Tardigrades, often called water bears or moss piglets, are microscopic invertebrates known for their distinctive appearance and remarkable durability. These tiny animals, typically measuring around 0.5 millimeters in length, possess plump, segmented bodies with eight legs, each ending in claws. They inhabit a wide array of environments across Earth’s biosphere, ranging from the highest mountaintops to the depths of the ocean. Tardigrades are commonly found in damp mosses, lichens, and various aquatic settings.
Entering the Tun State
Much of the tardigrade’s extraordinary resilience stems from a state of suspended animation known as cryptobiosis, specifically anhydrobiosis, which is life without water. When faced with adverse conditions like desiccation, tardigrades retract their legs and head, curling into a dehydrated, shrunken, dormant form called a “tun.” In this tun state, their metabolic processes nearly cease, allowing them to endure periods without food or water for years, and even decades.
Unlike some organisms that use trehalose for desiccation tolerance, tardigrades primarily utilize unique tardigrade-specific intrinsically disordered proteins (TDPs). These TDPs help protect cellular structures and DNA from damage during dehydration and subsequent rehydration by forming a protective, non-crystalline, glass-like matrix within the cell or by associating with lipid membranes.
Enduring Environmental Extremes
Tardigrades in their tun state can survive a wide range of environmental extremes that would eliminate most other forms of life. They demonstrate an exceptional tolerance to extreme temperatures, enduring conditions as cold as -272 degrees Celsius, just above absolute zero, and as hot as 149 degrees Celsius for short periods.
Their resistance to radiation is also notable, as they can withstand doses far exceeding what is lethal to humans. Tardigrades have been shown to survive ionizing radiation doses of 3 to 10 kilograys (kGy), which is approximately 1,000 times the lethal dose for an average human. This protection is partly attributed to a damage suppressor (Dsup) protein, which shields their DNA, and a robust DNA repair system that activates post-exposure.
Experiments have also demonstrated their capacity to survive the vacuum of space. Dehydrated tardigrades exposed to the harsh conditions of low Earth orbit, including vacuum and solar ultraviolet radiation, have been reanimated back on Earth, with many producing viable offspring.
Tardigrades exhibit tolerance to both high and low pressures. They can withstand immense pressures, such as those found at six times the depth of the Mariana Trench, the deepest part of the ocean. They have also shown survival after high-speed impacts, experiencing momentary shock pressures of up to 1.14 gigapascals.
Significance of Tardigrade Resilience
The extraordinary resilience of tardigrades holds substantial implications for various scientific fields. Researchers are keenly interested in understanding the molecular mechanisms behind their survival to explore potential applications in medicine. Insights from tardigrades could lead to improved methods for the long-term preservation of vaccines, organs, and blood without refrigeration.
In biotechnology, studying tardigrade proteins may offer avenues for developing more resilient crops capable of withstanding drought conditions. The ability to enhance the resilience of other organisms by transferring tardigrade genes or proteins is an active area of research.
From an astrobiological perspective, the tardigrade’s survival capabilities provide insights into the potential for life in extreme extraterrestrial environments. Their ability to withstand space conditions suggests that life might be more widespread in the universe than previously thought.