These microbes could be the first lifeforms to munch on infectious viruses

Halteria were caught chowing down on large numbers of chloroviruses in a recent lab experiment.
Yellow chlorovirus particles infecting microscopic green algae

Chlorovirus particles infecting microscopic green algae. Kit Lee and Angie Fox

The world is entering yet another year of a global pandemic caused by a virus—but COVID-19 is hardly the only virus out there. While some viruses can be deadly, others help keep organisms alive and there is even debate about whether viruses themselves should be considered living things.

However, it looks like a hungry microorganism may be a natural foe to some viruses. In a study published in December 2022 in the journal Proceedings of the National Academy of Sciences, a team from the University of Nebraska-Lincoln found that a virus-only diet is enough to fuel the growth of a species of Halteria. Halteria is a one-celled ciliate that lives in freshwater around the world and the team found that they can eat huge numbers of infectious chloroviruses at a time.

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The team calls this virus exclusive diet “virovory.”

Chloroviruses are known to replicate by infecting microscopic green algae that normally live inside a species of paramecium. They can then burst these single-celled hosts like balloons, causing carbon and other elements critical to sustaining life to spill out into the water. That carbon then gets sucked up by other microorganisms.

“That’s really just keeping carbon down in this sort of microbial soup layer, keeping grazers from taking energy up the food chain,” said John DeLong, associate professor of biological sciences at Nebraska and a co-author of the study, in a statement.

However, if ciliates like Halteria are eating those same viruses, then virovory could act as a counter balance to the carbon recycling that the viruses are known to perpetuate. Vivory might be helping carbon move up the food chain, according to DeLong.

“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),” said DeLong. He estimated that ciliates in a small pond might eat 10 trillion viruses a day.

DeLong built on the university’s previous work on chloroviruses and was already familiar with how they can entangle themselves in a food web. Research by co-authors ecologist James Van Etten and virologist David Dunigan showed that chloroviruses can gain access to algae, which are normally covered in a genus of ciliates called Paramecia. This access can only happen when small crustaceans eat the Paramecia and poop out the the newly exposed algae.

DeLong thought that viruses could be a source of food and energy. To test the hypothesis, he collected samples at a nearby pond and then corralled all of the microorganisms into drops of the water and added a lot of chlorovirus.

After sitting for 24 hours, the drops were searched for signs that any of the present species were reacting to the chlorovirus present. It turns out, Halteria was treating the virus more like a snack instead of a physical threat.

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“At first, it was just a suggestion that there were more of them [the ciliates],” DeLong said. “But then they were big enough that I could actually grab some with a pipette tip, put them in a clean drop, and be able to count them.”

In only two days, the number of chloroviruses dropped up to 100-fold. The Halteria had no other source of food other than the virus, and its population was growing 15 times larger, while the chlorovirus population wasn’t expanding at all.

The team tagged some of the DNA in the chlorovirus with a green dye to confirm that the Halteria was eating the virus. The ciliate equivalent of a stomach called the vacuole, was soon glowing green.

The research from this study may have some important implications. These viruses play an integral part in shaping their freshwater environments in the way that they recycle carbon and other nutrients and this recycling can prevent the energy from reaching other, larger forms of life. However, if something is eating these viruses, that are then consumed by bigger organisms, some of the nutrients and energy that would normally be recycled might work their way up the food chain instead.

“[Viruses are] made up of really good stuff: nucleic acids, a lot of nitrogen and phosphorous,” DeLong said. “Everything should want to eat them. So many things will eat anything they can get ahold of. Surely something would have learned how to eat these really good raw materials.”

The next steps for the team will happen once winter ends in the Midwest. They plan to go back to the pond to see if virovory is occurring in the wild, not just a lab setting.