Biologists do not consider viruses to be alive. They are smaller than bacteria (20-300 nanometers compared to 500-1500 nanometers) and cannot replicate on their own - instead they must invade a host cell and use its genetic tools to aid its replication. Yet viruses completely dominate the planet - hypochondriacs might tremble at the fact that there are ten million trillion trillion viruses existing on Earth right now, with a tenth of them found in the oceans. Given their total dependence on cellular life in order for them to replicate, it is little surprise that wherever life is found on our planet, viruses are right there with them.
Griffin, a microbiologist at the US Geological Survey at St Petersburg in Florida, thinks that we can expect to find a similar situation where life exists on other planets. “I would think that the evolution of cellular life on another planet would be very similar to what occurred on Earth,” he says. “If this is the case and cellular life is present on any given planetary body then I would expect viruses to also be present in superior numbers.”
Yet he doesn’t believe that astrobiologists have fully cottoned onto this fact and that exo-viruses are just as viable a topic for speculative study as extraterrestrial cellular life. The reason for this may be partly because the study of viruses on Earth beyond those that cause illness in humans and animals has really only taken off in the last decade or so. As Stedman alludes to, studying viruses is not easy.
Now that things are changing, Griffin thinks it is time to consider extraterrestrial viruses. Stedman agrees. “Life on Earth is clearly heavily influenced by viruses,” he says. “The jury is still out on whether viruses are essential for life, but certainly life on Earth would be very different without viruses. Finding life in the absence of viruses would surprise me, but would be very interesting.”
The key point, Griffin argues, isn’t that viruses will exist where life also exists, as we will have noticed that life long before we find the accompanying viruses, but rather that multiple host types of viruses could be found at both the beginning and the end stages of life on a planet.
Of course, viruses can cause damage but they can also bring benefits to the cell. For instance, if the cell has been damaged by ultraviolet light, a virus carrying ultraviolet-resistant genes can introduce those genes to the damaged cell, helping it to repair. Vice-versa, damaged viruses can have their replicating abilities restored if a cell is infected by multiple viruses that swap enough genetic information to produce a complete undamaged viral genome.
This makes viruses extremely hardy. “Viruses are robust and adaptable and they may be able to survive a long time in a dormant state,” says Impey. Although viruses are inert outside of host cells, on Earth they have been seen to survive in extreme conditions. For example, viruses have been found in hot spring water in Yellowstone National Park in temperatures reaching 93 degrees Celsius. Conversely, viruses have been discovered in briny sea-water at minus 12 degrees Celsius, while the influenza virus is stored in secure laboratories at temperatures as low as minus 70 degrees, with no apparent damage to the virus itself. Nor do they require water to survive outside of the cell – they simply remain inert down and assuming no damage from radiation for example, they just re-awaken when they invade a new cell.
As such, if there was once life on Mars, today all that may be left could be the generalized viruses, capable of infecting most cells they come across. These viruses could therefore present a biohazard for any future astronauts that land on Mars and perhaps while we are looking for life on Mars, we should also equip our robotic rovers to search for viruses too.
Griffin has some ideas about how we may begin searching for extraterrestrial viruses. One tool would be a MEMS-based concentrator [Micro-Electro-Mechanical Systems] which is a micron-sized technology that can be used in chromatography [the separating of mixtures in a laboratory setting] and spectroscopy techniques. This could be used in conjunction with particle-sized separators, microscopic imaging systems and nucleic sequencers to sift through the Martian dirt and analyze it for virus-like structures and nucleics.
There is one other location in the Solar System where viruses could remain the last holdout of organic entities, but it is only to be found in the far future, two billion years hence when our Sun has brightened. At this time Earth would have warmed, the plants withered and died, the oceans boiled away and life all but eradicated. Even here, in this smoldering world, viruses could remain. With little cellular material remaining the viruses could evolve to share genes altruistically, regardless of the host. As such, suggests Griffin, altruism in the microbial world may be the last state of life to be found on our planet, before the Sun grows so hot that even the viruses are snuffed out. Viruses and cells working in unison at the beginning and end of life stages on our planet – with billions of years of competitive conflict in between.
In the meantime, research into exo-viruses could be the seed for a new revolution in astrobiology. “The biological research community will continue to refine our understanding of viruses,” says Impey. “Meanwhile, it is useful to start seriously considering their role in the wider landscape of exobiology. Griffin’s excellent paper is a good starting point.”