Researchers discovered a microscopic organism that eats viruses

Even viruses are prey for some creatures.
Christopher McFadden
virovores.jpg
Virovores apparently do exist, according to this new study.

oto_feja/iStock 

Viruses have long been thought off the menu for almost all organisms, but a new study shows that viruses might be a delicious meal more often than once thought. Researchers have found that some animals have learned to eat certain viruses and use them for food and energy.

John DeLong and his colleagues at the University of Nebraska have discovered that a species of Halteria—microscopic ciliates prevalent in freshwater habitats worldwide—can consume a sizable number of infectious chloroviruses. For the first time, the team's laboratory tests have also demonstrated that a virus-only diet, or "virovory," can support an organism's physiological growth and even population increase.

James Van Etten of Nebraska University made the seminal discovery of chloroviruses, which are known to infect tiny green algae. The invasive chloroviruses eventually burst their single-celled hosts like balloons, releasing carbon and other essential components for life into the surrounding ocean.

According to DeLong, an associate professor of biological sciences at Nebraska, "that's just keeping carbon down in this sort of microbial soup layer, keeping grazers from transferring energy up the food chain."

However, virovory might be balancing the carbon recycling that viruses are known to propagate if ciliates are eating those same viruses for dinner. According to DeLong, it's likely that virovory is facilitating carbon's ascent from the bottom of the food chain by giving it upward mobility that viruses would typically hinder.

The newly discovered virovores eat trillions of viruses every day

According to DeLong, ciliates in a tiny pond might consume 10 trillion viruses daily. "If you multiply a rudimentary estimate of how many viruses there are, how many ciliates there are, and how much water there is, it comes out to this huge quantity of energy transfer (up the food chain)," he said.

"If this is happening at the scale we think it could be, it should completely change our view on global carbon cycling," he added.

DeLong knew how chloroviruses could become entangled in a food web. The ecologist collaborated with Van Etten and virologist David Dunigan in 2016 to demonstrate that chloroviruses can only access algae typically covered in a genus of ciliates called Paramecia when tiny crustaceans devour the Paramecia and expel the newly exposed algae.

This discovery changed DeLong's perspective on investigating viruses, putting him in "a different headspace." He reasoned that even putting aside infection, the presence of viruses and bacteria in the water made it unavoidable that the former would occasionally end up inside the latter.

It seemed clear that everyone and everything was constantly ingesting viruses, he added. Because there is so much of it in the water, it seemed certain that it must be taking place.

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Virovory had been postulated but never proven before

DeLong dug into the scholarly literature to find studies on aquatic organisms ingesting viruses and, ideally, what happened when they did. He had very little when he left. One study from the 1980s claimed that single-celled protists could eat viruses, but it stopped there. Later research from Switzerland revealed that protists appeared to be filtering viruses out of wastewater.

That's all there was, DeLong said.

The potential effects on the microbes, much less the food webs or ecosystems they belonged to, were not mentioned. DeLong was taken aback by this as he was aware that viruses were made not only of carbon but also of other essential life-building blocks. They were anything but junk food, at least in theory.

He noted that they include a lot of phosphorus, a lot of nitrogen, and nucleic acids. They are, in other words, precious potential food supplies.

The discovery used a basic yet robust methodology

DeLong was unsure about how to go with testing his idea because he is an ecologist who spends most of his time using math to describe predator-prey dynamics. He finally settled on keeping it straightforward. He would first require some volunteers. He took a car out to a neighboring pond and took water samples. He collected every type of microorganism he could manage in drops of water back at his lab, regardless of species. He then introduced copious amounts of chlorovirus.

DeLong would look through the drops after 24 hours for any indication that any species appeared to be having fun with the chlorovirus—that even one species was treating the virus more like a snack than a threat. He located it in Halteria.

The ciliates, according to DeLong, "at first it was simply a notion that there were more of them." But eventually, they grew large enough for me to count them by grabbing a few with a pipette tip and dropping them into a clean drop.

In just two days, the number of chloroviruses decreased up to 100-fold. Over the same period, the population of Halteria, which had only the virus to consume, was expanding by an average of about 15 times. Meanwhile, the chlorovirus-free Halteria showed no signs of growth.

Before introducing the virus to the ciliates, the team labeled some of the chlorovirus DNA with a bright green dye to ensure that the Halteria was indeed consuming the viral. Its vacuole, the ciliate equivalent of a stomach, started to glow green immediately.

The ciliates were consuming the virus. And they were living off of that infection.

Since this discovery, DeLong and his colleagues have discovered more ciliates that, like Halteria, can survive solely on viruses. Virovory, it seems, is not as rare as once thought.

You can read the study in full in the journal Proceedings of the National Academy of Sciences.

Study abstract:

"Viruses impact host cells and have indirect effects on ecosystem processes. Plankton such as ciliates can reduce the abundance of virions in water, but whether virus consumption translates into demographic consequences for the grazers is unknown. Here, we show that small protists not only can consume viruses they also can grow and divide given only viruses to eat. Moreover, the ciliate Halteria sp. foraging on chloroviruses displays dynamics and interaction parameters that are similar to other microbial trophic interactions. These results suggest that the effect of viruses on ecosystems extends beyond (and in contrast to) the viral shunt by redirecting energy up food chains."