Viruses play a major role in the functioning of ecosystems. They profoundly influence the dynamics of microbial communities, flow of matter and global biogeochemical cycles. Yet despite their abundance and ecological importance, many of them have long remained invisible to science.

This gap is largely due to the methods environmental virologists have used —isolating viruses by filtering out larger organisms from natural samples.

This approach was effective for isolating most viruses we knew about. Until the early 2000s, when an atypical virus was isolated by chance. Because it resembled a microbe, it was named mimivirus, for “microbe-mimicking” virus. Initially registered under the species name Acanthamoeba polyphaga mimivirus it was renamed Mimivirus bradfordmassiliense in 2024.

This initiated the discovery of a whole new group of “giant” viruses, called the Nucleocytoviricota. They are distinguished by their exceptional size, similar to that of small bacteria, and by massive DNA genomes that can reach up to 2.5 million base pairs, encoding genes from all domains of life.

Research now reveals these viruses — previously invisible to so-called traditional virology — as essential to the resilience of extreme polar environments.

Diversity across ecosystems

Giant viruses infect a wide variety of microalgae and small zooplankton. They have profoundly transformed our understanding of the nature of viruses, challenging the boundary between the living and the non-living and the extent of their dependence on the hosts they infect.

Some giant viruses carry part of their own replication machinery, which allows them to carry out most of their reproductive cycle within the host cell.

Today, the widespread........

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