Viruses are usually imagined as tiny biological particles with relatively small genomes.
Giant viruses challenge that picture.
Some have unusually large genomes and infect complex single-celled organisms such as algae and other eukaryotic microbes. Although they have been studied extensively in oceans, much less is known about how they are distributed across freshwater ecosystems.
A study published in Nature Communications on 23 September 2026 provides one of the broadest surveys so far of giant viruses in European lakes.
Researchers sampled 87 lakes spanning roughly 30 degrees of latitude from southern Italy to northern Norway and reconstructed 208 genomes belonging to the viral phylum Nucleocytoviricota.
Of those genomes, 205 were classified as putative new species.
The researchers also identified 33 previously undescribed potential virus–host relationships and found that the relative abundance of giant viruses was, on average, almost three times higher in southern than northern European lakes.
Lakes with greater giant-virus abundance and richness also tended to show several-fold lower gross primary production, suggesting that these viruses may be important components of freshwater food webs.
Giant viruses are unusual members of the viral world
Giant viruses are members of a broad group of large DNA viruses that infect eukaryotic cells.
They are called “giant” because many have unusually large particles and genomes compared with more familiar viruses.
Their genomes can contain hundreds or even thousands of genes, including genes involved in processes that were once thought to be largely restricted to cellular organisms.
Some infect algae, protists and other microscopic eukaryotes that play major roles in aquatic ecosystems.
That makes giant viruses potentially important for nutrient cycling, microbial population dynamics and the movement of energy through food webs.
However, most large-scale ecological work on these viruses has focused on marine environments.
The new study asks how their distribution changes across freshwater systems exposed to very different climates, resource conditions and biological communities.
The researchers sampled lakes across a large part of Europe
The study covered 87 lakes stretching from southern Italy to northern Norway.
This geographical range allowed the researchers to compare freshwater environments across roughly 30 degrees of latitude.
The lakes differed in depth, light availability, organic matter, nutrient conditions and the diversity of eukaryotic organisms living in them.
Rather than attempting to grow every virus in the laboratory, the team used metagenomic sequencing.
Metagenomics allows researchers to sequence genetic material directly from environmental samples and reconstruct genomes belonging to organisms and viruses that may never have been cultured.
This approach is particularly useful for giant viruses because many infect hosts that are themselves difficult to cultivate.
Most of the reconstructed viral genomes appeared to represent new species
The researchers recovered 208 Nucleocytoviricota genomes from the lake samples.
Of these, 205 were considered putative new species.
That does not mean 205 newly named virus species have already been formally established.
“Putative” indicates that the genomes were sufficiently distinct to be treated as likely new species according to the study’s genomic criteria, but formal viral taxonomy involves additional classification processes.
The finding nonetheless illustrates how much freshwater giant-virus diversity remains undescribed.
A large proportion of the viral diversity present in ordinary lake water may therefore still be absent from existing reference databases.
Giant viruses became more abundant toward southern Europe
The strongest geographical pattern was a north-to-south increase in relative abundance.
On average, the relative abundance of Nucleocytoviricota nearly tripled from northern to southern Europe.
Latitude itself is unlikely to be the biological mechanism.
Moving across latitude also changes temperature, light regimes, productivity, dissolved organic matter and the organisms available as potential hosts.
The researchers therefore examined environmental conditions to understand what might be associated with the geographical trend.
The results pointed particularly to differences in light and resource availability.
Deep lakes with aromatic organic matter emerged as hotspots
Some lake characteristics were especially strongly associated with giant-virus communities.
Hotspots tended to occur in deeper lakes with more aromatic organic matter and diverse eukaryotic communities.
Aromatic organic matter includes complex carbon-rich compounds often derived from decomposing terrestrial or aquatic material.
These compounds can alter water colour, light penetration and microbial resource availability.
The association does not mean that aromatic organic matter directly causes giant-virus abundance to increase.
Instead, these environmental conditions may help structure the eukaryotic communities that the viruses depend on, while also changing the ecological conditions under which infection occurs.
The viral gene repertoire also changed with latitude
The researchers did not only examine how many viruses were present.
They also analysed the functions encoded in the recovered genomes.
At lower latitudes, genes linked to autotrophic energy acquisition, stress responses and cell death were more prominent.
This suggests that giant-virus communities are not simply becoming more abundant toward the south.
The functional capabilities carried by those communities also appear to shift alongside environmental conditions.
Some giant viruses carry genes that can influence host metabolism during infection.
These genes may help viruses exploit host cells under particular environmental conditions, although the presence of a gene in a viral genome does not automatically prove that it is expressed or ecologically important in every infection.
The researchers predicted 33 new virus–host relationships
Viruses cannot reproduce without hosts.
Understanding which eukaryotic organisms are infected is therefore central to understanding what giant viruses are doing in lakes.
The study identified 33 novel potential virus–host linkages.
These relationships were inferred from genomic and ecological patterns rather than established by directly infecting each candidate host in a laboratory experiment.
