Environmental tolerance in tardigrades

In 1834, C.A.S. Schulze, giving the first formal description of a tardigrade, Macrobiotus hufelandi, explicitly noted the animal's exceptional ability to tolerate environmental stress, subtitling his work "a new animal from the crustacean class, capable of reviving after prolonged asphyxia and dryness". Tardigrades are not considered extremophilic because they are not adapted to exploit extreme conditions, only to endure them. This means that their chances of dying increase the longer they are exposed to the extreme environments, whereas true extremophiles thrive there. == Cryptobiosis and the dehydrated 'tun' state ==

, in the tardigrade Richtersius coronifer Tardigrades are capable of suspending their metabolism, going into a state of cryptobiosis. Terrestrial and freshwater tardigrades are able to tolerate long periods when water is not available, such as when the moss or pond they are living in dries out, by drawing their legs in and forming a desiccated cyst, the cryptobiotic 'tun' state, where no metabolic activity takes place. In this state, they can go without food or water for several years. Further, in that state they become highly resistant to environmental stresses, including temperatures from as low as to as much as (at least for short periods of time), lack of oxygen, vacuum, ionising radiation, and high pressure. Marine tardigrades such as Halobiotus crispae alternate each year (cyclomorphosis) between an active summer morph and a hibernating winter morph (a pseudosimplex) that can resist freezing and low salinity, but which remains active throughout. Reproduction however takes place only in the summer morph. == Specific environmental stresses ==

Extremes of temperature Tardigrades can survive in extremes of temperature that would kill almost any other animal, including: • A few minutes at • 30 years at • A few days at Tardigrades are however sensitive to high temperatures: 48 hours at kills half of unacclimatized active tardigrades. Acclimation boosts the lethal temperature to . Those in the tun state fare better, half surviving for one hour. Longer exposure decreases the lethal temperature. For 24 hours of exposure, kills half of the tun state tardigrades. Impact Tardigrades can survive impacts up to about , and momentary shock pressures up to about . Radiation Tardigrades can withstand 1,000 times more radiation than other animals, median lethal doses of 5,000 Gy (of gamma rays) and 6,200 Gy (of heavy ions) in hydrated animals (5 to 10 Gy could be fatal to a human). However, tardigrades, when hydrated, remain much more resistant to shortwave UV radiation than other animals; one reason is their ability to repair damage to their DNA. Exposure to space mission carrying the BIOPAN astrobiology payload (illustrated) exposed tardigrades to vacuum, solar ultraviolet, or both, showing their ability to survive in the space environment. Tardigrades have survived exposure to space. In 2007, dehydrated tardigrades were taken on the FOTON-M3 mission and exposed to vacuum, or to both vacuum and solar ultraviolet, for 10 days. In contrast, hydrated samples exposed to vacuum and solar ultraviolet survived poorly, with only three subjects of Milnesium tardigradum surviving. In 2011, tardigrades went on the International Space Station STS-134, showing that they could survive microgravity and cosmic radiation, and should be suitable model organisms. In 2019, a capsule containing tardigrades in a cryptobiotic state was on board the Israeli lunar lander Beresheet which crashed on the Moon. == Damage protection proteins ==

Tardigrades' ability to remain desiccated for long periods of time was thought to depend on high levels of the sugar trehalose, common in organisms that survive desiccation. However, tardigrades do not synthesize enough trehalose for this function. The proteins may also form a glass-like matrix that protects cytoplasm from damage during desiccation. Anhydrobiosis in response to desiccation has a complex molecular basis; in Hypsibius exemplaris, 1,422 genes are upregulated during the process. Of those, 406 are specific to tardigrades, 55 being intrinsically disordered and the others globular with unknown functions. Tardigrades possess a cold shock protein; Maria Kamilari and colleagues propose (2019) that this may serve "as a RNA-chaperone involved in regulation of translation [of RNA code to proteins] following freezing." The Dsup proteins of Ramazzottius varieornatus and H. exemplaris promote survival by binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals. The Dsup protein of R. varieornatus confers resistance to ultraviolet-C by upregulating DNA repair genes. == References ==