Fiona Jones


As the nights draw in and the mince pies are (almost) ready to come out, we’re bracing ourselves for the onset of some chilly weather. But next time you wish you’d packed an extra jumper, spare a thought for a little creature that is capable of an unfathomable feat of endurance…

They can overwinter on an Antarctic nunatak, survive being shock-cooled to -196°C and seem perfectly at home in frozen cryoconite water on the Greenland Ice Sheet1. I’m talking of course about the humble tardigrade – a ~1.5 mm long invertebrate, affectionately known as the ‘water bear’2. You can make up your own mind about this resemblance (see below) – describing them as ‘cute’ seems to divide people in my experience.

extreme_s
A tardigrade or ‘water bear’…cute or slightly scary? Image source: http://apod.nasa.gov/apod/ap130306.html

When it comes to surviving harsh conditions, the phylum Tardigrada – which contains roughly 600 species – has a pretty impressive résumé. (A ‘phylum’ is a taxonomic group, and is larger than a ‘class’, but smaller than a ‘kingdom’.) For instance, extreme levels of radiation, high pressure (600 MPa!), desiccation (removal of moisture), intense heat and freezing conditions can all be tolerated2,3. Of course, this has made ‘water bears’ the focus of much research, and when it comes to investigating cold-tolerance, tardigrades have really been put through the mill. In 1960, Dougherty et al. subjected specimens from Ross Island in Antarctica to repeated episodes of freezing and thawing4, while Sømme & Meier (1995) stored dehydrated specimens at -22°C for eight years1,5. Amazingly, two of the species tested in the latter experiment (Macrobiotus furciger and Diphascon chilenense) had survival rates of approximately 50%1. Interestingly, Ramløv and Westh (1992) found that cooling speed is an important determinant of survival, with tardigrades capable of withstanding temperatures of -196°C when cooled at a rate of 30°C per minute, but not able to survive being dropped into liquid nitrogen6. They could, however, cope with being shock-cooled to -196°C – as mentioned above – if they were already at a temperature of -10°C to -90°C, suggesting that the transition to a frozen state is the critical period1,6.

So it’s clear that tardigrades are somewhat special when it comes to dealing with freezing conditions. But how do they do it? While we still have a lot to learn in this area, it seems that tardigrades have evolved to cope with ice formation in their cells, rather than producing antifreeze proteins (as is seen in some fish)2. As such, they must protect their cells from damage, probably with the aid of ‘cryoprotectants’, which may include sugars and amino acids1,7. An interesting suggestion is that ‘cytoplasmic vitrification’ takes place – put simply, important parts of the cytoplasm become suspended in a “glass matrix”, thus preserving their structure1. Furthermore, ice-nucleating proteins have been identified in Adorybiotus coronoifer, which encourage ice formation outside cells, thus limiting damage to the inside7.

There is evidently still plenty to learn about these amazing creatures, which I suppose is what makes them so intriguing. It also seems that they have a lot to teach us, and – owing to their tolerance of extreme levels of radiation – they are even being considered as model specimens for space research (see Jönsson, 2007)8. Who would have thought you could find such an extraordinary survivor living in the mosses at the bottom of your garden…?

References

  1. Sømme, L. (1996). Anhydrobiosis and cold tolerance in tardigrades. European Journal of Entomology 93(3) pp. 349-357.
  2. Fox-Skelly, J. (2015). Tardigrades return from the dead. Accessed at: http://www.bbc.co.uk/earth/story/20150313-the-toughest-animals-on-earth. Retrieved on 11/11/15.
  3. Seki, K. & Toyoshima, M. (1998). Preserving tardigrades under pressure. Nature 395:6705 pp. 853-854.
  4. Dougherty, E.C., Chitwood, B.G. & Maggenti, A.R. (1960). Observation on Antarctic freshwater micrometazoa. The Anatomical Record 137 pp. 350.
  5. Sømme, L. & Meier, T. (1995). Cold hardiness of Tardigrada from Dronning Maud Land, Antarctica. Polar Biology 15 pp. 221-224.
  6. Ramløv, H. & Westh, P. (1992). Survival of the cryptobiotic eutardigrade Adorybiotus coronifer during cooling to -192°C: Effect of cooling rate, trehalose level, and short-term acclimation. Cryobiology 29(1) pp. 125-130.
  7. Wright, J.C. (2001). Cryptobiosis 300 years on from van Leuwenhoek: What have we learned about tardigrades? Zoologischer Anzeiger – A Journal of Comparative Zoology 240(3-4) pp. 563-582
  8. Jönsson, K.I. (2007). Tardigrades as a potential model organism in space research. Astrobiology 7(5) pp. 757-766.
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