The Invincible Tardigrade

Water bear, SEM

Tardigrades (Phylum: Tardigrada), also known as water bears, were first discovered by the German zoologist, Johann August Ephraim in 1773.  They are water-dwelling micro-animals with eight legs and can reach a maximum size of 1.5mm. Now over 1000 individual species of tardigrades have been discovered to this day.

Tardigrades are some of the most resilient animals known and can be found in almost every environment, ranging from tropical rainforests to Antarctica, and even your own backyard! It is said that these tardigrades get to all these different places by being carried by wind and water currents and then deposited somewhere far away. If you have a microscope, it is actually possible to find your own wild tardigrades!

tardigrades

These water bears are renowned for their ability to survive extreme environmental stresses that would kill almost any other animal, such as being dehydrated, exposed to excessive amounts of gamma radiation and coping with pressures as high as 600 mega-pascals (MPa) – pressures that are beyond anything they might encounter in nature. They can also tolerate being frozen to -272.8 °C (which is just above absolute zero), as well as being heated to 151°C for 15 minutes and still bounce back to life. Not to mention, they can healthily reproduce in outer space! If you were to go into outer space without protection, you will die in only a matter of 15 seconds. The low pressure would force the air out of your lungs and the fluids in your body will expand, causing you inflate. Your capillaries would rupture and the ionising radiation would destroy the DNA in your cells.

So, the question is, how are tardigrades able to survive in these extreme conditions?

Although the complete answer is unknown, scientists believe that to survive being frozen or dried out, the tardigrade must deal with the consequence of lacking liquid water inside the cell to carry out cellular respiration (the process whereby cells breakdown energy-rich molecules (e.g. sugars) to obtain energy for survival). Being one of the few groups of species that are capable of reversibly suspending their metabolism (which is made up of all the chemical reactions that take place in their cells), the tardigrade enters into a state of cryptobiosis (a physiological state in which metabolic activity is reduced to an undetectable level without disappearing altogether) to survive.

tardigrade-tun

Whilst in this state, the organism curls up into a dry husk called a ‘Tönnchenform’, but it is more commonly known as a “tun”. Here, their metabolism lowers to less than 0.01% of normal rate and their water content can drop to 1% of normal content. The tardigrade also synthesises special molecules, such as trehalose, which fill its cell to replace lost water by forming a matrix. Components of the cell that are sensitive to dryness, such as proteins, DNA and membranes, get trapped in the matrix. It is thought that this keeps these molecules locked in this position to keep them from unfolding, breaking apart or fusing together. Most species of tardigrades can live in this dehydrated state for up to 5 years, however some can survive for decades. Once the tardigrade is re-hydrated, the matrix dissolves and the organism reanimates.

Understanding how tardigrades survive in these environments can give us insight into the limitations of life. As these organisms can cope with conditions so extreme, that they don’t even exist on Earth, it indicates that life might be able to occur on planets that are much less habitable than the one that we call home. The real-world applications of knowing the mechanisms of how tardigrades function are significant. These include developing crops that can survive severe, long lasting droughts and manufacturing medications that can be stored at room temperature, instead of having to be continuously chilled – a factor that makes supply in remote communities very difficult.

Written by Mili Katneni, Year 12

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