For the first time, scientists discover organisms whose diets rely on eating viruses

Humanity loves a good villain, and few villains are as threatening and sinister as a virus. They are practically invisible, a brutal parasite that hijacks our own living cells, twisting their biological essence into more viral soldiers to keep up the massacre.

But viruses play an incredibly important role in the ecosystem. They are like the apex predators of the savanna, the lions that keep gazelle populations in check by thinning out their numbers.

This is the first time it has been shown that a virus-only diet can be enough for a microorganism to survive, a practice the researchers call “virovory,” meaning virus feeding.

For example, sometimes simple aquatic plants called algae will start breeding out of control, causing an algae bloom. Some blooms can be toxic, choking beaches or lakes and suffocating wildlife, often ballooning so large they can be seen from space. Blooms can occur naturally, but a variety of human activities, such as nutrient pollution from agricultural runoff or sewage discharge, can trigger a bloom or make it worse.

Then, along come the chloroviruses. Named for the Greek word for “green,” these are a large species of virus found everywhere in the world in freshwater environments. And they love to infect algae. Chloroviruses actually play an important role in keeping algae blooms in check.

Nonetheless, when a virus kills an algae cell, by popping it like a wet balloon, all those nutrients tucked inside spew out into the water, where it gets slurped up by other microbes. This is called the “viral shunt” and it means other organisms further up the food chain don’t get to benefit from this nutrient cycle.

They found that Halteria sp. not only hungrily devoured the viruses, but they had enough nutritional value to thrive and reproduce.

However, new research in the journal Proceedings of the National Academy of Sciences provides evidence that not only are chloroviruses regularly eaten, they are also nutritious, which has broad implications for how we think about food chains and the cycling of carbon earth. The paper was co-authored by a plant pathology professor James Van Etten, who first discovered chloroviruses in 1980.

To study this, researchers at the University of Nebraska-Lincoln scooped up some pond water containing a microbe called Halteria sp. These tiny creatures are called ciliates because they are covered in tiny hair-like projections call cilia. Then, they fed the ciliates heaping portions of chloroviruses, with some cultures not fed anything to serve as controls.

They found that Halteria sp. not only hungrily devoured the viruses, but they had enough nutritional value to thrive and reproduce. To really prove this was happening, they labeled the DNA of the viruses with a fluorescent green dye. And when they checked the “stomachs” of these microbes (technically called the vaculole), they found glowing viruses inside. This is the first time it has been shown that a virus-only diet can be enough for a microorganism to survive, a practice the researchers call “virovory,” meaning virus feeding.

“If you multiply a crude estimate of how many viruses there are, how many ciliates there are and how much water there is, it comes out to this massive amount of energy movement (up the food chain),” the study’s lead author John DeLong, an associate professor at University of Nebraska–Lincoln, said in a statement. He and his colleagues estimate that ciliates in a small pond might eat 10 trillion viruses a day. “If this is happening at the scale that we think it could be, it should completely change our view on global carbon cycling,” DeLong said.

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Indeed, this is a relationship that is sorely missing from food chain models. The authors argue that “current food web models are missing a critical interaction,” but there is surprisingly little research in this area.

“I was motivated to determine whether or not this was weird, or whether it fit,” DeLong said. “This is not weird. It’s just that nobody noticed it.”

To learn more, DeLong wants to repeat this experiment outside the lab. Discovery of the first virovore is really underscores how little we know about the microbes all around us, to say nothing of their evolution. Creatures that eat viruses will put selective pressure on them, which over time will shift their genetics. Not only does understanding this relationship have big implications for how nutrients are cycled through the ecosystem, but it illuminates some of the fundamental mechanisms of life itself.

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