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A cryoelectron microscope image of a Mimivirus, the first and largest giant virus discovered

Xiao et al. (CC by 2.5)

Giant viruses aren’t alive. So why have they stolen genes essential for life?

Viruses might not be alive, but they may be altering life on a global scale. Researchers have found that a bizarre group of microbes known as “giant viruses” contain genes associated with metabolism, which they likely use to convert their zombified hosts into supercharged energy factories. Because many of their victims are important players in dampening climate change and in controlling ocean ecosystems, the megaviruses may be exerting an unexpected power over life on Earth.

There are more than 200,000 kinds of viruses in the world’s oceans. Some are giant viruses, so named because they tend to be about 10 times bigger than the average virus. They’re still tiny—the largest is only one-fifth the size of a red blood cell—which may explain why they went undiscovered until 2003. Since then, researchers have learned some basic facts—the viruses mostly infect amoebas and phytoplankton, for example—but scientists are still trying to figure out what makes them tick.

In the new study, microbiologists led by Frank Aylward of Virginia Polytechnic Institute and State University didn’t scour the oceans. Instead, they went hunting in public databases, scanning thousands of mostly marine genomes for the DNA fingerprints of giant viruses.

They extracted 501 suspected giant virus genomes, mapping them against 121 known reference genomes to create a family tree. Their results, published this month in Nature Communications, show that giant viruses are extremely diverse, splitting into 54 distinct groups. Several genomes were new to science and likely represent new species.

In addition to the usual genes that allow a virus to infect its host and multiply, many also contained genes for metabolism, the process that converts food into energy in all living cells. This was a surprise because viruses don’t eat. And strangely, these genes weren’t a recent addition, says first author Mohammad “Monir” Moniruzzaman: Many had been evolving in the viruses for millions of years. “If your goal is simply to find a new host and multiply,” he asks, why do you need these genes?

All of this could have a dramatic impact on sea life. Phytoplankton suck the greenhouse gas carbon dioxide from the atmosphere as they photosynthesize. They also form the base of the entire food web, the interconnected “who-eats-whom” relationships between predator and prey. Still, researchers are just beginning to study what the larger impacts could be.

Frederik Schulz, a microbiologist at the Joint Genome Institute, says the new work comes to “similar conclusions” as those found by his own team. But he cautions against speculating too broadly. Just because a gene helps living organisms metabolize, doesn’t mean it does the same thing for viruses.

Both groups agree that cultivating giant viruses in the laboratory would allow researchers to see what these genes really do. “As much as I can tell lots of stories,” Moniruzzaman says, “you really need to verify them in the lab.”