In the last issue of the  Water Bear web base  we have seen how a desiccated water bear comes back to active life just by means of a minute droplet of water. When keeping in mind the extremely low water content of the dry forms (tuns) there is no doubt that all biochemical processes come to a halt at this time because enzymatic reactions are no more possible due to the lack of solvent.

Red Echiniscus tardigrade in dry state.
Ideal symmetry after desiccation
under favourite conditions.
Ventral side of the 'tun'. Length of dry water bear: ca. 0.1 mm.

A problem with the definition of the term 'life'
In Germany there is a popular pupils' textbook, the 'Linder'. It contains a basic definition of life well known to most of us:

Any form of life is linked to chemical reactions.
At the moment when those chemical reactions stop
life of an amoeba ends;
thus biochemical reactions habe to be considered as a characteristic sign of life.

Already some time ago scientists have noticed that such definitions put the dry water bears in the reign of death, not life, as there is no noticeable chemical activity within the tuns.
Crowe (see literature) even goes a step further, when stating that - following those definitions in the strict sense - a tardigrade not coming back to active life from the dry state might have died after having been dead already before!
Crowe offers a simple way out of all those self-made definition problems: it seems to be a mistake of the definition to link life and biochemical activity so closely. Life can be but needn't be connected to biochemical activity. Following Crowe life might be better defined by the highly organized and very complex chemical molecule structures which bear the chance to live within their complexity. As a consequence death would have to be assumed only after a complete breakdown or destruction of those complex structures.

The most important facts about the water bear dry state
The following informations are based on the article by Crowe cited below. It is important also for the microscope amateur to understand that the tardigrades need a certain minimum of time and favourite environmental conditions in order to cope with the enormous difficulties to freeze their active life in a such a way that a revival will still be possible, even after years. As a consequence it must be understood that it would be stupid and unfair to let the tardigrades dry on a slide in a centrally heated room. They would have no chance to survive such a sudden dryness attack which is far from the natural situation within a moss cushion well buffered with respect to humidity. There is plenty of time until such a wet moss cushion becomes completely dry even in plain sunlight. Furthermore the tardigrades cling to the moss plants and hide within the cavities among the moss leaves. This is one of the reasons why it can be so difficult to find the tardigrade tuns in dry moss cushions. After what has been said it should be quite understandable that a rather high relative humidity value of about 80% seems to be realistic for optimum revival survival rates. A further result from science is that the tuns are better prepared for revival after having been stored for some time at elevated humidity. So, when you have stored your tardigrade samples within a dry place like a drawer you should put them at a slightly more humid place, e.g. in a sheltered place outside for some days in order to provide them with the best revival chance.
The tardigrades have an inbuilt desiccation protection, their skin: during the process of drying the tardigrades tend to reduce their outer surface area by retraction of the head and limbs and the skin seems to get less permeable to water vapour. The inner organs have to be packed in a manner that they do not become damaged and dysfunctional during the process of further shrinking.


A litte bit of water bear chemistry (again following Crowe, see literature)

Molecular model of the disaccharide trehalose; schematic, so-called Haworth projection. The actual molecule geometry is a little more complex. Trehalose is well-known as a natural component of mosses (Römpps Chemie-Lexikon).
Red: oxygen atoms
White: hydrogen atoms
Blue: molecule skeleton made up of carbon atom chains

Molecule model of glycerine. Glycerine is a water-like, low-volatile liquid and can be mixed with water in any arbitrary percentage. The small dots symbolize the extension of the electron clouds, to get a better understanding of the overall molecule volume. It is known from practical experience that glycerine tends to elastify many materials, so scientists assume that also water bear tissue is being elastified by glycerine to avoid a brittle structure in the dry state.

Molecule model of glycogene (schematic). Each ring is part of a glucose (grape shugar) molecule which are linked to one another and so form a bigger molecule which is called glycogene (in reality the glycogene molecule are made of a higher number of glucose molecules but the structure principle is the same). It works like a table of chocolate: in case of emergency (hunger, energy lack) you can break away a little piece of the bulk chocolate (until you have to go the supermarket again).

As science has proven or still assumes the desiccation process uses a series of special tricks, which will be mentioned here in short.
During desiccation the amount of trehalose steadily increases. The residual water within the water bear body is being replaced by trehalose. This molecular exchange helps to build a mechanically and chemically stable, completely dry tissue structure.
Furthermore it was noticed that the glycerine content of the water bears increases during desiccation, a fact which can be interpretated in terms of slow exchange of water by glycerine too. Another explication for the high glycerine content is the need for elastification of the dry tissue. And last but not least chemists know that glycerine is a chemical stabilizer which is able to catch so-called 'radicals', i.e. extremely reactive chemical compounds which are generated by UV radiation and other environmental influences.
The high need for energy during desiccation and revival is covered by glycogene which is stored within the water bear cavity in special storage cells (similar to fat in the human liver).

We had a lot of theory in this issue. The next Water Bear web base will be more down-to-the-earth and will present an amazing early colour portrait of a water bear in its natural environment.

Literature

John H. Crowe: The physiology of cryptobiosis in tardigrades. Memorie dell' Istituto Italiano di Idrobiologia 1975, pp. 37-59 (if you should happen to have a little bit of spare time left you should read the original, it is worth while!).
Hartmut Greven: Die Kryptobiose der Bärtierchen. Mikrokosmos 62 (1973) pp. 65-69.
Hermann Linder: Biologie. Lehrbuch für die Oberklassen der höheren Schulen. 13th ed. p. 3. Stuttgart 1963.

© Text, images, and video clips by  Martin Mach  (webmaster@baertierchen.de).
Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

Main Page