Eternal sleep? - a dry state with expiry date

Two years ago the  Hydrobiologia  magazine published the proceedings of the 9th Symposium on Tardigrada, held in Florida, U.S.A.

Cover of the  Hydrobiologia  special issue
with the proceedings of the 9th international Symposium
on Tardigrada, Tampa, Florida, 28 Juli - 1 August 2003.
Dordrecht 2006. ISSN 0018-8158.

A thrilling article, well suited for amateurs as well, was published therein by Lorena Rebecchi and other tardiologists: it is a study of the actual survival endurance of tardigrades in the dry state . We recommend to read the publication in its original form if ever possible but nevertheless will present a rudimentary discussion of the contents and results here.

Rebecchi et al. took a big moss sample on April 14, 1999 and kept it under dry indoor conditions from this moment. In regular time intervals fractions of the moss bulk were removed in order to measure the percentage of tardigrades that were able to return to active life after rehydration. Over a period of four years, until September 5, 2003 the revival ability of impressive 10,730 (!) tardigrade individuals with respect to dry state length was assessed.

At the end of the experiment it became apparent that the survival ability of the Eutardigrades decreased markedly with time (cf. the following table with data by L. Rebecchi et al., abridged).
A survival rate of  0.3 %, as on the bottom of the table, last line on the right, does imply that only one (!) out of 344 Ramazzottius oberhaeuseri tardigrades was able to come back to active life after 1604 days in the dry state.

Date	Species	Dry days	Individuals	Still alive [%]
03.04.2002	R. oberhaeuseri	1085	586	40.4
22.07.2002	R. oberhaeuseri	1192	645	21.7
05.09.2003	R. oberhaueseri	1604	344	0.3

The situation looks even worse with regard to the Echiniscus tardigrades (data by L. Rebecchi et al. as well). Already before the end of the experiment, on March 20, 2001, i.e. after two years (706 days), only ten percent of the tardigrades came back to active life, later the survival rates apparently approached a zero value:

Date	Species	Dry days	Individuals	Still alive [%]
20.03.2001	Echiniscus spp.	706	161	9.9
17.10.2001	Echiniscus spp.	917	90	0.0
03.04.2002	Echiniscus spp.	1085	74	1.4
22.07.2002	Echiniscus spp.	1192	92	0.0
05.09.2003	Echiniscus spp.	1604	36	0.0

Lorena Rebecchi et al. thus have proven on a solid experimental basis that the tardigrade dry state survival cannot last infinitely, because, with increasing length of the dry state the number of tardigrade survivors drastically decreases. Infinite preservation for centuries, like in those science fiction movies with long-sleeping astronauts appears to be impossible, even for tardigrades. We must assume that also in the dry state some destructive decay chemistry does happen. Those processes, like e.g. tissue oxidation that can be repaired in a living tardigrade turn out to be fatal for the "sleeping" tardigrade in the long run.

Popular literature often quotes a tardigrade revival after 120 years of desiccation but this single observation couldn't be repeated, possibly it was just some kind of "rehydration shivering" due to tissue swelling.
More realistic estimates in literature claim that a 10 years' dry state survival can be possible - but even this interval is probably an exception from the rule which might be best summarized as: tardigrade revival will be possible without problems after months. Everything beyond this time range might cause serious problems for the tardigrades.

Comment: There can be abysses behind all table figures. The mental world of man is increasing in complexity every year. As a consequence there is a proportionally increasing desire for clarity, for guidelines, for understandability. We tend to listen to people who seem to be able to break down the complexity to simple news like: "use my diet xyz and you will stay healty. This is the result of a highly complex study based on scientific results from 100,000 students". Not to speak of some politicians that are lingering from press conference to press conference eagerly pretending to be able to understand and even to solve the problems of the whole world.

Tables with well ordered, cleanly single cell imprisoned figures are consoling in this chaotic situation and they look promising on our way to mental clarity. But they can be elusive and deceptive as well. Why? Let's look at our tardigrade tables once more:

The tardigrade enthusiast will possibly interprete the experiment in a different way: a survival value of 0.0% would imply that less than one of thousand tardigrades does survive after rehydration. But, as the 0.0% values in the table are based on groups of 90, 92 or 36 individuals, at least the last digit of the result is definitely overdone: in the case of 36 individuals one might state that "less than 2%" of the tardigrades do survive, not 0.0 %. Remember, one would need at least 1,000 individuals to confirm a 0.0 % survival value. On a first sight one might think that this point of view might just be a narrow-minded precision discussion. But, at second thought you will come to the conclusion that it is not:

In nature there can be a tremendous difference between a value of 2 percent and 0.0 percent. It might be the difference between the survival of a population and its death. Even a single female tardigrade might be sufficient to continue the thread of life. We are all aware of the fact that "mother" nature exerts tremendous brutality when generating individuals in high numbers and risking at the same time that only a few of them will actually survive. We call it selection or evolution. In fact it is some kind of life insurance for life itself - shared among those many individuals which are desperately trying to carry on - some lucky, many less lucky. Coming back to our percentages this means that a difference between 0.0% and 0.1% percent might be much more decisive than the rest of the scale between 0.1% and 100%. We live in a strange world anyway but it tends to become even more strange when we are investigating the environment of zero values on a linear scale. What if one out of a billion tardigrades would be able to come back to life after 100 years? We would no be able to find out in an experiment with 10,000 tardigrades but it might be possible. And we do have no doubt that a billion of tardigrades are nothing when compared to the world tardigrade population.
But, obviously, we will never be able to run an experiment with a billion tardigrades. So Rebecchi's experiment gives a reliable tendency but no one on earth will be able to actually tell whether it provides safety in the 0.000 to 0.001% range.

A similar assessment, though certainly less professional might be performed as a task at school or college, or by microscopy amateurs all over the world.

The impressive research by Rebecchi et al. was an incentive for us to try a smaller scale experiment ourselves: after reading the Hydrobiologia article we remembered that there was an old moss sample on our wardrobe forgotten since the year 2002.

Moss from an old roof in France, recovered in late summer 2002, since that date stored in this former "instant cappuccino" glass container.

And now, have a guess please, after four years of dry storage, in 2006, how many living Echiniscus tardigrades did we find in this glass container?

0.0% ?

You will get the answer in our next issue. See you.

Literature

J.H. Crowe: The physiology of cryptobiosis in tardigrades.
Memorie dell'Istituto Italiano di Idrobiologia 32 Suppl. (1975) p. 37 - 59.

L. Rebecchi, R. Guidetti, S. Borsari, T. Altiero, R. Bertolani: Dynamics of long-term anhydrobiotic survival of lichen-dwelling tardigrades. In: The Biology of Tardigrades.
Hydrobiologia 558 (2006) p. 23 - 30. ISSN 0018-8158.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
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