Halteria: The Virus-Eating Microorganism

 

by Hannah Watts

Halteria (image by Don Loarie - https://www.inaturalist.org/photos)

Halteria is a genus of microscopic planktonic ciliates (single celled organisms that at some point have cilia, small projections used for movement and food gathering) that live in freshwater environments. Until recently, these microorganisms were seen only as a food source but a recent study published in December 2022 in the Proceedings of the National Academy of Sciences journal suggested that Halteria were capable of sustaining themselves on viruses only. It is hoped that these  microorganisms could be used to remove viruses from fragile ecosystems and create balance in unstable food chains. 

Halteria was placed in freshwater with Chloro Viruses. These viruses are known to infect microscopic green algae until the algae burst, releasing carbon and other substances into the water.  As such they play an important role in the termination of algal blooms and supplying nutrients to the water column. Despite this, there can be problems with the viruses preventing the carbon which might have travelled up the food chain via the predators of the algae and is then consumed by other microorganisms. This can cause an imbalance in ecosystems causing destabilising and destruction of habitats. 

As well as this, Chlorovirus ACTV-1 DNA has been found in the oropharyngeal of humans and there is limited knowledge about the effects this has on the body. People who were infected had delayed memory and decreased attention. Humans infected with the ATCV-1 form also showed a decreased visual processing ability and reduced visual motor speed.[1] Since the discovery in humans , tests on mice have also been carried out and showed that there were changes in the structure of the brain. Within the hippocampus (area responsible for memory and learning), changes in gene expression occurred and the infection caused a change in the pathways of immune cell functioning and antigen processing.This perhaps indicates the immune response to the ACTV-1 virus causes inflammation which may be the cause for cognitive impairments.[2] 

So how was the virus eating ability of these organisms discovered? The research team collected samples at a nearby pond and created droplets of water that contained nearly all of the microorganisms present in the pond. Then they added the chlorovirus. After 24 hours the drops were tested and the number of Chlorovirus dropped significantly whilst the Halteria population had grown 15x larger. The Halteria deprived of the chlorovirus, meanwhile, wasn’t growing at all. The team then tagged some of the DNA in the chlorovirus with green dye to confirm that it was only the Halteria causing the decreasing population.The vacuole inside the Halteria organisms turned green proving that the Halteria were consuming the Chlorovirus.

Therefore, ‘if ciliates like Halteria are eating these viruses then it could balance the carbon recycling and ensure that carbon moves up the food chain’, according to John DeLong, the co author of the study. Unfortunately, the research isn’t developed enough to suggest Halteria could combat chlorovirus infection in mammals and it is unlikely that funding would be supplied unless there were mass outbreaks of Chlorovirus in humans (which is seen as low risk). 

The researchers have since identified other ciliates that thrive by only consuming viruses and consequently the next step is to see whether this occurs in the wild and whether virus eating organisms could have the power to easily remove viruses from fragile ecosystems. Viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by approximately 3 gigatonnes of carbon per year[3]. Consequently, this work may also benefit the global ecosystem. 

The full findings can be found published the journal - Proceedings of the National Academy of Science

References

[1] Yolken RH, Jones-Brando L, Dunigan DD, Kannan G, Dickerson F, Severance E, Sabunciyan S, Talbot CC, Prandovszky E, Gurnon JR, Agarkova IV, Leister F, Gressitt KL, Chen O, Deuber B, Ma F, Pletnikov MV, Van Etten JL (November 2014). "Chlorovirus ATCV-1 is part of the human oropharyngeal virome and is associated with changes in cognitive functions in humans and mice". Proceedings of the National Academy of Sciences of the United States of America. Accessed 12/1/23

[2] Petro, Marilyn S.; Agarkova, Irina V.; Petro, Thomas M. (August 2016). "Effect of Chlorovirus ATCV-1 infection on behavior of C57Bl/6 mice". Journal of Neuroimmunology. Accessed 13/1/23

[3]https://en.m.wikipedia.org/wiki/Marine_viruses#:~:text=It%20is%20estimated%20marine%20viruses,often%20kill%20other%20marine%20life. Accessed 14/1/23