That distinction matters.
Predicted host associations can reveal promising relationships, but experimental infection remains the strongest way to confirm that a particular virus infects a particular organism.
Even so, the new links substantially expand the range of freshwater eukaryotes that may interact with giant viruses.
More giant viruses were associated with lower primary production
One of the most ecologically important findings involved gross primary production.
Gross primary production measures how much carbon is fixed by photosynthetic organisms before accounting for the carbon they use through respiration.
In lakes, much of that production is generated by algae and other photosynthetic microorganisms.
The researchers found that increasing giant-virus abundance and richness were associated with several-fold reductions in gross primary production.
This is consistent with the possibility that viral infection influences populations of eukaryotic microorganisms involved in photosynthesis.
However, the study does not establish that giant viruses directly caused the lower primary production.
The same environmental conditions could influence both viral communities and productivity, and the observational design cannot completely separate these possibilities.
Viruses could influence freshwater food webs through their hosts
If giant viruses infect abundant algae or protists, their ecological effects can extend beyond the infected cell.
When viral infection damages or kills a host, organic material can be released back into the surrounding water.
That material can become available to bacteria and other microorganisms rather than moving directly to larger consumers.
Changes in host abundance can also alter who eats whom and which organisms dominate the plankton community.
Giant viruses may therefore influence freshwater ecosystems indirectly by changing the abundance, metabolism or mortality of their hosts.
The new study does not quantify every one of these pathways, but it strengthens the case that giant viruses should be included when researchers model freshwater food webs.
The north–south pattern raises questions about environmental change
The geographical gradient is especially interesting because European lakes are already changing through warming, altered rainfall, nutrient inputs and shifts in dissolved organic matter.
If giant-virus communities respond strongly to light, resources and host composition, future environmental change could alter where particular viruses thrive.
That does not mean northern lakes will simply become copies of southern lakes as temperatures rise.
Latitude reflects many interacting environmental conditions, and the study does not provide a direct experimental forecast of how individual lakes will respond to climate change.
Its value is showing that giant-virus abundance and functional composition are strongly structured across large environmental gradients.
Metagenomics reveals diversity that traditional sampling can miss
Another important contribution is methodological.
Traditional virology often depends on isolating a host and then growing its virus in the laboratory.
That approach is powerful but biased toward organisms that can be cultured successfully.
Environmental sequencing can reveal viral genomes without first knowing which host to grow.
The recovery of 205 putative new species from 87 lakes demonstrates how much hidden viral diversity can become visible through this approach.
At the same time, genome reconstruction has limitations.
A reconstructed viral genome can show that genetic material is present, but it does not always reveal whether the virus was actively infecting a host at the moment of sampling.
The study is a snapshot across many lakes rather than a long-term experiment
The breadth of the study is one of its major strengths.
Sampling 87 lakes across a large geographical gradient allows researchers to detect ecological patterns that would be invisible in a single lake.
But breadth also comes with trade-offs.
The study compares different lakes under different environmental conditions rather than manipulating viral abundance experimentally.
Relationships between viral abundance, host diversity, organic matter and primary production should therefore be interpreted as ecological associations.
Long-term sampling and controlled experiments will be needed to determine how stable these relationships are and which mechanisms drive them.
The bigger lesson is that freshwater viruses are part of ecosystem function
Viruses are often discussed primarily in terms of disease.
In aquatic ecosystems, their role is much broader.
By infecting microorganisms that produce biomass, recycle nutrients and support food webs, viruses can influence the structure and function of entire ecosystems.
The new study shows that giant viruses are not rare curiosities confined to a few unusual environments.
They are widespread across European lakes, highly diverse and strongly associated with environmental conditions and ecosystem productivity.
Understanding freshwater ecology may therefore require paying much closer attention to some of its least visible inhabitants.
Source Information
Study Title: The biogeography of giant viruses and interactions with Eukaryotes across European lakes
Authors: Olesya V. Kolmakova, Frederik Schulz, Yumary M. Vasquez, Jeremy A. Fonvielle, Lucas P. P. Braga, Simon Roux, Tanja Woyke and Andrew J. Tanentzap
Journal: Nature Communications
Published: 23 September 2026
Dataset: Environmental metagenomic samples from 87 European lakes spanning approximately 30 degrees of latitude from southern Italy to northern Norway.
Method: The researchers reconstructed Nucleocytoviricota genomes from environmental sequencing data, compared their abundance and functional gene composition across lakes, inferred potential virus–host relationships and tested associations with environmental conditions and gross primary production.
Main finding: The study recovered 208 giant-virus genomes, including 205 putative new species and 33 novel potential virus–host linkages. Giant-virus relative abundance nearly tripled from northern to southern Europe, while greater abundance and richness were associated with several-fold lower gross primary production. Hotspots occurred particularly in deep lakes with aromatic organic matter and diverse eukaryotic communities.
DOI: 10.1038/s41467-026-77765-w
