Marine viruses, including those found in the Arctic Ocean (pictured), help the ocean take up carbon from the atmosphere.Credit: Parshina Olga/Getty

The world’s oceans harbour nearly 200,000 virus species — two orders of magnitude more than scientists had previously recorded, according to a survey of marine microbes. Researchers also found an unexpected pocket of viral diversity in the Arctic Ocean.

The results1, published on 25 April in Cell, provide scientists with a foundation for understanding how viruses affect marine ecosystems — including the effect they have on the way organisms interact and the ocean’s response to climate change.

Every spoonful of seawater is filled with millions of viruses. And although most are harmless to people, they can infect a variety of marine life such as whales, crustaceans and bacteria. Mapping viral biodiversity will provide a more accurate depiction of what’s happening in the ocean and enable researchers to better predict its future, says Ahmed Zayed, a microbiologist at the Ohio State University in Columbus and co-lead author on the study.

A viral hotspot

Researchers collected seawater samples from nearly 80 sites around the world between 2009 and 2013, from surface waters to depths of 4,000 metres. The effort was part of two larger projects, called Tara Oceans and Malaspina, which study carbon dioxide and climate change in Earth’s oceans. A previous analysis2 of marine viruses by these missions had identified more than 15,000 species.

Zayed and his colleagues analysed the viral DNA in the latest samples and separated the sequences into ‘viral populations’ — the closest equivalent of species for viruses. They found nearly 200,000 populations in five ocean zones around the globe, with the species in each zone comprising their own viral community.

The most diverse communities were in temperate and tropical surface waters, as well as in the Arctic Ocean — a geographically and politically difficult region to access, and one of the most at risk from climate change.

Using this new map of virus diversity, scientists could manipulate specific areas of the ocean to boost the viral community’s ability to move carbon dioxide from shallow waters into the deep ocean, says Matthew Sullivan, a microbiologist at Ohio State University and senior author on the study. The oceans absorb half of the carbon dioxide that people pump into Earth’s atmosphere. And previous analyses3 have found that marine viruses can help to drive carbon in the ocean’s surface waters to the depths, locking it away from the atmosphere.

It’s scary to consider engineering the ocean like this, Sullivan says. “But we need to be thinking about seriously impactful ways to stem the coming climate issues we are facing.”

The tip of the iceberg

The revelation that viruses form communities in the world’s oceans emphasizes how little we know about them, says Curtis Suttle, a marine virologist at the University of British Columbia in Vancouver. And even though researchers now have a massive amount of data on ocean viruses, there are still many unsampled regions of the sea, he adds. Untapped areas include vast portions of the western Indian Ocean and the eastern Pacific Ocean.

Other researchers want more data to flesh out the question of viral diversity in the world’s oceans. The project team sampled Arctic waters repeatedly in a small area over six months, whereas samples other regions came from single-collection trips, says Rebecca Vega Thurber, a microbiologist at Oregon State University in Corvallis. It’s remarkable that the Arctic was a diversity hotspot, she says, but repeated sampling could reveal greater diversity at other locations.

Despite that, Vega Thurber is excited about this latest marine-virus data set. The study researchers “are creating the encyclopaedias that we need to look through to understand what we’re studying”, she says.