In just one day, in the still waters of a single pond, a million virus particles could enter a single-celled organism known for its tiny hairs, or cilia, that propel it through those waters.
For the past three years, John DeLong of the University of Nebraska-Lincoln has been busy uncovering a potential secret that could turn the tide: These virus particles are a source not only of infection, but of nourishment as well.
In a Pac-Man twist, DeLong and his colleagues found that sort of Halteria -;the microscopic ciliates that populate fresh waters all over the world -; can eat large numbers of infectious chloroviruses that share their aquatic habitat. For the first time, the team’s laboratory experiments have also shown that a viral-only diet, which the team calls ‘viroviry’, is sufficient to fuel physiological growth and even population growth in an organism.
Chloroviruses, a career-defining discovery of James Van Etten of Nebraska, are known to infect microscopic green algae. Eventually, the invading chloroviruses blow up their single-celled hosts like balloons, spewing carbon and other vital elements out to sea. This carbon, which may have gone to the tiny creatures’ predators, is instead being sucked up by other microorganisms –; a sinister recycling program in miniature and seemingly forever.
It only reduces the carbon in this type of microbial soup layer, preventing grazers from absorbing energy further up the food chain. »
John DeLong, associate professor of biological sciences, University of Nebraska-Lincoln
But if ciliates eat those same viruses for dinner, then virovory could counterbalance the carbon recycling that viruses are known to perpetuate. It’s possible, DeLong said, that virovory helps and encourages carbon to escape from the food chain, granting it an upward mobility that viruses would otherwise suppress.
“If you multiply a rough estimate of the number of viruses, the number of ciliates and the amount of water, you get this huge amount of energy movement (up the food chain),” said DeLong, who estimated that ciliates in a small pond could eat 10 trillion viruses a day. “If this happens on the scale we think it could be, it should completely change our view of the global carbon cycle. »
“Nobody Noticed”
DeLong already knew the ways chloroviruses can get entangled in a food web. In 2016, the ecologist teamed up with Van Etten and virologist David Dunigan to show that chloroviruses have access to algae, which are normally enclosed in a genus of ciliates called Parameciumonly when small crustaceans eat the Paramecium and expel the newly exposed algae.
This discovery put DeLong in “a different headspace” when it came to thinking about and studying viruses. Given the abundance of viruses and microorganisms in the water, he thought it inevitable that –; even setting aside the infection –; the former was sometimes found within the latter.
“It seemed obvious that everything must have viruses in its mouth all the time,” she said. “It looked like it was going to happen, because there are so many of them in the water. »
So DeLong dove into the research literature, intending to emerge with any studies of aquatic organisms that eat viruses and, ideally, what happened when they did. He came out with little. One study, dating back to the 1980s, reported that unicellular protists were capable of consuming viruses, but went no further. A handful of documents from Switzerland later showed that protists appeared to remove viruses from wastewater.
“And that’s it,” DeLong said.
There was nothing about the potential consequences for the microorganisms themselves, let alone the food webs or ecosystems to which they belonged. This surprised DeLong, who knew that viruses were built not only on carbon, but also on other elementary building blocks of life. They were, hypothetically at least, anything but junk food.
“They’re made of very good stuff: nucleic acids, lots of nitrogen and phosphorus,” he said. “Everyone should want to eat them.
“So many things eat whatever they can find. Surely he would have learned something by eating these delicious raw materials. »
As an ecologist who spends much of his time using math to describe predator-prey dynamics, DeLong wasn’t entirely sure how to go about investigating his hypothesis. In the end, he decided to keep it simple. First, he would need volunteers. He drove to a nearby pond and collected water samples. Back in his laboratory, he collected all the microorganisms he could handle, regardless of species, in drops of water. Finally, he added generous portions of chlorovirus.
After 24 hours, DeLong was looking in the drops for a sign that any species seemed to enjoy the company of chlorovirus -; that even one species treated the virus less as a threat than a snack. In Halteriahe found it.
“At first it was just a suggestion that there were more,” DeLong said of the ciliates. “But they were big enough that you could grab a few with the tip of a dropper, put them in a clean drop and be able to count them. »
The number of chloroviruses decreased almost 100 times in just two days. The population of Halteriathat they had nothing to eat but the virus, which increased on average about 15 times more in the same period. Halteria deprived of the chlorovirus, meanwhile, it did not grow at all.
To confirm that the Halteria actually consuming the virus, the team labeled some of the chlorovirus DNA with a green fluorescent dye before introducing the virus into ciliates. Sure enough, the ciliate equivalent of a stomach, its vacuole, quickly turned green.
It was unequivocal: the ciliates ate the virus. And this virus was supporting them.
“I was calling to my co-writers, ‘They’ve grown! We did it! ‘” DeLong said of the findings, now detailed in the journal Proceedings of the National Academy of Sciences. first time. “
DeLong wasn’t finished. The math side of him wondered if this particular predator-prey dynamic, strange as it might seem, might have points in common with the more pedestrian pairs he was accustomed to studying.
He started by mapping the decline of chlorovirus against the growth of Halteria. This relationship, DeLong found, generally matches one that ecologists have observed between other microscopic hunters and their prey. the Halteria also converted about 17% of consumed chlorovirus mass to clean new mass, which is exactly the percentage observed when Paramecium eat bacteria and millimeter crustaceans eat algae. The rate at which the ciliates fed on the virus and the roughly 10,000-fold disparity in their size also matches other aquatic case studies.
“I was motivated to figure out if it was weird or not, or if it was okay,” DeLong said. “It is not strange. It’s just that nobody noticed. »
DeLong and colleagues have since identified other ciliates who, such as Halteria, they can thrive on viruses alone. The more they find, the more likely it is that vivory can occur in the wild. It is a perspective that fills the ecologist’s head with questions: how could it shape the structure of food webs? The evolution and diversity of species within them? Their resilience in the face of extinctions?
Again, however, he’s chosen to keep things simple. As soon as the Nebraska winter settles down, DeLong will return to the pond.
“Now,” he said, “we have to go and see if that’s true in nature. »
