A notable microbe called Deinococcus radiodurans (the name comes from the Greek Deinos, which means terrible, coconut means grain or berry, radius means radiation, and durare means survival or resistance) has survived on board (but NOT inside) the International Space Station. This brave prokaryote is affectionately known by fans as Conan the Bacterium, as seen in this classic NASA article from the 1990s.
The ISS module JAibA (Japanese Aerospace Exploration Agency) Kib? has an unusual feature for spacecraft, a veranda! This outer part of the space station is equipped with robotic equipment to carry out various experiments in the brutal conditions of space. One of these experiments was to expose D. radiodurans cells for a year and then test the cells to see if they would not only survive but could reproduce effectively afterwards. D. radiodurans has risen to the challenge and what a challenge!
Not only does the exterior of a spacecraft offer no protection from the vacuum of space and the associated wild temperature fluctuations, but the porch is also exposed to enormous amounts of space radiation. These microbes survived massive ultraviolet radiation, ionized charged particles in the solar wind, and withstood galactic cosmic rays.
The Japanese kib? Module of the ISS. Photo credit: NASA
This article, published October 29, 2020 in the journal Microbiome, describes how these simple, unicellular life forms endured conditions that would kill a human in seconds for a full year. How does this robust bacterium do it? The authors note in their work that in an environment with high radiation the number of nucleic acid fragmentations (breaks in the DNA chain) that D. radiodurans experiences is no different from that of the known E. coli. In other words, D. radiodurans has no radiation-proof shielding like a microscopic lead vest from a dental practice. Instead, it is extraordinarily capable of repairing the damage it has suffered, making it 50 times more resistant to ionizing radiation than E. coli.
Illustration of the multifaceted response of D. radiodurans to exposure to space. Photo credits: Ott, E., Kawaguchi, Y., Kölbl, D. et al.
In addition to directly repairing its DNA, Conan the Bacterium also has to cope with the production of reactive oxygen species or ROS. The cells use complex protein interactions to encapsulate ROS and related cellular debris and debris. These capsules, called vesicles, are made from the cell membrane and can be seen on the outside of cells brought back from the ISS exposure. Compared to earth-bound control cells, the bacterial astronauts are covered with survival spots. It appears that the bacteria have an extensive, multi-faceted bag of tricks when it comes to dealing with the stress of exposure to space.
Why is this type of research important? Not only is it fascinating to see the remarkable feats life forms like D. radiodurans are capable of, but it is vital to the effective sterilization of future space programs. Should an extraterrestrial world like Europa or Enceladus contain life, terrestrial exploration probes must not contaminate the environment. It would be a tragedy to find life on another world only to accidentally wipe it out with blind bacteria that can survive the trip.
Artist concept of the spaceship Europa Clipper at Jupiter's moon Europa. Europe is believed to contain a large ocean of liquid water and is an excellent candidate for extraterrestrial life. Such programs must be careful with blind bacteria and the risk of contamination. Photo credit: NASA / JPL
A less disastrous yet frustrating option would be to find a false positive for living in another world. Imagine the excitement of finding alien microbes in the seas of Europe only to later discover they were just hitchhikers to Earth.
Another compelling reason to study the viability of extremophiles like D. radiodurans is to test the feasibility of the panspermia theory. Panspermia is the idea that sturdy life forms could survive in rocks or other material ejected from one planet (e.g. from a meteor hitting Mars) and then landing on another planet (like Earth ) could survive. If such a journey were possible, one could imagine life permeating the solar system from a single source and spreading from one world to the next through a random interplanetary space system, facilitated by the extreme durability of certain microbial astronauts.
Artist's impression of panspermia. A crashing body delivers microbes to another planet. Photo credit: Ralph Crewe
The type of survival and transportation events required to facilitate pansper rental theory seem extremely unlikely. For a given microbe at a given point in time, this is very unlikely. At first glance, the whole concept seems pretty far-fetched. The reason the theory should be considered in the first place is because of the enormous time scales and populations of microbes. Imagine the probability of a panspermia event being one in ten billion. There are trillions and trillions of microbes on earth, they have existed for billions of years and the earth has been hit by billions of meteors large enough to touch the ground during that time. The odds don't seem as astronomical given the size of the microbial world and the breadth of geological time spans.
It is easy to ignore the microbial world; Bacteria like D-Radiodurane are invisible and often live in worldly or even disgusting ways. D-radiodurans were first found in a test to determine whether minced meat could be sterilized in a can with gamma radiation (1950s food technology was at a different level). From one tin can to the next, admittedly more advanced tin cans on the ISS (who can resist a space oddity reference?), This simple form of life continues to amaze and inform, and ultimately helps us to draw a richer picture of the universe around us and our place in it .
Full article in Microbiome