Amazing Invertebrates: Water Bears (Phylum Tardigrada)- Survival in Extreme Conditions
Tardigrades, also known as water bears, manage to make the world of microbiology a little more adorable. Don't let their appearance fool you though- despite looking cute and squishy water bears are some of the toughest invertebrates on the planet. Tardigrades are able to go into a state of suspended animation, referred to as a tun state, for 10 years or more. During this hibernation-like state they can withstand conditions that very few other organisms can. Some scientists estimate that the typical tardigrade life span could exceed 60 years when including different phases of suspended animation in the life cycle (Barnes et el 2001).
Water bears are free-living microscopic organisms that can be found everywhere from the seas to mountaintops. They can also live within thin films of water that develop on vegetation (such as dew on a leaf) and are sometimes referred to as "moss piglets." When their habitat dries up, tardigrades lose water through their cuticle and shrivel up into tun formation. While in tun form water bears can withstand extreme environmental conditions because they are in a state of such severe dehydration. They are also eutelic, meaning they are born with a fixed number of cells, so cell growth doesn't have to be halted and restarted as the water bears go from a tun state to an animated state and vice versa. When their habitat is again suitable for survival the tardigrades rehydrate, coming back to life.
While in tun form water bears are able to withstand almost completely anoxic conditions, using only 1/600 of the oxygen they would normally consume. In the tun state tardigrades can also withstand extreme temperatures, ranging anywhere from 150C (305F) to -272C (-457.6F) (Barnes et el 2001). The tardigrade easily survives where a human (or any other mammal for that matter) most certainly would not. Due to their ability to survive in such harsh conditions, tardigrades can live virtually anywhere on the Earth. This led scientists to theorize that perhaps water bears could survive in space.
Animals are usually sent to space in nice pressurized, oxygen-filled capsules that are shielded from deadly radiation given off by the sun. Kristianstad University in Sweden launched 3000 tardigrades into orbit
after inducing dehydration (thus prompting them into tun form) but the water bears didn't get quite the same treatment as their mammalian predecessors. They were traveling in a crude capsule that offered no oxygen, no protection from the vacuum of space and, for some select groups, no protection from solar radiation. The tardigrades orbited the planet for ten days under some of the harshest conditions a living organism can be subjected to, and then returned to Earth. Upon return, they were rehydrated and most of the water bears bounced back to life (many even healthy enough to successfully reproduce!) though the groups not exposed to solar radiation had significantly higher survival rates than those groups that were exposed to the radiation (Binns 2008).
Before the study only some species of lichens and bacteria were known to be able to survive both the vacuum of space and exposure to solar radiation. This makes the water bear the first animal known to survive under such conditions (Jonnson et el 2008).
The water bear is a reminder that life can persist in some of the harshest environments found on our planet and perhaps beyond. The definition of what is necessary or suitable for sustaining life is changing, and the more we learn, the cooler it gets.
Thanks for Reading! Literature Consulted:
Barnes, R., P. Calow, P. Olive, D. Golding and J. Spicer. (2001). The Invertebrates: A Synthesis . Oxford, UK: Blackwell Publishing.
Binns, C. (Dec 2008). Bears in Space. Popular Science , 273(6), 26.
Jonsson, K., E. Rabbow, R. Schill, M. Harms-Ringdahl and P. Rettberg. (8 Sep 2008). Tardigrades Survive Exposure to Space in Low Orbit. Current Biology , 18(17), 729-731.