The tun state of suspended animation
Tardigrades, also known as water bears, are microscopic invertebrates famous for their ability to enter a state of suspended animation called cryptobiosis. The most studied form is anhydrobiosis, a response to the loss of water. When its mossy home dries up, a tardigrade retracts its eight legs and contracts its body into a desiccated, barrel-shaped form called a tun. In this state, its metabolism slows to undetectable levels.
This process works because is vitrification, or turning into glass. As the tardigrade desiccates, its cells produce large amounts of the sugar trehalose along with unique tardigrade-specific intrinsically disordered proteins (TDPs). This mixture replaces the water inside the cells, forming a glass-like, non-crystalline solid. This "bioglass" locks sensitive molecules like proteins and DNA in place, preventing them from breaking or unfolding. This protective state allows the tardigrade to wait out environmental conditions that would be lethal to almost any other form of life.
Tested to the limits
In the tun state, tardigrades are remarkably resilient. They have survived for minutes at temperatures as high as 150°C (302°F) and as low as -272°C (-458°F), just one degree above absolute zero. Their radiation tolerance is exceptionally high; they can withstand 5,000 to 6,200 grays (Gy) of ionizing radiation. For comparison, a dose of 5 to 10 Gy is fatal to humans.
Their durability has been tested in the most extreme environment known: outer space. In 2007, the European Space Agency's FOTON-M3 mission carried desiccated tardigrades into low Earth orbit. For 10 days, the animals were exposed to the vacuum and cosmic radiation of space. Some were also exposed to the full spectrum of solar UV radiation. After returning to Earth and being rehydrated, many of the tardigrades revived, and some even produced viable offspring. This was the first animal an animal was shown to survive the combined exposure to space vacuum and solar radiation. The record for long-term survival belongs to a sample of tardigrades frozen in moss in 1983; they were successfully revived after more than 30 years at -20°C.