A New Prey On the Menu: Viruses

The relationship between predator and prey is one that is present in all ecosystems. Whether a whale and plankton or an amoeba and protozoa, billions of heterotrophs hunt other organisms each day to obtain nutrients. Recently, however, scientists have discovered a new type of prey on the menu: viruses. Over the last three years, John Delong of the University of Nebraska-Lincoln has intensely researched microscopic aquatic organisms — their diets, behaviors, and reproductive stages. One specimen that he observed, Halteria, can eat copious amounts of chloroviruses. Halteria is classified as freshwater ciliate, a group of protists classified by the hair-like organelles called cilia on their membranes. The cilia on microorganisms like Halteria function similarly to their eukaryotic counterparts but are shorter and present in greater numbers. 

Chloroviruses are instrumental carbon recyclers in aquatic ecosystems. They infect algae, and eventually rupture their single-celled hosts, spilling carbon and cellular substances into the water. This carbon, which otherwise would have been taken by predators, is now released into the water for microorganisms to absorb. Consequently, it prevents predators from obtaining carbon, which balances the amount that travels up the food chain.

Here’s where Halteria comes in. If chloroviruses stifle the upward flow of carbon, then these virus-eating organisms (virivores) will stimulate it. “If you multiply a crude estimate of how many viruses there are, how many ciliates there are, and how much water there is, it comes out to this massive amount of energy movement,” DeLong says. This could overhaul our current view on global carbon cycling, and adjust the way we understand aquatic ecosystems’ role in the transferral of carbon.

DeLong was familiar with chloroviruses’ role in the food web prior to his discovery. Seven years ago, in collaboration with Nebraska ecologist James Van Etten and virologist David Dunigan, he discovered that chloroviruses are able to attach to algae because of a genus of ciliates named Paramecium. Tiny crustaceans who dwell on the floors of ponds, lakes, and rivers eat the paramecia, and secrete the algae within them, allowing the choroviruses to easily latch on and multiply on their host. 

This propelled DeLong’s study on viruses, and he figured that given the infinitely complex populations of microorganisms in aquatic environments, it was inevitable for viruses to be consumed. Scientists before him had barely begun to grasp this possibility. Initially, all DeLong dug up was a 1980s study that reported protists eating viruses and a few Swiss papers that documented protists removing viruses from water. No research had ever proven any consequences of virivory*. This left DeLong pondering: Why? He understood that viruses contained carbon and other key biological components like nucleic acids, nitrogen, and phosphorus. “Everything should want to eat them,” DeLong said.

*Virivory is the nature of an organism to specifically prey upon viruses.

DeLong kept his search for the first virivore simple. A handful of volunteers collected water samples from a local pond, and brought them to his lab. There, he put as many microorganisms as possible into a drop of water, and then added chlorovirus. The water sat for 24 hours as DeLong wanted to see if any species consumed the virus or simply co-existed. Halteria was key to unlocking virivory. After concentrating more Halteria in a test tube, DeLong observed that the number of chloroviruses dropped 100-fold in as little as two days, and Halteria’s population grew 15 times larger during this span. In addition, when the Halteria was separated from the chloroviruses, their population did not increase at all. 

Since he discovered Halteria, DeLong has found numerous other ciliates that are also classified as virivores. How might these new organisms affect the food web? What complex behaviors and evolutionary traits do these organisms have? How might they change our understanding of the carbon cycle? DeLong plans to return to the pond and answer these questions, along with numerous others that originate from this discovery. Halteria is just the tip of the iceberg — there are thousands of other species waiting to be discovered.

Works Cited:

Introduction to the Alveolates. (n.d.). Introduction to the Alveolates. Retrieved March 22, 2023, from https://ucmp.berkeley.edu/protista/alveolates.html

ScienceDaily. (2023, January 3). Eating viruses can power growth, reproduction of microorganism. ScienceDaily. Retrieved March 20, 2023, from https://www.sciencedaily.com/releases/2022/12/221227103521.htm

Wikimedia Foundation. (2023, January 30). Chlorovirus. Wikipedia. Retrieved March 20, 2023, from https://en.wikipedia.org/wiki/Chlorovirus

Wikimedia Foundation. (2023, March 14). Ciliate. Wikipedia. Retrieved March 20, 2023, from https://en.wikipedia.org/wiki/Ciliate