The world’s oceans are teeming with infection.
Each liter of seawater on this planet is home to about 100 billion viral particles, adding up to about a nonillion (in the U.S., that’s 1 followed by 30 zeros) worldwide. Lined up end to end, Earth’s marine viruses would stretch 10 million light-years beyond Earth, bypassing some 50 nearby galaxies and tumbling deep into interstellar space.
Viruses are arguably the most successful biological entities in the sea, where they outnumber microbes—their typical hosts—10 to one. But scientists still have no idea what most of them do, or even how many genetically distinct populations of viruses may speckle the seas.
That may soon change. In an unprecedented global survey of the viruses in Earth’s oceans, an international team of scientists has now expanded the number of known marine virus populations to nearly 200,000, most of which don’t match any previously characterized family of virus. Their research, published today in the journal Cell, sets the stage for work that could explain how marine viruses shape their surroundings—including, perhaps, the ways they affect our planet’s climate.
“This is very impressive work,” says Paul Turner, a virologist and evolutionary biologist at Yale University who was not involved in the study. “They’ve produced a hugely valuable data set here...that will be a tremendous resource for others to dive into.”
Three years ago, the most up-to-date catalog of marine viruses boasted 15,000 genetically distinguishable populations—and study author Matthew Sullivan, a viral ecologist at Ohio State University, felt pretty confident that the work was close to done.
At the time, Sullivan and his team had already amassed a hefty repository of viral DNA from seawater collected around the globe through the Tara Oceans Expedition. But when more powerful computational tools became available to assemble and analyze genomes, the researchers decided to re-analyze their data, alongside a set of previously untested samples from the Arctic and deep sea.
Sullivan quickly realized that his initial estimates would be dwarfed. On top of confirming most of the original 15,000 populations, the new and improved algorithms spat out more than 180,000 additions, bringing the grand total to 195,728.
And the vast majority of the newcomers bore no resemblance to any viral family that had been described before. “That’s stunning,” says Bonnie Hurwitz, a viral geneticist at the University of Arizona who wasn’t involved in the study. “This shows there’s really a lot for us to explore both in terms of the function of viruses, and how they’re affecting ecosystems.”
There was an additional challenge to orienting these viruses on their family tree: Defining a viral “species” remains contentious. In animals, species are often delineated by physical characteristics, genetic relatedness, or which organisms can (or can’t) reproduce with one another. These rules fall apart when it comes to viruses, which can take on deceptively similar appearances and frequently exchange genetic information with each other and their myriad hosts.
Still, viruses do tend to cluster by the number of genes they share. And by continuing to identify genetically distinct populations, Sullivan and his team hope to someday sharpen the boundaries between lineages. “This is the closest we’ve been to a sequenced definition of a viral population,” says Melissa Duhaime, a viral geneticist at the University of Michigan who conducted some of the virus work that emerged from the Tara Oceans project, but was not involved in the new study.
With these newly defined populations in hand, the team then mapped where viral communities had settled around the globe. Unsurprisingly, viruses were pretty much everywhere—but the spots they preferred weren’t the most obvious: Of the 180,000 new populations described, more than 40 percent hailed from the cold climes of the Arctic. This bucked the typical trend of biodiversity, wherein creatures tend to flock to the equator, steering clear of the frigid poles.
“This is one of the most striking features of this study,” Hurwitz says. “This cradle of diversity in the Arctic might be something that’s been overlooked in other studies.”
When the researchers next looked across ecosystems, they found that temperature had a big say in determining the number of viral populations that could thrive in a given region. This probably has something to do with how sensitive ocean microbes—the primary prey for these viruses—are to their environment, says study author Ann Gregory, a virologist at KU Leuven in Belgium. It’s also something to be wary of, given that global temperatures are increasing, she says. “Climate change could end up having a huge impact on viral diversity.”
That may not sound like a big deal. But viruses are constantly pulling the strings of their microbial hosts—and marine microbes are a veritable tour de force. By producing atmospheric oxygen, shuttling carbon away from the ocean surface, and forming the basis of countless food chains, the sea’s smallest living residents play critical roles in buffering the world against the effects of climate. Which means their viral puppet masters do, too.
When they manipulate the metabolisms of photosynthetic bacteria, viruses can ramp up the rate of carbon capture by seawater. Viral killing sprees also supply the sea with nutrients and distribute carbon into deeper waters, where it can’t escape into the atmosphere.
“If you care about the [air] you breathe, then you care about viruses too,” Sullivan says.
Viruses can even shape entire communities of microbes through ritual culling. Up to 40 percent of marine bacteria will lose their lives to a virus in a given day—a form of underwater warfare that turns out to be an important system of checks and balances, says Marcie Marston, a viral ecologist at Roger Williams University who was not involved in the study. “You get a much more diverse microbial community with viruses than you would if you didn’t have them.”
Viral diversity, then, is probably a decent barometer for microbial diversity—and, perhaps, ocean health overall. “Viruses are a huge player in the realm of earth systems, but they’re completely ignored in all models,” Hurwitz says. “Being able to include these data...means we can really start to understand the impact of global change.”
Someday, in the very distant future, this information could even aid interventions to abate the consequences of climate change, Sullivan says. “I can imagine trying to manipulate food webs [to address carbon cycling]...and I think viruses would be the lever upon which you could pull most effectively.”
This kind of environmental virus “therapy” is a long way off. First, researchers must understand communities well enough to “convince [themselves] what the viruses are, and how much health they bring to an ecosystem,” Turner says.
And bridging that gap could be tough. Scientists may still just be scratching the surface of the viral populations in ocean waters, Duhaime says. Given that samples from the Arctic alone contained such staggering diversity, future research in new locations (like parts of the Antarctic, which wasn’t sampled as heavily as the Arctic) will likely yield further shake-ups. The study also only looked at double-stranded DNA viruses, leaving entire branches of the viral tree, like RNA viruses and single-stranded DNA viruses, unexplored.
But in a way, those gaps make the future all the more exciting. “There is nothing like being right at the precipice of the known and unknown,” Duhaime says. “When you first begin looking at new data like this, it’s akin to landing on Mars and looking around for the first time...but a Mars with little critters never described before staring back at you.”