A Bug's Life

July 16, 2008

There were many things I loved about Wall-E, Pixar's latest animated offering, such as the little robot's obvious affection for its tiny cockroach friend -- the only living creature, apparently, capable of surviving in the harsh environment of that futuristic version of Earth. Cockroaches are pretty hardy insects, able to go for a month without food -- or to survive by consuming the glue on the back of postage stamps, for example. In fact, it's often been said that in the event of a nuclear world war, the lowly cockroach would be the only species to survive.

It's quite true that these tiny insects can withstand higher doses of radiation than humans: 6 to 15 times as much, in fact. That's because they have a slower cell cycle than humans, only dividing once each time they molt (once a week, tops). This is crucial because cells are most easily damaged by radiation when exposed during the dividing process. Acute bursts of radiation would never be able to take out the entire population of cockroaches, since it would be strange indeed if all the insects were molting at the exact same time. But as the Mythbusters proved earlier this year, fruit flies have an even higher resistance. I guess fruit flies will inherit the earth... or maybe the two insect families can learn to share their joint inheritance.

There is a class of creatures that could give even the hardy fruit fly a run for its money in terms of surviving extreme environments. They're known as extremophiles: organisms (usually microbes) that can survive under seemingly impossible conditions: ultra-high, or ultra-low temperatures and pressures, ionizing radiation, high acidic levels, and so forth.

One of the most impressive extremophiles is the tiny tardigrade, otherwise known as a water bear, first observed back in 1773. (The name means "slow walker.") There are some 1000 different species found all over the world, from the deepest parts of the ocean to the peaks of the Himalayas. Mostly, they hang out on moist, comfy lichens or moss -- although some prefer beaches and sand dunes, and the occasional lake or pond.

Tardigrades have numerous survival skills. They can withstand 570,000 rads of x-ray radiation, compared to the 1000 to 2000 rads that would be fatal to a human. Provided it's only a few minutes' exposure, they can withstand temperatures of up to 151 degrees Celsius and -272 degrees Celsius (about 1 degree warmer than absolute zero). As for pressure, they can even survive the vacuum of space.

No water? Not a problem! The tardigrade can survive almost a decade in a dehydrated state. It can do all this because it's one of those rare creatures capable of reversibly suspending their metabolism and going into a weird kind of hibernation if need be, by expelling all the water from their cells. Ironically, in their active state, they are surprisingly short-lived, lasting a month or so. Last fall, the European Space Agency sent a few tardigrades into space with the BioPan-6 mission, just to see how they fare in the even harsher environment of outer space. They are eagerly awaiting the results.

Astrobiologists are intrigued by extremophiles because similar organisms might be capable of surviving known conditions on other planets in our solar system: the deep subsurface permafrost of Mars, for example, or the ocean that many astronomers believe may lie below the surface of Jupiter's moon, Europa. Proponents of the theory of panspermia even believe these creatures might be evidence that life actually originated in outer space. It seems a bit far-fetched for the tardigrade: water bears share a genetic lineage with c. elegans, the common earthworm -- i.e., the most terrestrial of creatures.

Photo: Artist's rendition of a tardigrade in space. Source: Brett's Blog (via io9).

Comments

[Brig Klyce]

Thanks for noticing panspermia! Two picky comments.

You wrote: "Proponents of the theory of panspermia even believe these creatures might be evidence that life actually originated in outer space."

They are evidence that not just simpler things like viruses and bacterial spores, but even a multicelled critter, can _survive_ in space. Evidence that life can _originate_, anywhere, is still missing. If you mean only ..._came from_ space, I have no complaint.

You wrote: "It seems a bit far-fetched for the tardigrade: water bears share a genetic lineage with c. elegans, the common earthworm -- i.e., the most terrestrial of creatures."

If life comes from space, we would still expect it all to be genetically related. To life on Mars even, if any. Marsworms?

[jennifer Ouellette]

The scientific jury is still very much out on the question of panspermia, and the tardigrade in particular. There's some interesting circumstantial evidence that seems to support it, and believe me, all eyes are turned towards Mars Phoenix as it continues to analyze soil samples, etc. from that planet. But until more evidence comes in, it's a tenuous hypothesis.

[x-ray fluorescence]

Hi,
Thanks for your great blog.It's quite true that these tiny insects can withstand higher doses of radiation than humans...