All of us, at least once, have asked this question: there is life on other planets?
When thinking about extraterrestrial life we usually imagine little green creatures with antennae on their head, but what scientists are looking for are microscopic organisms, more similar to bacteria than to humans.
The American National Aeronautics and Space Administration (NASA) defines Astrobiology as the study of the origin, evolution, and distribution of life in the universe. Not only the astrobiologists look for habitable environments on other planets of the Solar System, but they also try to understand the origin, evolution, and diversity of life on the Earth.
Some organisms on our planet can live under extreme conditions (extremophiles), for example at very high or very low temperatures (thermophiles like Thermus aquaticus or psychrophiles like Psychrobacter), in the vacuum (like tardigrades) or in the absence of oxygen (anaerobic like Chlostridium tetani). Since what we know about living organisms come from our knowledge on our planet, the current studies are based on the search on other planets of elements similar to those found on the Earth, even in environments that can look extremely inhospitable at a first sight.
The ExoMars program of the European Space Agency (ESA), for example, aims at studying the environmental conditions on Mars, to determine whether it can host living beings, or if it could have been possible in the past. The program consists of two missions, the first one launched in 2016 and the second one scheduled for 2022, to collect samples of Mars soil, search for traces of life, analyse hydrogeologic changes in the planet and study its atmosphere composition.
Previous missions on Mars have detected methane (CH4). Methane is also present in our atmosphere, and 90% of it has a biotic origin, meaning that it is produced by living organisms. The ExoMars missions want, among other things, confirm the presence of gaseous CH4 in Mars atmosphere and determine its origin. If its biotic origin were confirmed, it could have been produced by bacteria millions of years ago and stored under the surface of the planet to be gradually released to date, or it could be produced by bacteria currently living in Mars extreme environment.
The two missions will also search for sugars and amino acids, two building blocks of the terrestrial living organisms. An important characteristic of both these two classes of molecules is that they exist in two forms that are the mirror image of one another (enantiomers); however, terrestrial organisms can produce and use only one of those forms to build their carbohydrates and proteins (homochirality), while when they are chemically synthesized an equal mixture of both enantiomers is produced (racemic mixture). The presence of homochirality of sugars and amino acids on Mars would suggest that there is (or there was once) life on the red planet.
But if there are living organisms similar to bacteria on Mars, would be they able to survive also on Earth? Could they infect humans or other terrestrial organisms?
And conversely, would terrestrial microorganisms be able to survive on other planets?
Recently, scientists of the Radboud University Medical Center in the Netherlands, have tried to answer this latter question.
They cultured four different species of terrestrial bacteria (P. aeruginosa, B. cepacia, K.pneumoniae, and S. marcescens) in a medium containing water, ammonium, sulfur, phosphorus, and iron, and supply as a source of carbon different sugars previously identified on meteorites. The bacteria were able to grow on those culture media, even if slower than in the presence of a terrestrial sugar (glucose). Bacteria exposed to carbon sources other than glucose in these experiments showed structural changes in their external membrane, that in turn altered their interaction with human immune cells. Human cells responded in a stronger of weaker manner upon contact with those bacteria, depending on the sugar they were fed with.
These experiments suggest that terrestrial microorganisms that could arrive on other planets during space missions (for example through contaminated tools), might adapt two new conditions and modify some of their characteristics. This is why deep contamination of all the tools and objects used in space missions is extremely important.
Given that on Earth all living organisms including bacteria have viruses able to infect them, and that viral particles are the most abundant biological entities on our planet, if the presence of bacteria on other planets was confirmed, it would be reasonable to speculate about extraterrestrial viruses. However, at the moment, the search for viruses in the universe is not among the goals of any of the current space programs.
Figure taken from http://www.publicdomainpictures.net
Bibliography
European Space Agency: https://exploration.esa.int/web/mars/-/43608-life-on-mars
NASA Astrobiology Institute: https://nai.nasa.gov/about/
Immune recognition of putative alien microbial structures: Host–pathogen interactions in the age of space travel, Netea M.G. et al., PLoS Pathogens 2020 https://doi.org/10.1371/journal.ppat.1008153
Growth on Carbohydrates from Carbonaceous Meteorites Alters the Immunogenicity of Environment-Derived Bacterial Pathogens, Domínguez-Andrés J. et al., Astrobiology 2020 https://doi.org/10.1089/ast.2019.2173
Astrovirology: Viruses at Large in the Universe, Berliner A.J. et al., Astrobiology 2018, https://doi.org/10.1089/ast.2017.1649
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