Halteria, a genus of microscopic planktonic ciliates that are found in many freshwater environments, can eat huge numbers of infectious chloroviruses — up to one million viruses per day — that share their aquatic habitat.
DeLong et al. estimate that each Halteria in their experiments ate 10,000 to one million viruses per day. Image credit: Proyecto Agua.
Chloroviruses are known to infect microscopic green algae. Eventually, the invading chloroviruses burst their single-celled hosts like balloons, spilling carbon and other life-sustaining elements into the open water.
That carbon, which might have gone to predators of the tiny creatures, instead gets vacuumed 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 lead author Dr. John DeLong, a researcher at the University of Nebraska-Lincoln.
“But if ciliates are having those same viruses for dinner, then virovory could be counterbalancing the carbon recycling that the viruses are known to perpetuate.”
“It’s possible that virovory is aiding and abetting carbon’s escape from the dregs of the food chain, granting it an upward mobility that viruses otherwise suppress.”
“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).”
“If this is happening at the scale that we think it could be, it should completely change our view on global carbon cycling.”
For their study, Dr. DeLong and his colleagues collected samples of the water from a nearby pond.
Back at the lab, they corralled all of the microorganisms they could manage, regardless of the species, into drops of the water. Finally, they added generous portions of chlorovirus.
After 24 hours, they would search the drops for a sign that any species seemed to be enjoying the company of the chlorovirus — that even one species was treating the virus less like a threat than a snack. In Halteria, they found it.
“At first, it was just a suggestion that there were more of them. 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,” Dr. DeLong said.
The number of chloroviruses was plummeting by as much as 100-fold in just two days.
The population of Halteria, with nothing to eat but the virus, was growing an average of about 15 times larger over that same timespan.
Halteria deprived of the chlorovirus, meanwhile, wasn’t growing at all.
To confirm that Halteria was actually consuming the virus, the authors tagged some of the chlorovirus DNA with a fluorescent green dye before introducing the virus to the ciliates.
Sure enough, the ciliate equivalent of a stomach, its vacuole, was soon glowing green.
It was unmistakable: the ciliates were eating the virus. And that virus was sustaining them.
“I was calling up my co-authors: ‘They grew! We did it!’ I’m thrilled to be able to see something so fundamental for the first time,” Dr. DeLong said.
The researchers have since identified other ciliates that, like Halteria, can thrive by dining on viruses alone.
The more they uncover, the more likely it seems that virovory could be occurring in the wild.
“It’s a prospect that fills the ecologist’s head with questions: How might it shape the structure of food webs? The evolution and diversity of species within them? Their resilience in the face of extinctions?” Dr. DeLong said.
The team’s findings appear in the Proceedings of the National Academy of Sciences.
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John P. DeLong et al. 2022. The consumption of viruses returns energy to food chains. PNAS 120 (1): e2215000120; doi: 10.1073/pnas.2215000120
