Tardigrades are very small creatures ranging in size from 0.2 to 1.2 mm. They live almost everywhere, but prefer to live in aquatic or humid environments. They are also called water bears. But they are known for their ability to survive in almost any conditions. They manage to survive at extremely low temperatures approaching absolute zero, but also in extreme heat of up to 150 degrees Celsius. They can withstand pressure of more than 6,000 atmospheres. They will live without water for decades. It is the only life form known to us that remains in space.
Scientists from Marshall University in West Virginia and the University of North Carolina at Chapel Hill say they have determined in a recent study how tardigrades can survive extreme conditions. According to them, small molecular sensors in their cells can detect when too many harmful molecules called free radicals are being produced. When this situation occurs, these animals enter a state of rest that allows them to survive.
The results and description of the research were published in the journal “PLOS ONE” (DOI: 10.1371/journal.pone.0295062).
Derek Collinge, co-author of the publication from Marshall University, exposed tardigrades to high levels of hydrogen peroxide, sugar, salt, and temperatures as low as minus 80 degrees Celsius in order to put the invertebrates into a state of hibernation. He then put the tardigrades into a machine that detects harmful and highly reactive free radicals, which are atoms or molecules containing unpaired electrons. He actually saw such atoms being created in tardigrades. This is not very surprising, since an animal's normal metabolic processes, as well as environmental stressors such as smoke and other pollutants, create free radicals in cells.
Free radicals strip electrons from their surroundings to stabilize them. In the process, they damage cells and other compounds such as DNA or proteins. When Leslie Hicks, a chemist at the University of North Carolina at Chapel Hill, learned of Collinge's experiments, she began to wonder whether free radicals might play a role in these animals' amazing survival abilities.
Under the influence of adverse environmental conditions or other stressors, tardigrades shrink, curl up, and enter a state of hibernation, which helps them survive unfavorable conditions. The researchers monitored the tardigrades as they entered this protective state, then checked whether and how they could wake up and resume normal activity when environmental conditions improved.
Hicks studies signaling interactions between free radicals and cysteine, a major component of proteins. The researchers exposed tardigrades to different types of molecules known to prevent cysteine oxidation.
Scientists have determined that cysteine oxidation leads to changes in the structure and function of proteins, signaling entry into a dormant state. This process also acts as a kind of regulatory sensor in response to stress. When there was no cysteine, the tardigrades were defenseless and could not enter a state of protective dormancy.
Kazuharu Arakawa of Keio University in Japan, who studies tardigrades, says the new work is consistent with previous research showing the role of cysteine oxidation in a particular fly known to be resistant to complete dehydration. The similarities suggest that this mechanism may be a common inducer of dormancy.
However, scientists say there is still a lot of work to be done to understand how free radicals affect tardigrades. The resilient state of cryptobiosis is not the only tactic tardigrades use to survive environmental stresses. The researchers plan to examine the remaining strategies in detail. Hicks hopes that this discovery will help in the long term in research into aging and space travel, which involves the occurrence of free radicals that destroy important cellular mechanisms such as DNA and proteins.
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