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The word virus conjures up hard-to-shake colds and swine flu, but these tiny microbes are far more than just the culprits behind annoying human ailments. They play an integral role in life on the planet, and a new study has shown that for marine viruses, that role is more complex than we thought.

Viruses are the most numerous life forms on Earth. And in the ocean, they outnumber bacterial cells a staggering 10 to one. But what exactly causes them to thrive or die? Since viruses reproduce by infecting the cells of other organisms (of all sizes, but mostly microbes), and each type of virus infects very specific hosts, it’s logical to assume the more hosts available to infect, the higher the number of viruses. But no one has actually studied the relationship between the ocean environment and virus abundance, says Curtis Suttle, an oceanographer at the University of British Columbia. So Suttle and his colleagues* delved into this question and published their findings in the journal Viruses.

The team counted viruses and their hosts in Canada’s temperate and polar seas. At the same time, they measured environmental variables such as temperature, salinity, oxygen levels, and available nutrients. They found that the physical environment has as much influence over viral abundance as the number of hosts available to infect, a finding that has big implications in a changing ocean.

Viruses have a simple structure—essentially a protein capsule filled with genetic material (DNA or RNA). They’re such oddities that researchers still debate whether they should be considered a life form. Yet viruses are a critical part of the ocean’s ecosystem.

Viruses help maintain marine biodiversity by keeping populations in check: in particular, bacteria and algae, which make up the vast majority of living matter in the ocean. “Viruses are probably responsible for killing about 25 to 30 percent of the living material in the oceans every day,” says Suttle. “The wheels in the ocean spin very quickly in terms of life and death.”

Viruses also perform a key service called the “viral shunt.” When they infect and kill organisms, nutrients such as carbon and phosphorous are released into the ocean. This keeps microbes and phytoplankton thriving. “If you take the viruses out of seawater, photosynthesis stops,” says Suttle. And photosynthesis by marine algae and phytoplankton is responsible for over half the oxygen on the planet. So viruses may be tiny, but the scale of the services they perform is global.

As the climate changes, temperature, salinity, and other physical variables will change too. Suttle says the next step will be to look at these changing ecosystems and the virus-host relationship in even closer detail.

“It’s a very impressive amount of work,” says Jed Fuhrman, a marine biologist at the University of Southern California. He says that with only this preliminary data, it’s not yet possible to predict the specific effects of climate change. “But it’s certainly not premature to do the research,” he says.

The more we untangle the complex relationships between individual viruses, their hosts, and their environment, says Fuhrman, the better. After all, viruses are an absolutely integral part of life—which may be cold comfort the next time you’re up all night sneezing.

*The research of Curtis Suttle and his colleagues was funded in part by a grant from the Tula Foundation, which also funds Hakai Magazine and the Hakai Institute. The magazine is editorially independent of the institute and foundation.