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An Organism That Can Dine Exclusively on Viruses Has Been Found in a World First : ScienceAlert

A species of freshwater plankton is the first organism to thrive on a viral diet, according to a new study by researchers at the University of Nebraska-Lincoln in the US.

Viruses are often ingested randomly by a variety of organisms and can even flavor the diets of certain marine protists. However, to qualify as a true tier in the food chain – known as a virovory – viruses should deliver a significant amount of energy or nutrients to their consumer.

The microbe Halteria is a common genus of protists known for darting about when their hair-like cilia propel them through the water. Lab samples of the ciliated creature not only consumed chlorine viruses added to its environment, but fueled the giant virus Halterias growth and increased population.

The knock-on effects of widespread consumption of chlorine viruses in the wild could have profound implications for the carbon cycle. Chloroviruses, known for infecting microscopic green algae, burst their hosts and release carbon and other nutrients into the environment – a process that could be curbed by serious amounts of virus eaters.

“If you multiply a rough estimate of how many viruses there are, how many ciliates there are, and how much water there is, you get this enormous amount of energy moving up the food chain,” says University of Nebraska ecologist John DeLong -Lincoln.

“If this happens at the scale we think it could be, it should completely change how we view the global carbon cycle.”

The research took three years and was based on the idea that the sheer number of viruses and microorganisms that can be found in water could well mean that the former are eaten by the latter – although there haven’t been many previous studies on the subject which scientists could turn to for reference.

There are some good things in viruses if you’re an organism that wants to feed, including amino acids, nucleic acids, lipids, nitrogen, and phosphorus. Surely they would want to make something out of it for a meal, the researchers reasoned.

The team collected pond water samples and added chlorine viruses to them to see if a species was treating the viruses as food rather than a threat. That led her to it Halteria and parameciaboth thriving in water.

During paramecia gnawed at the viruses, its size and number scarcely bubbling. Halteria, on the other hand, they ate and used the chlorovirus as a nutrient source. The ciliate population increased 15-fold in two days, while the virus population decreased 100-fold.

“At first it was just a hint that there would be more of it [the Halteria organisms]’ says DeLong. “But then they were big enough that I could actually grab a few with a pipette tip, put them in a clean drop and count them.”

Fluorescent green dye was used to label chlorinevirus DNA before it was introduced into the two plankton species. This confirmed that the viruses were being eaten: the vacuoles – the microbial equivalent of stomachs – were glowing green from feeding.

Further analysis showed that the growth of Halteria compared to the decline in chlorovirus was consistent with the ratio seen in other microscopic predator-prey relationships in aquatic environments, giving the team more evidence as to what was happening.

There is much more to discover here. Next, the researchers plan to study how virovores might affect the food web, species evolution, and population resilience. But before that, they need to gather evidence that this is happening in the wild.

“I was motivated to figure out if that was weird or not, or if it was right,” says DeLong. “It’s not weird. It’s just that nobody noticed.”

“Now we have to find out if that’s true in nature.”

The research was published in PNAS.

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