Viruses—already the bane of human existence—may also be having an impact on Earth’s climate. A new study reveals that the pathogens steal energy from ocean bacteria, preventing them from sucking up the greenhouse gas carbon dioxide. As a result, viruses could be responsible for billions of metric tons of extra carbon dioxide every year, though the impact on the environment is unclear.

“I think it's a great study,” says Adam Martiny, a microbial ecologist at the University of California, Irvine, who was not involved in the work. “It adds to this growing notion that to understand the global carbon [cycle], there's just a lot of different biological interactions.”

The ocean is full of microbes that breathe in carbon dioxide and get their energy from the sun. In fact, by converting this carbon into the building blocks of bodies, so-called cyanobacteria and other ocean microbes sequester—or “fix”—about half of the carbon dioxide on Earth.

But viruses can impact this process. When they attack cyanobacteria, they inject their genetic material, turning the microbes into virtual virus factories. The injected DNA includes photosynthetic genes, suggesting that the viruses—known as cyanophages—might be changing how the bacteria process carbon.

To find out whether that’s the case, scientists at the University of Warwick in Coventry, U.K., infected one of the smallest and most abundant photosynthetic microbes on the planet—Synechococcus—with cyanophages from either the English Channel or the Red Sea. They fed the bacteria baking soda, or sodium bicarbonate, which served as a source of CO 2 . By using radioactive carbon in the baking soda, the researchers were able to track precisely how much carbon dioxide the bacteria were sequestering—or “fixing”—over time.

Several hours after the scientists introduced the viruses, carbon fixation stalled, they report this month in Current Biology . Bacteria infected with the Red Sea and English Channel phages fixed 4.8 and 2.3 times less carbon, respectively, than uninfected hosts. Carbon fixation fell no matter how much light was hitting the bacteria.

Critically, the team found that the reduction in carbon fixation didn’t stem from changes early in the photosynthesis process, when cyanobacteria use sunlight to generate energy. Infected cells converted sunlight into energy just as well as uninfected cells. Instead, it appeared that the viruses interfered during the later stages, when the bacteria normally use the energy from the sun to fix inorganic carbon by transforming it into sugars. By redirecting that energy toward their own reproduction, the viruses block the bacteria’s ability to fix carbon.

“Viruses are just selfish machines,” says Dave Scanlan, a marine microbiologist at the University of Warwick, and one of the study’s senior authors. “The energy that's being made is being siphoned off by the virus to make more of itself.”

The team estimates that the cyanophages are preventing the fixation of between 20 million and 5.39 billion metric tons of carbon each year. At its upper end, that would be equivalent to about 10% of the carbon fixed every year by the entire ocean, or 5% of the carbon fixed globally. The true number depends on how many bacteria are infected at any one time—something scientists don’t yet have a good handle on. Previous studies indicate that anywhere from 1% to 60% of cyanobacteria in the ocean could be infected at once.

There are other questions as well. For example, how might different viruses affect other species of cyanobacteria in the open ocean? “This is a lab study of a very specific virus and a very specific host cell,” Martiny says. “How that translates to an ocean environment I think is a big unknown.”