Tardigrades, often called water bears or moss piglets, are among Earth’s most resilient microorganisms. These tiny invertebrates, typically no larger than half a millimeter, can endure conditions lethal to most other life forms. Scientists have even confirmed their ability to survive direct exposure to the vacuum and radiation of outer space, an astonishing feat for a multicellular animal. This unique hardiness has propelled them into the spotlight of scientific inquiry, particularly in understanding the boundaries of life and its potential beyond Earth.
The Space Mission Experiment
The groundbreaking experiment that sent tardigrades into space occurred in 2007 as part of the European Space Agency’s (ESA) FOTON-M3 mission. This mission carried the BIOPAN astrobiology payload, which included the “Tardigrade Resistance to Space Effects” (TARSE) experiment. For 12 days, dehydrated tardigrades of the species Macrobiotus richtersi and Milnesium tardigradum were transported into low Earth orbit, at an altitude of 250-290 kilometers.
The experiment subjected these animals to harsh open space conditions. Some desiccated tardigrades were exposed to the vacuum of space, shielded from solar ultraviolet (UV) radiation. Others faced both vacuum and direct solar UV radiation. The specific setup aimed to isolate the effects of these distinct space stressors on the tardigrades’ survival.
Cryptobiosis and Extreme Survival
Tardigrades use cryptobiosis, a reversible state where their metabolism slows to near undetectable levels. When faced with extreme environmental stressors, such as desiccation or freezing, tardigrades undergo a physical transformation, retracting their legs and shriveling into a compact, barrel-shaped form called a “tun”. In this state, their body water can drop to 1-3%.
Tun formation involves molecular changes, including the reversible oxidation of cysteine thiols in their proteins, signaling the tardigrade to enter dormancy. Within this desiccated form, unique proteins play a significant role in their cellular protection. Cytoplasmic Abundant Heat Soluble (CAHS) proteins, for example, are highly expressed during tun formation and polymerize to create gel-like structures that help prevent desiccation-sensitive proteins from losing function.
Tardigrades also produce a unique nuclear protein called Damage Suppressor (Dsup). Dsup directly protects their DNA by binding to nucleosomes, the structures that package DNA. This protein shields chromosomal DNA from harmful agents like hydroxyl radicals, generated by ionizing radiation or oxidative stress. This mechanism contributes to their remarkable radiation tolerance, allowing them to withstand doses thousands of times higher than lethal for humans.
Findings from Space Exposure
After returning to Earth, the tardigrades were rehydrated, and scientists observed their recovery and viability. Results showed varying survival rates based on exposure conditions. Over 68% of tardigrades exposed to space vacuum but shielded from solar UV radiation reanimated within 30 minutes.
Though shielded survivors had a high post-rehydration mortality rate, a significant number of them were still able to produce viable embryos. This indicated functional reproductive systems despite space vacuum stress. In contrast, tardigrades enduring both vacuum and direct solar UV radiation had a much lower survival rate.
Only a few UV-exposed specimens survived; intense UV radiation significantly reduced their overall survival and egg-laying capacity. Despite the severe challenge posed by unshielded solar radiation, the fact that any tardigrades survived and reproduced after such exposure underscored their extraordinary resilience to the combined stressors of the space environment.
Implications for Astrobiology
Tardigrades’ ability to survive outer space has profound implications for astrobiology, the study of life in the universe. Their resilience supports panspermia, the theory that life, or its building blocks, could travel between planets or even star systems. Organisms like tardigrades, encased within meteoroids or cometary debris, might theoretically endure the journey through space and potentially “seed” life on other celestial bodies.
Studying tardigrade protective mechanisms, such as the Dsup protein and their cryptobiotic state, offers valuable insights for future human space exploration. Scientists research how these mechanisms could be adapted to protect human astronauts from the dangers of space radiation during long-duration missions, such as journeys to Mars. For instance, recent research has explored delivering tardigrade proteins via messenger RNA (mRNA) into mammalian cells, demonstrating a reduction in radiation-induced DNA damage. These findings suggest potential avenues for developing countermeasures against the cellular damage caused by cosmic and solar radiation, enhancing the safety and feasibility of extended human presence beyond Earth.