Life Beyond Earth: Deinococcus radiodurans demonstrates resilience in low-earth orbit

By: Ileana Fournier '21 and Rachel McNeil '21

Figure 1. A Russian cargo craft just after undocking from the International Space Station. Cargo Crafts bringing food and other supplies and taking away waste are an important part of how humans are able to live on the International Space Station.

Space is generally considered to be incompatible with life, but that may be changing. In recent years, scientists have begun to consider the possibility of continuing human life on other planets. Scientists have also been searching for information about how life might be able to exist beyond earth. While nothing conclusive has been found about life on other planets, we do know some of the challenges that space travel/living in space pose for humans.

One major barrier to extended stays on other planets is the production of food. This may sound simple enough, but existing in outer space presents new challenges to organisms, which must now survive and grow in unfamiliar temperatures, gravitational strengths, external pressure levels and radiation levels. One way scientists are approaching this challenge is by studying a few rare organisms that are naturally resistant to these stressors.

In a recent study, Molecular repertoire of Deinococcus radiodurans after 1 year of exposure outside the International Space Station within the Tanpopo mission, scientists took one such organism, a gram positive bacteria called Deinococcus radiodurans, into orbit outside of the International Space Station to study its mechanisms for radiation resistance. The International Space Station was chosen for its ease of accessibility and its ability to provide the conditions of low Earth orbit (LEO). D. radiodurans was already known to be resistant to radiation damage, which was first discovered after it was isolated from a can of SPAM that had spoiled from X-ray exposure. This radiation resistance had subsequently been observed in more traditional laboratory settings many times before this recent experiment. In some past experiments D. radiodurans and E. coli were both exposed to UVC radiation lamps and ionizing radiation lamps that mimicked LEO levels of radiation. After 3-4 hours of exposure, the E. coli samples died from DNA fragmentation while the D. radiodurans samples sustained the same amount of DNA fragmentation but were somehow able to completely repair and restart their normal growth cycles soon after exposure despite having very similar DNA repair mechanisms as E. coli. This finding suggests that D. radiodurans is specifically adapted to be resistant to long term radiation damage, prompting researchers to investigate exactly how long it can withstand radiation and still recover and whether D. radiodurans maintains its radiation resistance in LEO.

After one year at the space station, samples of D. radiodurans were allowed to recover for two hours so the repair process could begin and then the samples were observed. They found that while less new colonies were produced compared to a control sample left on Earth, there was only an approximately 25% decrease in the overall number of D. radiodurans cells in the liquid sample that had spent 1 year in LEO. While the concentration of the samples was impressively similar, there were some equally impressive changes observed at the cellular level.

Figure 2. Scanning and transmission electron microscopy (SEM and TEM) images of D. radiodurans cells recovered after LEO exposure in complex medium. a, b SEM images of recovered D. radiodurans cells after LEO exposure. c, d TEM images of recovered D. radiodurans cells after LEO exposure. e, f SEM images of ground control D. radiodurans cells. g, h TEM images of ground control D. radiodurans cells

Scanning electron microscopy (SEM) imaging observations revealed a substantial amount of small particles on the surface of the LEO exposed cells (figure 2 a and b) that were not present on the control samples (figure 2 e and f), providing evidence that these particles are related to the resistance to damage or repair from LEO. After looking at the surface of the cells, researchers then looked inside the cells to look for intracellular responses, specifically proteomic changes. They found that genes related to catalytic activity were significantly more active in the LEO exposed cells than the control cells, and that there were virtually no mRNA associated with replication in the LEO exposed cells. Instead, there were more mRNA found associated with cellular and DNA repair in the LEO exposed cells than the control cells. This explains why there were less new colonies observed: cells divert energy away from replication in favor of supporting repair and increasing metabolism, likely to keep up with the increased catalytic activity seen during the early stages of repair.

One important takeaway from the paper (and previous research) is that the resiliency of D. radiodurans to radiation and other environmental stressors is due to efficient repair/recovery, not prevention/protection or rapid cell multiplication. In other words, radiation does a similar amount of damage to the DNA of D. radiodurans as it does to other bacteria, but D. radiodurans is more readily able to repair that damage, partially (as demonstrated in this study) by diverting its resources away from reproduction while it recovers. This suggests that future research into the development of food that can endure space travel should focus on DNA repair, as increased DNA repair is sufficient to keep one organism alive in LEO. Future researchers can, and likely will, focus on the mechanisms for increased DNA repair in D. radiodurans as a blueprint for modifying agricultural organisms to survive space travel.