By Sarah Choi
Tardigrades, also known as water bears or moss piglets (due to their adorable appearances), are microscopic animals that have become well known as “the most indestructible” living creature on Earth. They have been found to be resilient to many stresses, such as being frozen, fried, dried, and deprived of many of the basic necessities of life such as food and air. In 2007, researchers even found them capable of surviving in space (Jönsson et al., 2008). So, while their cuteness may be debatable, they are undeniably the owners of some extraordinary abilities.
For tardigrades to survive being released into space, they would need to be equipped with mechanisms to overcome a number of stresses. Solar radiation severely damages DNA, so tardigrades would need to protect or rapidly repair their genetic material. The low temperature in space causes ice crystal formation in internal fluids, so tardigrades would need some type of cryoprotectant. There are also extremely low pressures to consider, and the absolute lack of everything vital to life. As outer space does seem to be the most hostile environment there is, it is worth taking a look at how tardigrades manage to remain alive and well after direct exposure to the space vacuum.
Tardigrades are able to survive temperatures lower than -180 °C by undergoing cryptobiosis. Cryptobiosis is a reversible state of suspended animation, in which metabolism is decreased to the point of being undetectable yet still present. There are different types of cryptobiosis to counter different stresses, including three types that involve the tardigrade forming a “tun” (when they tuck themselves into a small ball). These types are: anhydrobiosis to cope with a lack of water; cryobiosis to combat low temperatures; and osmobiosis to counter extreme salinity.
The most studied type of cryptobiosis in tardigrades is anhydrobiosis. During anhydrobiosis, tardigrades remove water from within themselves (making themselves anhydrous), and curl up into a compact tun. Tardigrades are able to survive in this form because they synthesize and secrete trehalose, a sugar that replaces the water in cells. This is like gel, protecting the membranes and organelles within cells (Stromberg, 2012). Tardigrades also produce glycerol and other proteins to prevent ice crystals from damaging their cells. Furthermore, some tardigrade species that lack trehalose use tardigrade-specific intrinsically disordered proteins (TDPs) to survive desiccation. These proteins vitrify, forming non-crystalline glass-like solids that trap and protect cells like an armour (Boothby et al., 2017). Tardigrades can persist for years or even decades as a desiccated tun, patiently waiting until conditions improve. In studies, this would be when they are rehydrated and subsequently show signs of life (instead of dying).
Tardigrades can prevent their DNA from being damaged by harmful UV radiation. UVC-induced thymine dimers in DNA interfere with DNA synthesis and can lead to mutation, therefore the less thymine dimers, the better. While undergoing anhydrobiosis undoubtedly helps protect DNA, antioxidant proteins also actively protect DNA (and cells in general) from UVC-induced oxidation. Moreover, if preventing DNA damage falls through, hydrated tardigrades can repair the damage once thymine dimers are formed using light-dependent photolyases (Horikawa et al., 2013). The many strategies that tardigrades can use demonstrate the diversity and resilience this phylum have.
Besides UV radiation, tardigrades can also be revived after exposure to 5000 Gy gamma radiation and cosmic radiation in general (with certain limits). They can lose DNA, make protective genes, and increase expression of special proteins that protect and repair the delicate strands of DNA. In particular, a nuclear damage protection protein, Dsup, shields DNA from damage by radiation and hydroxyl radicals. By making and comparing a model of Dsup with a similar protein, researchers have found that Dsup’s structure allows it to bend and fit the shape of DNA, wrapping around the helical structure. Its nucleosome-binding domain also allows the protein to bind to nucleosomes. This protein therefore seems to support DNA, while deflecting or absorbing radiation, thereby protecting the genetic information from harm (Chavez et al., 2019; Mcrae, 2020). This is why, due to a crashed spacecraft (containing tardigrades) on the Moon, there may be live tardigrades on the Moon at this very moment, provided that the tardigrades managed to get somewhere below the surface (Woordward, 2019).
The ability of tardigrades to survive in such extreme conditions for such a long period of time opens up the question of whether tardigrades or other living creatures could travel to other planets or celestial bodies, and even possibly live there (actively, not in cryptobiosis). There is still much to discover about these amazing animals, and knowledge of their strategies for survival (in space as well as in other extreme conditions, of which there are numerous) may be used to better human lives.
References:
Boothby, T. C. et al. (2017) ‘Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation’, Molecular Cell, 65(6), pp. 975-984.e5. doi: 10.1016/j.molcel.2017.02.018.
Chavez, C. et al. (2019) ‘The tardigrade damage suppressor protein binds to nucleosomes and protects dna from hydroxyl radicals’, eLife. doi: 10.7554/eLife.47682.
Horikawa, D. D. et al. (2013) ‘Analysis of DNA Repair and Protection in the Tardigrade Ramazzottius varieornatus and Hypsibius dujardini after Exposure to UVC Radiation’, PLoS ONE. Edited by G. Maga, 8(6), p. e64793. doi: 10.1371/journal.pone.0064793.
Jönsson, K. I. et al. (2008) ‘Tardigrades survive exposure to space in low Earth orbit’, Current Biology. Elsevier, pp. R729–R731. doi: 10.1016/j.cub.2008.06.048.
Mcrae, M. (2020) Tardigrades Have DNA Armour, And We Just Got Closer to Understanding How It Works, Science Alert. Available at: https://www.sciencealert.com/we-re-a-little-closer-to-understanding-how-the-tardigrade-s-dna-armour-works (Accessed: 9 September 2020).
Stromberg, J. (2012) How Does the Tiny Waterbear Survive in Outer Space?, Smithsonian Magazine. Available at: https://www.smithsonianmag.com/science-nature/how-does-the-tiny-waterbear-survive-in-outer-space-30891298/ (Accessed: 9 September 2020).
Woordward, A. (2019) There Could Be a Bunch of Tardigrades Alive on The Moon Right Now, Science Alert. Available at: https://www.sciencealert.com/a-group-of-tardigrades-crashed-into-the-moon-in-april-they-could-still-be-alive (Accessed: 9 September 2020).
Imperial Bioscience Review 