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published on 10 July 2017 in life

Some like it extreme

In summer we need air conditioning, in winter heating. To climb the highest peaks in the world we need oxygen and if we go underwater we have to be very careful how we return to the surface. The examples one could make are many, but what we want to emphasise is very simple: the environmental conditions in which man can survive without the support of technology are limited in terms of temperature, humidity, pressure, atmospheric composition, exposure to sunlight, etc. Let’s provide some numbers: we can survive between 4 and 35 degrees Celsius, above 7925 metres of altitude we depend on cylinders to breathe and if we lived on a planet 14 times larger than Earth, the gravity to which we would be subjected would jeopardise the integrity of our internal organs.

Lichens

But not all living beings have the same “tastes” and the same limits as man. Those that for us are lethal physical-chemical conditions are quite bearable for other species (called extremotolerant). Other organisms (called extremophiles) find their ideal habitat in extreme conditions. There are organisms living in the Arctic ice (at -20°C), in hydrothermal springs (at 122°C), in oceanic abysses (at pressures of 110 MPa), in the bowels of the planet (6.7 km below the Earth’s surface) and even in our intestines. There are representatives of these organisms both among the eukaryotes as well as among the prokaryotes, and some of them are termed polyesterophiles because they can survive under extreme conditions from different points of view.

The case of tardigrades
Among the eucaryotes, one of the most impressive examples of adaptability and versatility is that of tardigrades, also known as “water bears” or “moss piglets”, due to their resemblance, in miniature, to bears and caterpillars. They are a very common invertebrate – which can be divided into at least a thousand different species – which rarely reach one millimetre in size and which can survive from a minimum of -272°C (only one degree Celsius above absolute zero, which is -273°C!) up to a maximum of 151°C, at pressures of 6,000 atmospheres and X and γ ray exposure.

Tardigrade

To survive in extreme conditions, these organisms have developed a very particular strategy, known as quiescence. Already observed by Spallanzani, the well-known 18th century biologist, Tardigrade quiescence consists of cryptobiosis (which literally means “hidden life”), i.e. a slowdown (up to almost suspension) of metabolic activity and a modification of the form of the organism following certain environmental changes. This phenomenon thus takes on different names depending on the environmental conditions in which it occurs. In particular, it is referred to as anhydrobiosis when there is a loss of almost all of the water present in the body due to dehydration, as cryobiosis in the case of freezing, as anossibiosis when there is oxygen deficiency and osmobiosis when the tardigrade is subjected to significant variations in the osmotic pressure of the solution in which it is immersed. Even after quite long periods of suspension of the normal metabolism, tardigrades, on the return of favourable conditions, resume their activity.

Anhydrobiosis
Perhaps the most well-known form of cryptobiosis is anhydrobiosis, which occurs when water starts to lack and a slow dehydration begins, in which the animal assumes a new form, called “barrel shaped”, and loses 97% of its body water. At this point, the biological necessity for survival is to find a way to prevent the cellular membranes from breaking, making it impossible for any recovery after the “fake” death. Thanks to the synthesis of a particular sugar, trealose, tardigrades manage to maintain the cellular structures intact and can withstand the worsening of other physical-chemical conditions such as freezing, high concentration of certain substances or UV radiation .
A very recent discovery has highlighted a specific group of genes which encodes the proteins that govern this drying process: the “Tardigrade-Specific Intrinsically Disordered Proteins” (TDPs).
Thanks to this stratagem, tardigrades can remain in quiescence for years and then resume their activity in a time varying from half an hour to little more than an hour, depending on how long the cryptobiosis lasted. Thanks to this ability, these tiny animals have succeeded in colonising dry land, even that most hostile to life, and are the subject of much NASA research into life in space.

 By Anna Pellizzone

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