Astrovirology

Viruses are a major driving force in evolution; the arms race between viruses and their host, or the Red Queen hypothesis, causes strong evolutionary pressures in both the host and viruses.[1] The host evolves to evade and destroy viruses, while the virus evolves mechanisms to continue infecting the host. Evolution is also influenced by viral horizontal gene transfer. Viral genes can be inserted into the host genome (ex. Retroviruses) and sometimes these genes are evolutionarily favorable. One common example of beneficial horizontal gene transfer in humans is the gene for syncytin, which came from ancient viruses and is important in placenta development.

Though unproven, some virologists posit that viruses may have played an important role in major evolutionary events, including the emergence of a DNA genome from an RNA world, divergence from LUCA to the three domains of life, archaea, bacteria, and eukarya, and development of multicellularity.[1] Emergence of a DNA genome and divergence from LUCA may have been aided by horizontal gene transfer of polymerases and other gene-editing enzymes from viruses. Meanwhile, viral selection pressures could have also aided divergence from LUCA to defend against different viruses, while multicellularity provides greater cell population protection from viruses.[1]

Viruses cause nutrient cycling in the ocean via the viral shunt, and up to 25% of the available carbon in the upper ocean is attributed to virus-induced cell lysis.[1]

Around 5% of Earth's oxygen is thought to be produced by cells infected by viruses encoding photosynthetic genes otherwise absent from the cell.[1] For example, some viruses of cyanobacteria contain genes for Photosystem II, which allows those cyanobacteria to photosynthesize and live in a different part of the ocean as their non-infected counterparts. Some viruses encode other metabolic genes that allow new metabolic functions in their host, for example, phosphate, carbon, and sulfur metabolism.

Viruses have been found in extremely hot, cold, and acidic natural environments, up to 93 °C (199 °F), down to −12 °C (10 °F), and down to pH 1.5.[2]

Viruses including tobacco mosaic virus, poliovirus, and bacteriophage T1 have maintained infectivity after being exposed to space-like conditions including interstellar radiation, low temperature, and low pressure.[1] Further studies are needed to assess the risk of viral hitchhikers, but any virus infecting an organism inside a habitable spacecraft can survive as long as that organism survives.

Latent viruses such as herpes virus, prevalent in humans, can become reactive during spaceflight due to spaceflight stressors. While astronauts experienced few if any symptoms, the potential for other viruses to become reactivated or more virulent is a substantial threat.[3]

Furthermore, some bacteria (Serratia marcescens) have been found to be more virulent in spaceflight conditions, leading to a question of whether viruses could also become more virulent.[4]

Limiting forward contamination is critical to be confident in the results of life detection efforts. Bacteria pose a significant contamination challenge in spacecraft assembly clean rooms despite decontamination procedures.[5] However, viruses were found to be present at relatively low levels, based on a metagenomic analysis.[6] Another metagenomic study detected viable human viruses, including herpesvirus and cycloviruses.[7]

Life (and viruses) on other planetary bodies have two important potential origins: from Earth or from a second genesis (life originated on that planet). Ancient viruses could have been transported from Earth to another planetary body, perhaps following a massive meteorite impact or volcanic eruption.[1] If this occurred, these viruses would likely be very biological similar to modern organisms.[2] There may be minimal or no immunity among Earth life against the ancient virus, and whatever organism it can infect may be crippled by its re-introduction.

If extraterrestrial viruses are part of a second genesis, their infectivity of Earth life depends on how they encode their genetic information. While their encoding could be incompatible with Earth life, it is also possible that RNA, DNA, or similar molecules could encode for life in the second genesis. In this case, Earth life may be a suitable host.[2]