Are viruses alive? This is a question that arises time after time and debate continues to rage on between researchers to this day. The general consensus is no – viruses are only able to reproduce within host organisms and lack the essential machinery required to consider them living.
One such machinery is the viruses’ lack of ability to produce their own energy. To get the energy they need to complete replication, they hijack the host’s energy supply to reproduce within the host cell, before exploding out and embarking on their way to infect another cell – or so we thought.
In a pre-print study published to bioRxiv (meaning it has not yet been peer-reviewed), researchers from the French National Institute for Sustainable Development (IRD) have discovered that a species of giant virus is possibly capable of independent energy production, representing the first time a virus has been demonstrated to have energy production capabilities.
"In our work, we have shown experimentally that pandoravirus (a giant virus of amoebas) have electrical membrane potential, an essential component to the survival of all living cells. It allows cells to function as a battery to generate energy," Professor Bernard La Scola, corresponding author of the study, told IFLScience.
The virus in question is quite the beast. With a virus particle diameter of 1 micrometer and a massive virus genome of 2.5 million base pairs, the Pandoravirus genus broke every size record possible after its discovery in 2013. Moreover, its massive size prompted scientists to rethink what exactly viruses are, as it blurred the lines between viruses being considered floating particles or single-celled organisms.
Since their discovery, scientists have found that many definitions of what a virus is do not apply to these giant viruses. They have some form of immune system, which helps fight off smaller viruses, and they have genes that allows them to process transcribe DNA into mRNA all on their own, which other viruses cannot do without a host.
Following discoveries of multiple proteins within these giant viruses that are not typically found in other viruses, the researchers decided to challenge Pandoravirus further to see whether it deviates from the norm in metabolism too.
Using technology that allows researchers to see whether there is an energy difference between the inside of a cell and outside, Sarah Aherfi and colleagues tested the virus Pandoravirus massiliensis for a voltage difference across the virus membrane (called membrane potential). They found that there was a difference in voltage, particularly with mature virus particles, suggesting a mechanism that may create energy within the virus itself.
The researchers then delved into the genome of P. massiliensis to try and identify genes commonly associated with energy production in other organisms. Eight genes were discovered that are all activated during the end of the virus’s replication cycle and share similarities (albeit low similarities) with genes used by other organisms in a key stage of energy production. When isolated and inserted into bacteria, some of these genes could produce the essential enzymes required for energy production, and supplying the virus with a regulator of energy production increased the membrane potential.
If these results are verified, a virus will have been demonstrated to produce energy independently and it's possible scientists will need to rethink the definition of viruses. Pandoraviruses are certainly viruses, as they still require host cells for replication, but to what extent do they conform to other viruses?
"Classical viruses were taken out of the three domains of life because they do not fulfill certain criteria characterizing life: they can only reproduce inside living host cells by using its translation machinery and its energy," said Professor La Scola.
"With our finding in pandoravirus (membrane potential and candidate genes of tricarboxylic acid cycle, the central hub of energy metabolism), it is increasingly evident that these organisms are very different from classical [definitions of viruses] and they are more close to the domain of life. That's why we are convinced to look seriously into their classification."
Some virologists are not yet convinced. New Scientist reported David Wessner from Davidson College in North Carolina stated that the research was limited to just viruses that have been released from cells, and some did not show energy production at all. IFLScience reached out for third-party comments, but have not recieved a response at this time.
Being a preprint, the study will need to be peer-reviewed before the textbook is rewritten and caution must be taken when drawing conclusions from initial studies such as this. Hopefully, further study into these viruses will yield new insights.
[H/T: The New Scientist]