Scientists discover first known organism thriving exclusively on diet of viruses

Previous studies provide scarce evidence of other organisms eating viruses, researchers say

Scientists have found that a species of plankton that populate freshwater worldwide is the world’s first known organism that survives and thrives by dining on viruses alone, an advance that sheds new light on the role of viruses in the global food web.

The study, published last week in the journal PNAS, found that this virus-only diet – which they call “virovory” – is enough to fuel the growth and reproduction of a species of Halteria, a single-celled organism known for the minuscule hairs.

“It seemed obvious that everything’s got to be getting viruses in their mouths all the time. It seemed like it had to be happening, because there’s just so much of it in the water,” study co-author John DeLong from the University of Nebraska-Lincoln said in a statement.

Previous studies provide scarce evidence of aquatic organisms eating viruses, researchers say.

Most research place viruses as the top “predator” in food chains, but Dr DrLong and his team say, viruses also can serve as food like most predators.

“They’re made up of really good stuff: nucleic acids, a lot of nitrogen and phosphorous. Everything should want to eat them,” he explained.

“So many things will eat anything they can get ahold of. Surely something would have learned how to eat these really good raw materials,” Dr DeLong added.

While viruses are incidentally consumed by other microbes, to qualify as a step in the food chain, researchers say, an organism need to gain a significant amount of energy or nutrients from consuming viruses.

In the study, Dr DeLong collected samples of the water from a pond, corralled all of the microorganisms into drops of the water, and added “generous portions” of chlorovirus – known to infect microscopic green algae.

Researchers found that laboratory samples of the Halteria not only consumed the chloroviruses added to its environment, the virus also fueled the plankton’s growth and increased its population size.

While the number of chloroviruses was plummeting by as much as 100-fold in just two days, scientists observed that over this time span, the population of Halteria – with nothing to eat but the virus – was growing an average of about 15 times larger.

They also found that the Halteria, when deprived of the chlorovirus, “wasn’t growing at all.”

Researchers estimated that each Halteria in the experiment ate about 10⁴ to 10⁶ viruses per day, “suggesting that 10¹⁴ to 10¹⁶ virions could be consumed per day in a small pond.”

Halteria, scientists say, also converted nearly a fifth of the consumed chlorovirus mass into new mass of its own.

The study also hints that the evolution of viruses may be impacted by the pressures exerted on them by other predator microbes in their environment.

“Our results suggest that virion persistence in the environment depends not only on environmental factors but also on grazing by predators,” scientists wrote in the study.

“It is therefore possible that grazers exert selective pressure and influence the evolution of viral phenotypes in a way that interacts with pressure on viruses to effectively infect and replicate within hosts,” they added.