The First ‘Virivore’: An Organism That Can Feed Only on Viruses, Up to a Million a Day

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    There are plants that digest amphibians, algae that feed on fish, or viruses that infect bacteria. But among the relationships between predator and prey there is one that is barely known and that could be essential in the cycle of life: beings that eat viruses. The virivores. Although the word does not yet exist, a group of American researchers have discovered two groups of microorganisms that are neither animals, nor plants, nor fungi, but neither are they bacteria, called ciliated protists, and that feed on viruses. Although they are not the first virus-eating organisms identified, they have shown that organisms can thrive by feeding exclusively on viral material.

    During the last three years, a group of researchers from the University of Nebraska in Lincoln (United States) has been investigating viruses from a different perspective than usual: not as pathogenic biological entities (there is no consensus on whether or not they are living beings), but as basic nutrients in the life cycle. Next to aquatic bacteria, viruses are the most abundant organisms on Earth. Being so many, it is normal for filter-feeding organisms, those that feed by filtering the water, to ingest all kinds of organic matter it contains, viruses included. But what John DeLong, a scientist from the American university, has done has been to show that there are at least two types of truly virivorous beings that can live only by eating viruses.

    “Several studies had already documented the consumption of viruses,” recalls DeLong in an email. “But, as far as we know, it is the first time that it has been demonstrated in these two species,” he adds. It refers to the Paramecium bursaria and the Halteria sp, two aquatic ciliated protists. They were suspected of eating viruses, although it was not known if accidentally. What DeLong’s team has done is observe it in the laboratory, under controlled conditions. Thus, in small droplets of water obtained from a pond near the university, they released large amounts of chlorovirus, a relatively large virus that infects the chlorophyll of algae in lakes and freshwater reservoirs all over the planet.

    Significant increase

    At 24 hours, they carefully studied the water droplets. The results of the experiments, published in the scientific journal PNAS, showed that in the presence of both species, the amount of virus in the medium was reduced up to 100 times. What they needed to know then was whether they had eaten the viruses. Using a staining technique (adding dye for contrast), they turned several of them fluorescent and watched the vacuoles of protists (which serve a similar function to the stomach) turn bright green. They came to estimate that each Halteria sp. he was capable of ingesting between 10,000 and a million chloroviruses a day.

    But you had to relate how badly the viruses fared with an improvement for the protists. The scientists observed that the population of paramecia stayed the same, they ate viruses, but it didn’t seem to do them much good. However, after 48 hours of exposure, they saw that the population of Halteria sp. increased while the amount of chlorovirus was drastically reduced. In numbers, the viral load plummeted 100-fold in just two days, while the population of the protista, with nothing to eat except the virus, grew an average of 15-fold over that same period of time.

    “We think that viruses are probably very nutritious, having high levels of protein and phosphorus,” says DeLong. A study on the composition of viruses published a few years ago also mentions that they contain nucleic acids, lipids, and amino acids. In the case of chloroviruses, in addition, they could contain the carbon that they steal from the chlorophyll of the algae.

    Ecologist Joshua Weitz co-authored this latest study on what’s in viruses. Weitz, who leads a group of researchers at the Georgia Institute of Technology (United States) focused on the ecology of viruses, has also published several papers on the role of viral entities in the cycle of life, including his latest book, Quantitative Viral Ecology. In an email, Weitz, unrelated to the study of chloroviruses and the two protist species, argues that “viruses are potentially nutritious if swallowed by a microbe that digests them and is not infected by the virus.” In general, viruses are made up of genetic material (DNA or RNA) with an envelope that protects it. “Because viral genomes are relatively densely packed and because genetic material is rich in phosphorus, viruses have a relatively higher phosphorus content than typical microbes and therefore might have a nutritional bonus for their size”, Weitz details.

    Review all that we know

    The fact that there are microorganisms that eat viruses forces biologists to review what is known about the life cycle. It was known that viruses are keys in the base. By infecting all kinds of living beings, they cause a process called viral shunting. In the phase in which they emerge from the infected cell (or single-celled organism), they burst the cell, releasing organic matter and nutrients into the medium that would otherwise be lost. Now, as Weitz points out, “this research is an important step in advancing our understanding of the ways in which viral particles are involved in moving energy (and nutrients) both up and down microbial food webs.” That is, the role of viruses in the food chain of microorganisms.

    Changing our view of the global carbon cycle

    But quantifying the role of viruses as transmitters of energy and nutrients will not be easy. Delong exemplifies this in a note from his university: “If you multiply a rough estimate of how many viruses there are, how many ciliates there are, and how much water there is, we would get an enormous amount of energy movement (in the food chain).” Delong estimates that ciliates in a small pond can gobble up 10 trillion viruses a day, adding: “If this is happening on the scale we think it could be happening, it should completely change our view of the global carbon cycle.”

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