The discovery of liquid water on some planetary bodies could be a sign that the Earth is not the only one to harbor life. But if micro-organisms from far away reached us, would we be immune? The answer is most likely not. Indeed, while having an immune system that protects us from a vast variety of pathogens sounds like an agreeable thing to have, the interaction between it and ET turns out to be more problematic than you might think.
I live for the day when I see humans set foot on a planet other than Earth. This is the type of long-term project that makes astronomers and entrepreneur’s dream.
Research is multiplying in the quest for a planet that could harbor life in any form, Europa, moon of Jupiter, and Enceladus, moon of Saturn, already offer interesting leads. These satellites could indeed satisfy one of the essential conditions for life developing thanks to the presence, still under investigation, of liquid water on their surface. Water hides under the surface of Mars, the neighboring planet on which humans have the ambition to go. However, before such a breakthrough takes place, NASA and space exploration companies such as SpaceX, Virgin Galactic and Blue Origin are promoting commercial space travel and exploring other planets, accompanied by the sending of long-range probes to recover samples.
I read the report issued by NASA’s Planetary Science Division in August 2016 with great interest. It describes several scenarios exploring Europa, Enceladus and Mars. And it is fascinating to imagine the first human footsteps on another planet; to see humans set foot on a planet other than Earth is truly an exciting idea!
However, simulations of these missions soon showed that such a breakthrough would also pose profound questions related to quarantine issues and to planetary protection.
If there is indeed water elsewhere than on Earth, life will have followed. What if, in samples, we introduced this life on Earth? This is the question that British scientists, authors of a study published in the journal Microorganisms, have asked themselves.
Extraterrestrial life brought back to Earth
Life as we know it requires water in liquid form. Celestial bodies outside of Earth that have or may have hosted an aqueous environment are the focus of attention in the search for extraterrestrial life. Mars’ dry river valleys are evidence of its past oceanic history, and liquid water may still exist on the Martian polar caps or beneath its surface. According to observations from the Cassini mission and images from the Hubble telescope, large oceans of liquid water may also exist under the ice caps of Jupiter’s moon Europa, as well as Saturn’s moon Enceladus.
The number of Earth-like planets orbiting Sun-like stars in our galaxy is estimated to be over six billion, and spectroscopic signatures of water vapor have been reported in the atmosphere of many exoplanets, including K2–18b.
Space missions such as Mars 2020 are looking for evidence of an “exovia” where water may exist, and may attempt to recover samples from planets or moons where life may exist. Such expeditions could contaminate the “exo-environments” with terrestrial microorganisms, bacteria or viruses, which would have survived the hard conditions imposed by space transportation, such as radiation, vacuum, variable temperatures… However, the reverse is also possible: would it be possible that non-terrestrial microorganisms that may have grown in aqueous environments could interact with humans and other animals upon arrival on Earth? This raises the question of biosecurity risks and problems of contamination of the terrestrial ecosystem, or even infection by ex-microorganisms.
“The world is all too aware of the immune challenge posed by the emergence of entirely new pathogens,” said Professor Neil Gow, deputy vice chancellor of the University of Exeter who led the study, describing the essence of the project, “As a thought experiment, we asked ourselves what would happen if we were exposed to a microorganism taken from another planet or moon where life has evolved.
Amino acids, building blocks of life
“Some very unusual organic elements exist outside of planet Earth, and could make up the cells of these extraterrestrial microbes,” introduces Neil Gow. “Would our immune system be able to detect proteins made from these non-terrestrial building blocks if such organisms were discovered and brought back to Earth and then accidentally escaped? We hypothesized that life forms that evolved in an environment of different amino acids might contain them in their structure.”
To simulate this hypothesis, the scientists synthesized in vitro peptides, chains of amino acids, that are rare on Earth, and then examined the ability of these peptides to activate and induce the action of T cells, immune cells, in mice, whose immune systems are very similar to ours. “Life on Earth relies on 22 essential amino acids,” explains lead author Dr. Katja Schaefer of the University of Exeter, who adds, “we chemically synthesized ‘exo-peptides’ containing Earth-rare amino acids, and tested whether a mammalian immune system could detect them.”
“Contact with extraterrestrial microorganisms could present an immunological risk.”
To synthesize the “exo-peptides”, the researchers used meteorite remains. Indeed, chondrites, stony meteorites, contain up to 5% of carbon, mainly inorganic, and a high proportion of organic compounds. Several amino acids, polyols, sugars, sugar alcohols, and sugar acids have been found on carbonaceous chondrites, including the Murchison meteorite that fell in Australia in 1969, which contained over 100 amino acids among thousands of organic molecules composed of two to nine carbon atoms. Among the most common amino acids in the meteorites, isovaline and α-aminoisobutyric acid were incorporated into peptides and subjected to the immune system of mice. “Our investigation showed that these “exo-peptides” were still processed, and that T cells were still activated, but that these responses were less effective than for “ordinary” terrestrial peptides.
T cells are activated at only 15% when the peptide containing isovaline is entered and at 61% when the peptide contains α-aminoisobutyric acid. For peptides comprising naturally abundant amino acids on Earth, these rates rise to 82% and 91%. “We therefore assume that contact with extraterrestrial microorganisms could pose an immunological risk for space missions aiming to retrieve organisms from exoplanets and moons,” concludes Dr. Katja Schaefer.
Although it has been shown that astronauts can survive in good health after many months in space, research has shown that spaceflight can weaken immune responses. The effects of exposure to a new microbe, absent on Earth, could thus be exacerbated in astronauts whose bodies and immune systems have already been exposed to extreme conditions and sustained environmental stress.
In the classic sci-fi novel, “The Hitch-Hiker’s Guide to the Galaxy”, there is a humorous view of transferring microbes from Earth to space and back again using self-delivering space suit washing units, whereupon they settle on Earth when water mixes with air in an astronaut’s helmet or spacesuit.
It is not yet certain that an extraterrestrial microorganism adapted to an extreme non-terrestrial environment could be pathogenic to a human host. It is likely that such an organism would be so poorly adapted to human body conditions that its ability to colonize and infect us would be limited. However, even if the pathogenic potential of the microorganisms were inherently attenuated, it is still possible that they could induce allergic reactions or create new toxigenic compounds.
Thus, if microbial life is discovered outside of Earth, and the protein content of the cells includes rare organic molecules found in meteorites, an immunological challenge would arise for humans and other animals.
Space explorations that aim to sample aqueous environments in our solar system therefore owe it to the researchers to recognize and mitigate the potential immunological threats posed by accidental exposure to new “exo-microorganisms.”