The oceans contain almost 200,000 different viral populations, according to the latest count.
Though most are harmless to humans, they can infect marine life, including whales and crustaceans. And scientists are only just starting to understand how these tiny organisms play a role in the life and chemistry of the seas.
The viruses fall into just five groups based on their location and depth.
"When we examined the genes of the viruses in each of those communities, we found evidence of genetic adaptation to the different zones of the ocean," said researcher Ann Gregory, now of KU Leuven in Belgium.
The second surprise was that the Arctic Ocean had lots of different types of viruses. It had been thought that hotspots for microbial diversity would be at the equator.
What do viruses do in the oceans?
According to previous discoveries there are "giant" marine viruses, which can infect green algae. A litre of seawater typically contains billions of viruses - the vast majority of which remain unidentified. In the latest dataset, 90% of the populations could not be classified to a known group.
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