There has been much debate as to whether viruses constitute living organisms. In a magazine article from the Microbiology Society titled “Are Viruses Alive”, two microbiologists discussed whether viruses should be considered living organisms. Nigel Brown, the first interviewee, explains that viruses need a host cell to replicate, which goes against one of the eight characteristics of life. Additionally, Brown explains that many viruses lack ribosomes and proteins needed to sustain a proper metabolism. Contrastingly, David Bhella, the second interviewee, explains that viruses could be considered living if one defines life as the ability to evolve, not the metabolic-focused definition that other biologists use. In consideration of this definition, viruses are found to evolve rapidly which may suggest that they are indeed alive1. Though scientists are still discussing whether viruses should be considered living, a new type of virus, giant viruses, has added a quizzical new perspective on what viruses are. In 2003, a team discovered a virus named mimivirus from Acanthamoeba polyphaga that had a genome size of 800 kb. The overall size of the virus particle is 400 nm which rivals that of multiple species of bacteria2. These viruses have very unique gene sets when compared to other viruses and canonically known living organisms3. Additionally, these giant viruses are found to have some metabolic pathways which, to some, seem to suggest that viruses may be living organisms.
In the podcast “This Week in Virology: Endless Giant Virus Forms Most Beautiful”, Alexandra Worden describes a giant virus her team unintentionally discovered. Worden and her group found a 900 kb-sized giant virus in a choanoflagellate that has 862 predicted proteins and a GC content of 22%. It was the low GC content that facilitated the isolation and ultimately the sequencing of the viral genome as the team separated the low GC fraction of DNA from the choanoflagellate’s genome. This virus was found to have three rhodopsin-like proteins in its genome. These proteins were expressed in Escherichia coli where it was found that all three absorb different wavelengths of light. Upon the absorption of their specific spectra of light, they can pump protons across a membrane which is suggested to facilitate the choanoflagellate they infect with carbon fixation in the form of photoheterotrophy. Additionally, the virus has the full metabolic pathway for making the pigments needed in its rhodopsin proteins. This virus may bring a new metabolic pathway for the choanoflagellate which may provide a mutualistic relationship with this protist. However, both the choanoflagellate and the virus have not been cultured. Although transgenic expression suggest these rhodopsin proteins play a role in the choanoflagellate cell, the rhodopsin proteins must be studied in vivo before conclusions can be drawn as to how the virus interacts with its host cell4,5.
Though more work needs to be done to fully research this giant virus, Worden’s team brings a fascinating new virus-eukaryotic host interaction to light that is somewhat similar to coral and zooxanthellae. With regard to the current debate on the legitimacy of viruses being living organisms, giant viruses seem to provide examples where a virus can have a metabolism. Though the study of giant viruses does not entirely guarantee viruses may be accepted as a lifeform, they certainly bring a new perspective into sophisticated relationships with how these giant viruses interact with their host organisms. Additionally, the high diversity and uniqueness of giant viruses may provide clues towards the origins of life, information on the evolution of viruses, or new metabolic pathways never seen before.
- Society, M. (n.d.). Are viruses alive? Retrieved from https://microbiologysociety.org/publication/past-issues/what-is-life/article/are-viruses-alive-what-is-life.html
- La Scola, B., Audic, S., Robert, C., Jungang, L., de Lamballerie, X., Drancourt, M., ... & Raoult, D. (2003). A giant virus in amoebae. Science, 299(5615), 2033-2033.
- Giant Viruses. (2018, February 2). Retrieved from https://www.americanscientist.org/article/giant-viruses
- https://www.asm.org/Podcasts/TWiV/Episodes/Endless-giant-virus-forms-most-beautiful-TWiV-575
- Needham, D. M., Yoshizawa, S., Hosaka, T., Poirier, C., Choi, C. J., Hehenberger, E., ... & Kurihara, R. (2019). A distinct lineage of giant viruses brings a rhodopsin photosystem to unicellular marine predators. PNAS 116(41), 20574-20583.
