John B. DeLong, James L. Fan, Zina El Amily, Irina V. Agharkova, and David D. Dunigan, “Virus consumption restores energy to food chains,” Proceedings of the National Academy of Sciences, December 27, 2022. https://doi.org/10.1073/pnas.221500012
aThrough various peaks of the COVID-19 pandemic, viruses have been known to be devastating to public health systems and human lives. They have a strange biology – while they are dormant outside the living body, inside, they hijack the cellular machinery of feeding, reproduction and proliferation. This association with disease and death has come to define itself in the public imagination, not even compensated by the fact that there are other microbes destroying viruses. But a new study, published December 27 by researchers at the University of Nebraska-Lincoln, offers to rectify this.
The study authors report that a certain genus of plankton can consume viruses as well as “grow and divide if there are only viruses for it to eat.” We already know of other cells that can “consume” viruses in an effort to destroy them – such as the macrophage cells of the human immune system.
The difference lies in the ability to “eat” viruses to meet an individual’s biological requirements.
The researchers claim to have discovered that plankton of the genus Halteria can consume 10,000 to 1 million virus particles per day, increasing their numbers using metabolic energy, and providing more food for zooplankton that consume Halteria. This may be important to the marine food chain.
Halteria grandinella. | Image source: Picturepest / Flickr
What are plankton?
Plankton are microscopic organisms that can only move with a current. They do not have any facilities to actively push themselves. Halterian plankton are ciliates, which means they have hair-like structures called cilia on their surface. Sometimes they can outgrow some of these ciliates to jump short distances, but not often because they often can’t do it because it requires a lot of energy.
What do plankton do in the food chain?
A type of plankton – phytoplankton – is found near the surface of many bodies of water. It is autotrophic, meaning it can make its own food which it does by consuming carbon dioxide, among other compounds, through photosynthesis. Small fish and large plankton called zooplankton feed on phytoplankton for nourishment. They, in turn, are eaten by larger fish, and so on and so forth.
When phytoplankton die, they either drift around where they are, becoming part of the coastal nutrient cycle, or drift towards the sea floor, where they decompose. Their components then become available to microbes or are sequestered on the sea floor.
Therefore, phytoplankton bring carbon and other nutrients from the atmosphere and sea surface to the sea floor and help replenish the food chain (and also “lock” carbon in their bodies and as sediments). They are joined by bacteria that make their own food by oxidizing sulfur, iron, or hydrogen, in a process called chemosynthesis.
Role of Halteria
Halteria plankton is found in large numbers in freshwater bodies. They are heterotrophs which means they cannot produce their own food. Instead, they are recognizable bacteria—they consume bacteria to provide themselves with energy.
In the words of a 1999 paper, viruses “short-cut” the process of moving nutrients up the food chain. They infect and kill bacteria and plankton, and release organic matter that dissolves in water. Note that scientists have modeled this “viral shunt” and studied it in the laboratory; Investigations into its impact in the real world are ongoing.
In the new study paper, the authors write that by also consuming the viruses to feed, Halteria plankton can restore nutrients lost in the viral shunt and put them back into the food chain. “This influx will depend on virus size and nutritional content, which varies across strains,” they added, “but it is already clear that viruses of a wide variety of sizes can be accommodated.”
Redemption for viruses?
Viruses are the most populous form of life on Earth. Their cumulative biomass exceeds the biomass of humans by a factor of 10^10. In 2020 Curtis Suttle, a co-author of the 1999 paper, called them “wonderful food sources”.
The discovery was not unexpected. In 2020, a group from the United States and Spain published a paper after 10 years of taking and studying the data, positing that some protists (eukaryotes that are neither plants, animals, nor fungi; eukaryotes are organisms whose cells contain nuclei) might chew. on viruses. Their findings, they wrote, “suggest that the viral shunt completes a viral link in the marine microbial food web.”
There have been papers discussing this link since at least 1992.
In the new study, the researchers found that halytaria plankton reduced the number of chloroviruses it “feeded” on while their numbers increased — while paramecium ciliates consumed chloroviruses but did not reproduce. This means, to quote their paper, “Not all ciliates can grow on chloroviruses in these conditions, even when you consume them.”
Chlorine virus particles on algae. | Photo credit: Kit Lee and Angie Fox/University of Nebraska-Lincoln
Second, as the populations of viruses and helteria change over the course of their interaction (in the lab), the researchers used a modeling tool to understand if they fit with what we already know about predator-prey interactions in the wild. The model indicated that they did, but John DeLong, biologist and principal investigator, said in a press release: “Now, we have to go find out if this is true in nature.”
Delong et al. They also write that they don’t yet know how virus-eating Halteria can affect viral evolution.