Are viruses living organisms? A Researcher’s point of view

The quandary of the ontological status of the virus is not new. Despite its simple appearance, it shapes the complex question of our relationship with living organisms. Thierry Candresse, researcher at INRA, and Jean Michel Lett, CIRAD virologist and Alexander Bryun, doctor of virology, give us their views on the subject. Are viruses living organisms ?

Behind this deceivingly simple question lies a particularly complex problem which is impossible to give a response without defining what is meant by a "living organism". This definition, however, stems from our own understanding of life. Therefore, it can only be subjective and incomplete, changing as new scientific knowledge comes to light.

Biologically speaking, a "living organism" is a complex structure which, through homeostatic phenomena, avoids thermodynamic equilibrium^[1]. Living beings are characterized by their ability to self-replicate and the information necessary for this is encoded using nucleic acids in the form of DNA or RNA. Living organisms are also subject to natural selection and evolution.

"Viruses are unable to replicate on their own ..."

Viral information is indeed stored in DNA or RNA strands. However, viruses are unable to self- replicate. To multiply, a virus requires the replication apparatus of a living cell. It is an obligate parasite. Is that enough to separate viruses from other living organisms ? Jean Michel Lett, a virologist at the CIRAD, gives us his thoughts on the subject : 

"Viruses cannot self-replicate, but neither can most parasites. Parasitoidal wasps are widely used in biological control. No one questions their status as living organisms !”

Viruses are subject to the laws of evolution. Despite their inability to replicate in the absence of host cells, it would seem logical to consider them as living, albeit as an extreme parasite. This is the view of Alexander de Bruyn:

"For me, they are part of the living world because they have two key characteristics: they replicate and persist over time. However, the issue is complex because, evolutionarily speaking, certain viruses could be cellular organelles which have acquired their independence. Geminiviruses, for example, are assumed to be of plasmid origin, derived from a bacterium or arched prokaryote. Plasmids, when isolated, are not considered living. It really does not simplify the debate ... "

There are other features of the viral world that we need to consider 

In 1935, Wendell Stanley demonstrated the crystallizable nature of viruses and their ability to retain their virulence^[2]. A kitten undergoing the same treatment would encounter some difficulties in remaining alive. Crystallization is the criterion of purity of a chemical compound, viruses are, according to this definition, closer to the mineral world than the living world. In 1992, a team recreated the vacuum of space in a laboratory to observe the effect on viruses. Their conclusion is straightforward: after being in space, viruses remain infectious. In contrast, a cow, placed in the same conditions, would have little chance of remaining alive for long^[3] Philosophically, being alive might be considered as contextual, dependent on external factors. A cow in the vacuum of space no longer retains the elements necessary to be classified as living (especially in maintaining its homeostasis or breeding). Therefore, upon its return from space, a virus will seem more “alive” than a cow, for which the experience would have been infinitely more traumatic.

"In everyday life the question does not arise"

The status of the virus is still under debate in the scientific community, and more often appeals to passions than rationality. This is because it fundamentally affects life and raises the sensitive question of the origin of life. Henry Poincaré talks about day science and night science^[4], Thierry Candresse joins him in analyzing the problem :

"In everyday life the question does not arise. I prefer to speak of biological entities. They are pathogenic, causing damage to crops and this is what we are interested in. There is no debate. For us, they are part of the fruit’s ecosystem^[5].”

For Thierry Candresse a virus is defined as a biological entity, body "on the fringes of life." To the chagrin of the philosophers, the living world refuses to be simplified. It is characterized by complex and non-reducible interactions. Living beings are interdependent of each other and viruses are not an exception. According to Jean Michel Lett, things go further :

"New sequencing technologies and the growing interest in the marine world have revealed an extraordinary and unexpected diversity: viruses are everywhere and in great abundance! To such a point that they are a key factor in the evolution of the living world. On the other hand it seems that they are involved in ecosystems and the emission of greenhouse gases. Oddly enough, these parasites are a key element of life."

However, the ontological status of viruses is still not settled and will change depending on the definition we give to life. Max Planck used to say (about the acceptance of new scientific theories), it was not that they were insisted upon, but their opponents who eventually disappeared^[6]. It remains to be seen what position the next generation of virologists will take.For my part, I prefer relativism^[7]. A virus can be defined as a living organism as soon as it finds the conditions necessary for multiplication. Just like a cow drifting in the vacuum of space is regarded as an inert body, a simple cluster of nucleic acids, proteins and fatty acids. 

^[1] Due to their physical structure, living beings can cut loose from the environment in which they live. A bacterium does not have the same composition as the medium in which it is immersed. Homeostasis allows Man to retain water in his cells, which prevents us from having to live constantly in water.
^[2] For his work on the tobacco mosaic virus, Wendell Stanley was awarded the Nobel Prize for Chemistry in 1946.

^[3] The bovine metaphor comes from Jean-Jacques Kupiec and Pierre Sonigo who take all the credit.
^[4] The mathematician Henri Poincaré differentiated daily laboratory science, from the researcher thinking about his work. He defined the first as “day science”, the second “night science”.
^[5] Thierry Candresse is a virologist for the UMR Fruit Biology and Pathology, a collaboration between the INRA and the University of Bordeaux working on plant / virus interactions.
^[6] "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, followed by a new generation that is familiar with the new idea." Max Planck
^[7] Hard to be more relativistic than this CIRAD researcher who wishes to remain anonymous, who told us recently: "Virus are what keeps us, the virologists, alive."