The scale of the survey is what sets it apart. By sampling lakes spanning 30 degrees of latitude, the team captured an extraordinary range of environmental conditions, from the sun-drenched, nutrient-rich waters of the Mediterranean to the cold, dark basins of the Arctic Circle. From these samples, the researchers recovered 208 Nucleocytoviricota genomes, of which 205 represent putative new species. That figure alone signals how little is still known about viral diversity in inland waters. In addition to the genomes, the study identified 33 novel virus-host linkages, meaning the researchers were able to connect specific giant viruses with the eukaryotic organisms they likely infect, a notoriously difficult task in viral ecology.

Nucleocytoviricota are a phylum of large DNA viruses that replicate, at least in part, inside the nucleus or cytoplasm of eukaryotic cells. They are called giant viruses because their genomes, and in some cases their physical particles, rival those of small bacteria. Many of them infect algae and other single-celled eukaryotes that form the base of aquatic food webs. When these viruses lyse, or burst open, their hosts, they release carbon and nutrients back into the water, a process known as the viral shunt that redirects energy away from grazing food chains and toward microbial decomposition. Understanding where these viruses thrive and whom they infect is therefore central to predicting how lake ecosystems store or release carbon.

One of the clearest patterns to emerge from the study is a strong latitudinal gradient. The relative abundance of Nucleocytoviricota nearly triples, on average, from northern to southern Europe. This means that lakes in Italy and other southern regions carry substantially more giant viral genetic material, relative to the total microbial community, than lakes in Scandinavia. The gradient is not merely a matter of how many viruses are present. The researchers also found that functional genes related to autotrophic energy acquisition, stress responses, and cell death are predominant at lower latitudes, suggesting that southern giant viruses carry a different toolkit for manipulating their hosts than their northern counterparts.

These functional shifts appear to be driven by differences in light and resources between northern and southern lakes. Sunlight fuels photosynthesis, which in turn produces the oxygen and organic matter that both algae and their viruses depend upon. In southern lakes, where light is abundant and growing seasons are longer, viruses that can hijack their hosts’ photosynthetic machinery or modulate programmed cell death may gain a competitive advantage. Genes involved in autotrophic energy acquisition, for example, are thought to help viruses maintain host metabolism during infection, keeping the cellular factory running long enough to produce more viral copies. Stress-response genes may help both virus and host cope with the higher temperatures and variable conditions of southern climates.

The study also pinpointed the environmental conditions that create hotspots of giant viral diversity. Nucleocytoviricota were most abundant and rich in deep lakes containing aromatic organic matter and diverse eukaryote communities. Aromatic organic matter, which comes largely from plant material decomposing in the surrounding catchment, absorbs light and shapes the underwater light environment, potentially favoring certain viral strategies. Deep lakes, meanwhile, offer stratified water columns with distinct habitats, from sunlit surface layers to dark, oxygen-poor depths, providing more niches for both viruses and their eukaryotic hosts. The link with diverse eukaryote communities makes intuitive sense: more host species means more opportunities for viral specialization and coexistence.

Perhaps the most consequential finding concerns the relationship between giant viruses and the fundamental productivity of lakes. The researchers found that increasing Nucleocytoviricota abundance and richness are associated with several-fold reductions in gross primary production, the total amount of carbon fixed by photosynthetic organisms in the ecosystem. Gross primary production underpins the entire aquatic food web, feeding everything from zooplankton to fish. A several-fold reduction associated with viral presence suggests that giant viruses are not passive passengers but active regulators of how much energy flows through lake ecosystems. If viruses suppress the algae responsible for photosynthesis, whether by infecting and killing them or by altering their metabolism, the effects could cascade upward through the food web and downward into the carbon budget of the lake.

The mechanisms behind this suppression remain an open question, but the study’s virus-host linkages offer clues. Many Nucleocytoviricota are known to infect algae, and the 33 novel linkages identified in this study likely connect additional giant viruses with eukaryotic hosts whose identities were previously unknown. As genomic techniques such as metagenomics and single-cell sequencing improve, researchers can increasingly pair viral genomes with host genomes recovered from the same water sample, inferring infection relationships from shared abundance patterns and other signals. This approach, applied across dozens of lakes, is what allowed the European survey to move beyond a simple census of viral diversity toward an ecological account of who infects whom and with what consequences.

The findings carry broader implications in a changing climate. As temperatures warm, lake stratification intensifies, growing seasons lengthen, and the supply of organic matter from surrounding land shifts, conditions that may favor the expansion of giant viruses northward or alter their functional repertoire. If Nucleocytoviricota abundance rises in lakes that are currently less virus-dominated, the associated reductions in gross primary production could reshape carbon cycling across vast regions of the boreal and Arctic zones, where lakes are numerous and increasingly affected by thawing permafrost and changing vegetation. The authors emphasize that understanding the roles of these viruses will be essential for predicting aquatic food webs under environmental change, a statement that underscores how far viral ecology has moved from the margins of biology toward its center.

For decades, viral research in freshwater systems focused on smaller bacteriophages, leaving the giant viruses of lakes largely unexplored. This study demonstrates that the phylum Nucleocytoviricota is not only present in inland waters but ecologically relevant, with distributions, gene repertoires, and ecosystem impacts that vary systematically across geography. The recovery of more than 200 genomes, most of them new to science, and the establishment of dozens of host associations provide a foundation for future work on viral biogeography, host manipulation, and carbon cycling in lakes. As the authors and their collaborators continue to analyze these data, the giant viruses of Europe’s lakes are likely to yield further surprises about the hidden architecture of freshwater food webs, and about the microscopic battles that quietly govern how much carbon the world’s lakes produce, consume, and release.

Subject of Research: Biogeography and ecological interactions of giant DNA viruses (Nucleocytoviricota) in European freshwater lakes

Article Title: The biogeography of giant viruses and interactions with Eukaryotes across European lakes

Article References: Kolmakova, O. V., Schulz, F., Vasquez, Y. M., Fonvielle, J. A., Braga, L. P. P., Roux, S., Woyke, T., & Tanentzap, A. J. (2026). The biogeography of giant viruses and interactions with Eukaryotes across European lakes. Nature Communications. https://doi.org/10.1038/s41467-026-77765-w

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77765-w

Keywords: giant viruses, Nucleocytoviricota, freshwater lakes, biogeography, viral ecology, eukaryotes, gross primary production, metagenomics, virus-host interactions, carbon cycling, latitudinal gradient, Nature Communications