A remarkable microbe named Deinococcus radiodurans (the name comes from the Greek deinos meaning terrible, kokkos meaning grain or berry, radius meaning radiation, and durare meaning surviving or withstanding) has survived a full year in the harsh environment of outer space aboard (but NOT inside) the International Space Station. This plucky prokaryote is affectionately known by fans as Conan the Bacterium, as seen in this classic 1990s NASA article.
The JAXA (Japanese Aerospace Exploration Agency) ISS module Kib? has an unusual feature for spacecraft, a front porch! This exterior portion of the space station is fitted with robotic equipment to complete various experiments in outer space’s brutal conditions. One of these experiments was to expose cells of D. radiodurans for a year and then test the cells to see if they not only would survive but could reproduce effectively afterward. D. radiodurans proved to be up to the challenge, and what a challenge it was!
Not only does the exterior of a spacecraft offer no protection from the vacuum of space and the wild temperature fluctuations associated with it, but the front porch is also subject to an enormous amount of space radiation. These microbes survived massive ultraviolet radiation levels, ionizing charged particles in the solar wind, and withstood galactic cosmic rays.
The Japanese Kib? module of the ISS. Credit: NASA
This article, published on October 29, 2020, in the journal Microbiome, details how these simple, single-celled life forms endured conditions that would kill a human in seconds for a full year. Just how does this hardy bacterium do it? The authors remark in the paper that in a high radiation environment, the number of nucleic acid fragmentations (breaks in the DNA chain) experienced by D. radiodurans is no different than that of the familiar E. coli. In other words, D. radiodurans doesn’t have some kind of radiation-proof shielding, like a microscopic lead vest from the dentist’s office. Instead, it is exceptionally capable of repairing the damage it sustains, making it 50 times more resistant to ionizing radiation than E. coli.
Illustration of the multi-faceted response of D. radiodurans to exposure to outer space. Credit: Ott, E., Kawaguchi, Y., Kölbl, D. et al.
Along with the direct repair of its DNA, Conan the Bacterium must cope with reactive oxygen species or ROS production. The cells use complex protein interactions to encapsulate ROS and associated cellular waste and debris. These capsules, called vesicles, are made from the cell membrane and can be seen dotting the exterior of cells brought back from ISS exposure. Compared with Earth-bound control cells, the bacterial astronauts are covered in survival polka-dots. It seems that the bacteria has a comprehensive, multi-faceted bag of tricks at its disposal when it comes to coping with the stress of exposure to outer space.
Why is this kind of research important? Not only is it fascinating to see what kinds of remarkable feats lifeforms like D. radiodurans are capable of, but it is also critically important for the effective sterilization of future space programs. Should an extraterrestrial world like Europa or Enceladus contain life, any Earthly explorative probes mustn’t contaminate the environment. It would be a tragedy to find life on another world only to accidentally wipe it out with stowaway bacteria that can survive the trip.
Artist’s concept of the Europa Clipper Spacecraft at Jupiter’s moon Europa. Europa is thought to contain a large, liquid-water ocean, and is an excellent candidate for extraterrestrial life. Such programs must be careful about stowaway bacteria and the risk of contamination. Credit: NASA/JPL
A less catastrophic but still frustrating possibility would be to find a false positive for life on another world. Imagine the excitement of finding alien microbes in the seas of Europa, only to discover later on that they were just hitch-hikers from Earth.
Another compelling reason to study the viability of extremophiles like D. radiodurans is to test the feasibility of the theory of panspermia. Panspermia is the idea that hardy life-forms could survive within rocks or other material ejected from one planet (say from a meteor impacting Mars) and then survive landing on another planet (like the Earth). If such a trip were possible, one could imagine life permeating the solar system from a single source, spreading from one world to the next through an accidental interplanetary space travel system facilitated by the extreme durability of certain microbial astronauts.
Artist’s depiction of panspermia. An impacting body delivers microbes to another planet. Credit: Ralph Crewe
The kind of survival and transport events necessary to facilitate the panspermia theory seem like exceedingly unlikely happenings. For any given microbe at any particular time, it is very improbable. At a glance, the whole concept seems rather far-fetched. The reason the theory is even worth consideration is the tremendous time-scales and populations of microbes. Imagine that the odds of a panspermia event are one in ten billion say. There are trillions and trillions of microbes on Earth, they have existed for billions of years, and the Earth has been struck by billions of meteors large enough to hit the ground in that time. The odds don’t seem so astronomical when you consider the microbial world’s scale and the breadth of geological timespans.
It is easy to ignore the microbial world; bacteria like D radiodurans are invisible and often live in mundane or even disgusting ways. D radiodurans was first found in a test of whether ground meat could be sterilized in a can using gamma radiation (1950s food technology was on another level). From one tin can to another, admittedly more advanced tin can in the ISS (who can resist a Space Oddity reference?), this simple life-form continues to amaze and inform and ultimately helps us paint a richer picture of the Universe around us and our place within it.