mimivirus, then renamedMimivirus bradfordmassiliensein 2024.
This discovery marks the emergence of a new group of viruses called giant viruses,Nucleocytoviricota. These viruses are distinguished by their exceptional size, comparable to that of small bacteria, and by massive DNA genomes that can reach up to 2.5 million base pairs, the elementary units of DNA, encoding hundreds to thousands of genes from all domains of life.
Unlike bacteriophages, which regulate bacterial populations, giant viruses infect a wide variety of microalgae and small zooplankton. Since their identification, these viruses have profoundly disrupted our understanding of the very nature of viruses, challenging the boundary between living and non-living, as well as their level of dependence on the hosts they infect. Some giant viruses possess part of their own replication machinery, which allows them to carry out most of their reproduction cycle in the cytoplasm of the host cell.
Today, the democratization of DNA sequencing techniques, the implementation of aspecific taxonomic structureand the development ofbioinformatics toolsfor the detection of these viruses have demonstrated the important distribution and great diversity of giant viruses in very many ecosystems. Research has shown that they play a major role in the functioning and microbial dynamics on a planetary scale.
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Polar ecosystem engineers
The structure of polar food webs, often described as “truncated,” amplifies the ecological influence of these viruses. In the aquatic or frozen habitats of the poles, in the absence of large multicellular predators, life is dominated by unicellular microorganisms. Protists and microalgae play central roles there but are also the favored hosts of giant viruses, which thus position themselves at the top of the food pyramid.
But these viruses are not mere parasites. They act as true biogeochemical engineers through two key mechanisms:
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The viral short-circuit (dérivation virale)Â: By causing the lysis of their hosts, they massively release dissolved and particulate organic matter into the environment. This process reinjects nutrients directly into the microbial loop, thus supporting local microbial productivity.
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Metabolic reprogramming: Thanks toauxiliary metabolic genes(AMG), giant viruses actively modulate the physiology and metabolic activity of their host during infection. They thus seem able to optimize nutrient acquisition, manipulate lipid synthesis to maintain membrane fluidity, or even influence the host’s energy production.
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Parasitized giant viruses
However, the predominant influence of giant viruses at the poles is itself regulated by another, more discreet actor: the virophages (Lavidaviridae). These small viruses can only multiply by parasitizing the “viral factories” created by giant viruses (from the family ofMimiviridae) Inside infected host cells. By diverting resources intended for the production of new giant viruses, virophages reduce the infection capacity and their production of virions (the free form of the virus).
This mechanism of “parasite of a parasite” introduces additional complexity into the microbial loop and the viral shunt. This has major consequences on the stability of ecosystems. For example, models based on the Organic Lake system in Antarctica show that the presence of theOLV virophagereduces the mortality of microalgae. Paradoxically, it allows for more frequent algal blooms by limiting the virulence of giant viruses, thus acting as a stabilizer of the trophic network.
Even more surprising, some virophages can integrate directly into the genome of the microbial host in the form of endogenous viral elements (EVEs). These genetic sequences remain dormant until the cell is infected by a giant virus. They then reactivate to hinder viral replication, functioning like a trueantiviral defense system acquired by the host over the course of evolution.
These complex interactions between hosts, giant viruses, and virophages reveal a layer of biological regulation essential to the resilience of extreme environments. It is in this context of extreme dependence on cryospheric conditions that certain polar environments, such as theLast ice refuge(Last Ice Area), become unique reservoirs of viral diversity still largely unexplored.
A sanctuary of viral endemism
TheLast ice refuge(LRG) is the region of the Arctic Ocean expected to retain its multi-year sea ice longer than any other northern region in the face of current climate warming. Located along the northern coasts of Greenland and the Canadian Arctic Archipelago, the LRG is characterized by thethickest and oldest ice of the Arctic Ocean. This region is considered a future climate refuge for ice-dependent organisms and a bastion of long-term cryospheric stability.
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Along the terrestrial margin of the last ice refuge (LIR) lies a narrow coastal strip comprising various freshwater systems perpetually covered by ice (epi-platform lakes, ice-dammed lakes, meromictic lakes), fjords, coastal bays, and marginal terrestrial habitats. These systems are protected from changes by the persistent cold conditions maintained by the LIR.
They have experienced centuries, even millennia of uninterrupted cold, minimal hydrological connectivity, and extreme geographical isolation leading toextreme rates of endemism in giant virus populations. In these systems, they are distributed according toprecise ecological nichesdictated by gradients of light, oxygen, and salinity, demonstrating a fine adaptation to the extreme constraints of the Arctic.
As a refuge of ancient cryospheric conditions, the MDRG offers a natural laboratory to understand how viruses and their hosts developed and evolved under stable cold regimes.
Finally, it also serves as a climate sentinel: Arctic amplification and rapid warming threaten the perennial ice covers and the stratified water columns that maintain the isolation of the unique lakes of the MDRG, as well as the stability of the surrounding glaciers. Their rapid degradation risks causing the loss of unique microbial communities. The breakdown of these physical barriers could trigger a rapid ecological restructuring, a loss of endemic diversity, and long-term changes in the biogeochemical functioning of High Arctic ecosystems.
Catherine Girard has received funding from NSERC, NFRF, and Genome Quebec.
Thomas M. Pitot does not work for, advise, own shares in, receive funds from any organization that could benefit from this article, and has declared no affiliation other than his research institution.
–ref. Extraordinary viruses that orchestrate life at the poles –https://theconversation.com/extraordinary-viruses-orchestrating-life-at-the-poles-278366