Tuesday, September 16, 2008

space bears

A year ago, researchers sent two species of tardigrades (microscopic animals also known as water-bears) into a low Earth orbit, around 280 km above sea level for 10 days, to see how they cope. Tardigrades are remarkable for their resistance to extreme desiccation, so the idea is that they might be the only animals capable of surviving the high vacuum in space.

Now Ingemar Jönsson et al. from Kristianstad University in Sweden have reported the results of this space mission. It turns out that both species cope with space vacuum very well as long as they are protected from radiation, with survival rates close to 100 %. One species (M.tardigradum) also had a reasonable survival rate after exposure to space vacuum and UV radiation in the UVA and UVB range. Only three specimens of M. tardigradum survived the full unfiltered radiation spectrum typical of Earth orbit.

Tardigrades have now joined bacterial spores and lichens in the very small group of organisms that can survive space conditions. Mechanisms remain to be elucidated, but one may speculate that sophisticated ways of DNA packaging during desiccation and DNA repair afterwards play a role, as has been shown for the radiation resistance of Deinococcus radiodurans.

Current Biology 2008, 18, R729-R731

There’s more about the remarkable stress-resistance of tardigrades in the following chapter from my new book, The birds, the bees, and the platypuses:

Squeezy little bears

The crazy creatures at the extreme ends of life on Earth have fascinated me for many years. As both my PhD thesis and one of my books dealt with life under extreme conditions, I’m no longer that easily impressed by tales of life in boiling water, sizzling deserts, or permanent ice. However, the following story (which unfortunately came up too late for the original edition of “Life on the Edge”) beats them all. If anybody wants to send animals to Mars, I suggest they try the “little bears” or tardigrades. The following text is adapted from a postscript included in the paperback edition of “Life on the Edge.”

Tardigrades are microscopically small animals reminiscent of downsized bears, at most half a millimetre long. They live in water droplets suspended in moss and lichens and can be found on all continents. Now if you’re such a tiny little bear exposed to the elements, you need some very special survivial skills.

Tardigrades have at least two major emergency routines. If their habitat is flooded and there is a risk of oxygen shortage, they inflate to a balloon-like passive state that can float around on the water for days. If, however, the threat comes from a lack of water, they shrink to form the so-called tun state (because it looks like a barrel), which could be described as the animal equivalent of a spore. Researchers have managed to resuscitate tardigrades by rehydrating moss samples after up to 100 years of storage on museum shelves, which proves the quite remarkable long-term stability of this state.

It was this tun state that Kunihiro Seki and Masato Toyoshima (Kanagawa University, Japan) used in their studies of resistance against high pressures. As the presence of water would have converted the animals back to the active state, the researchers suspended the tuns in a perfluorocarbon solvent before they applied pressures of up to 6,000 atmospheres (more than five-fold the pressure found in the deepest trenches of the oceans). While active tardigrade populations in water are are killed off by 2000 atmospheres (already an implausibly high threshold for an animal), the tun state allowed 95 % of the individuals of one species and 80 % of another to survive the maximal pressure of 6,000 atmospheres.

This observation is unprecedented for any animal species. Only some bacterial spores and lichens could hope to compete with that. Still, tardigrade experts may have been only mildly surprised, as they knew already that the tuns can be revived after freezing in liquid helium -- they are frost resistant down to 0.5 Kelvin. Detailed mechanistic explanations for these record-breaking achievements are not yet available. One thing that is known for sure is that the tuns contain high concentrations of the sugar trehalose, which is known to improve the stress resistance of baker’s yeast.

The phenomenal shelf life of the tuns has aroused the interest of researchers in medical technology. Some are trying to copy the tardigrades’ recipe to achieve similar long-term stability for human organs to be used in transplantation.


Further reading

M. Gross, Life on the Edge

What happened next

I am pleased to report that researchers actually followed up on my suggestion and sent tardigrades to space. The TARDIS (Tardigrades in Space) experiment was part of the FOTON M-3 mission, that launched on 14 September 2007 and returned safely on the 26th, after 189 orbits. At the time of writing, the tardigrade passengers were awaiting detailed analyses that will surely reveal how well they are suited to withstand space conditions.

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