How Do Tardigrades Or 'Water Bears' Survive Freezing Temperatures? Study Gives Clues, Compares It To A Fairytale

Tardigrades can survive both heat and cold equally unscathed and no longer show any obvious signs of life, raising the question of what happens to the animals' internal clock in the resting state.

Tardigrades, or water bears, are microscopic creatures excellent at adapting to harsh environmental conditions. Barely one millimetre in size, tardigrades have managed to adapt perfectly to rapidly changing environmental conditions over the course of evolution.

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Tardigrades, or water bears, are microscopic creatures excellent at adapting to harsh environmental conditions. Also known as moss piglets, tardigrades are eight-legged invertebrates that have been to space and would likely survive the apocalypse. In 2019, Ralph Schill, a professor at the Institute of Biomaterials and Biomolecular Systems at the University of Stuttgart, Germany, showed that anhydrobiotic or dried tardigrades can survive undamaged for many years without absorbing water. It was previously unclear whether tardigrades age faster or slower in a frozen state, or whether ageing comes to a halt. However, the mystery has now been solved as researchers found that frozen tardigrades do not age in extreme conditions.

A new study describes how tardigrades survive freezing temperatures. The findings were recently published in the Journal of Zoology.

Tardigrades can dry out in extreme conditions

Tardigrades, which belong to the family of nematodes, have a gait that is reminiscent of that of a bear. This is the only similarity between a tardigrade and a bear. Barely one millimetre in size, tardigrades have managed to adapt perfectly to rapidly changing environmental conditions over the course of evolution and can dry out in extreme heat and freeze in cold conditions. 

In a statement released by University of Stuttgart, Schill explained that tardigrades do not die in extreme temperatures, but fall into a deep sleep.

What is the sleeping beauty hypothesis?

Freezing or drying out causes different kinds of stress for a cell organism. However, tardigrades can survive both heat and cold equally unscathed and no longer show any obvious signs of life, raising the question of what happens to the animals' internal clock and whether they age in the resting state.

Dried tardigrades wait many years in their habitat for the next rain. Several years ago, Ralph and his colleagues explained the ageing of tardigrades through a fairytale. They said that in a fairytale by the Grimm brothers, the princess falls into a deep sleep, and awakens when a prince kisses her 100 years later. After waking up, she still looks as young and beautiful as before. The team said it is the same with tardigrades in a dried state and therefore, this is also called the "Sleeping Beauty" hypothesis. 

Schill explained that during inactive periods, the internal clock stops and only resumes running once the organism is reactivated. Therefore, tardigrades, which usually only live for a few months without periods of rest, can live for many years or decades. 

Before the completion of the study, it was unclear whether the internal clock stops only in dried animals, or applies to frozen animals as well. It was not known whether frozen animals age faster or slower than the dried animals, or if ageing comes to a halt in frozen animals.

Does ageing stop even in frozen animals?

In order to know if ageing stops even in frozen animals, Schill and his colleagues conducted several experiments in which they froze a total of more than 500 tardigrades at minus 30 degrees Celsius, thawed them out, counted them, fed them, and froze them again. The researchers repeated this until all the animals died. 

They also kept control groups at constant room temperature at the same time. Apart from the time in frozen condition, the comparison with the control groups showed an almost identical lifetime. This means that even in ice, tardigrades stop their internal clocks like Sleeping Beauty, Schill concluded.