"PNAS" first discovered a microorganism that specializes in eating viruses for a living

In one day, in the calm water of a pond, a million viral particles may enter a single-celled organism known for tiny hairs or cilia that propel it through these waters
.
For the past three years, John DeLong of the University of Nebraska-Lincoln has been busy discovering a potential tide-turning secret: These viral particles are not only a source of infection, but also a source of
nutrition.

DeLong and his colleagues discovered a species of Halteria, a tiny ciliate that lives in freshwater around the world and can swallow large amounts of infectious chlorviruses
that share aquatic habitats with them.
The team's lab experiments have also shown for the first time that a virus-only diet (which the team calls "virovory") is sufficient to promote the physiological growth of organisms and even population growth
.

Chloroviruses are a business-defining discovery by James Vanetten of Nebraska and are known to infect microscopic green algae
.
Eventually, the invading chlorine virus burst their single-celled host like a balloon, leaking carbon and other life-sustaining elements into open water
.
This carbon could have been eaten by the predators of these tiny creatures, but was sucked up by other microbes – a tiny, seemingly permanent, cruel cyclical process
.

DeLong, an associate professor of biological sciences in Nebraska, said: "It's really just leaving carbon in this microbial soup layer, preventing herbivores from carrying energy up
the food chain.
" But if cilia treat these viruses for dinner, then virality may counteract the carbon cycle
that is known to be perpetuated by viruses.
DeLong said it's possible that virality is helping and abetting carbon escape from the dregs of the food chain, giving it the upward mobility
that the virus inhibites.

"If you roughly estimate how many viruses there are, how many ciliates there are and how much water there is, you get a lot of energy movement (along the food chain)," said DeLong, who estimates that ciliates in a small pond could eat 10 trillion viruses
a day.
"If this happens at the scale we think, it should completely change the way we think about
the global carbon cycle.
"

DeLong is already familiar with the way
the chlorine virus entangles itself in the food web.
In 2016, the ecologist teamed up with Van Etten and virologist David Duigan to demonstrate that chlorviruses only come into contact with algae when tiny crustaceans eat paramecium and expel newly exposed algae, which are normally enclosed in
a genus of cilia called paramecium.

The discovery puts DeLong in a "different space of thought"
when thinking about and studying viruses.
Considering the absolute abundance of viruses and microorganisms in water, he believes that even without taking into account infections, it is inevitable
that the former will sometimes enter the latter.

"It's clear that anything has to carry the virus in their mouth all the
time," he said.
It seems inevitable that this will happen because there are so many
in the water.
”

So DeLong delved into the research literature to try to find out any research on aquatic organisms eating viruses, and what happens
when they eat viruses.
He had
almost nothing.
A study in the 80s of the 20th century reported that single-celled protists had the ability to consume viruses, but no further research was conducted
.
Several later papers from Switzerland showed that protists appeared to be removing viruses
from wastewater.

Nothing is known about the potential consequences of microbes themselves, let alone the food web or ecosystem to which they belong
.
This surprised DeLong because he knew that viruses were built not only on carbon, but also on other basic elements of life
.
At least in theory, they are by no means junk food
.
"They're made up of very good substances: nucleic acids, a lot of nitrogen and phosphorus," he said
.
All animals should want to eat them
.
You can eat whatever you can grab a lot of things
.
Someone must have learned how to eat these really good ingredients
.
”

As an ecologist who spends a lot of time mathematically describing predator-predator dynamics, DeLong isn't entirely sure how to proceed to investigate his hypothesis
.
Eventually, he decided to keep it simple
.
First, he needed some volunteers
.
He drove to a nearby pond to collect water samples
.
Back in the lab, he concentrated all the microbes he could control, whatever they were, into droplets
.
Finally, he added a large amount of chlorvirus
.

After 24 hours, DeLong looks in the droplets for signs that any species seems to like being around the chlorvirus — and one species even sees the virus as a snack rather than a threat
.
This is Halteria, he found it!

"At first, it was just a hint because there were more ciliates here," DeLong said
.
"But then there were enough of them that I could grab some with the tip of a pipette, put them in clean droplets, and be able to count them
.
"

In just two days, the number of chlorviruses plummeted 100-fold
.
The number of Halteria, which has nothing to eat except the virus, has grown by an average of about 15 times
in the same time.
At the same time, Halteria, deprived of the chlorvirus, did not grow
at all.

To confirm that Halteria was actually consuming the virus, the team labeled some of the chloroviral DNA
with a fluorescent green dye before introducing the virus into the ciliate.
Sure enough, the ciliate's stomach — its vacuoles — soon
glows green.

There is no doubt about it: the cilia are devouring the virus
.
This virus sustains them
.

"I called my co-authors: 'They've grown up! We did it!'" DeLong said of the findings that details of the findings have been published in PNAS
.
"I'm excited to see such a fundamental discovery
for the first time.
"

DeLong hasn't finished
yet.
His rational thinking side wondered if this particular predator-predator dynamic, strange as it may seem, had something in common with the more ordinary pairings he was accustomed to
studying.

He first mapped the relationship
between the decline of chlorvirus and the growth of Halteria.
DeLong found that this relationship was essentially in line with the relationship
observed by ecologists between other microscopic hunters and their prey.
Halteria also converts about 17 percent of the consumed chlorvirus mass into its own new mass, in line with
the percentages seen when parameciums eat bacteria and millimeter-long crustaceans eat algae.
Even the rate at which cilia feed on viruses, and the roughly 10,000-fold difference in their size, dovetails with other aquatic case studies
.

DeLong said: "I'm motivated to determine if this is strange, or if it is appropriate
.
This is not surprising
.
It's just that no one noticed
.
”

DeLong and his colleagues later discovered that other ciliates, like Halteria, thrive simply by eating the virus
.
The more they discovered, the more likely it was that viral predation
would occur in the wild.
This prospect leaves ecologists wondering: How does it shape the structure of the food web? Where is the evolution and diversity of species? Their resilience in the face of extinction?

Still, he chose to keep it simple
.
Once the Nebraska winter subsides, DeLong returns to the pond
.

"Now," he said, "we have to figure out if this holds true
in nature.
" ”