Life is Tough

The word extremophile didn’t exist until the 1970s. It entered wide circulation only after 1979 when the US Navy’s submersible Alvin revealed ecosystems prospering in deep-ocean hydrothermal vents. The Alvin scientists discovered organisms living in superheated water and largely metabolising hydrogen sulphide, which until then had been thought toxic and incompatible with life. Interest in extremophiles has burgeoned in proportion as scientists have come to appreciate their abundance, as well as their novel physiology. There is a journal devoted to extremophiles, focusing on creatures that survive – even, thrive – in environments that are extremely hot, cold, highly acidic or alkaline, and so forth, circumstances that would be lethal for most living things.

Not surprisingly, extremophiles tend to be relatively simple creatures, notably invertebrates and especially bacteria and archaea, although there is no bright line distinguishing, say, arctic hares, which thrive in very cold habitats, from their rabbit relatives whose habitats are more temperate. But neither compares with those life forms whose existence excites the admiration and wonder of biologists. The concept itself is nonetheless anthropocentric, since denizens of, say, blisteringly hot hydrothermal vents would perish in our ‘moderate’ temperatures and pressures, which for them would doubtless be extreme. (...)

Purists don’t include tardigrades among extremophiles, since they don’t appear to be adapted to extreme environments per se – that is, like us, they do best in comparatively benign conditions, which, in the case of tardigrades includes the moist, temperate miniworld of forest moss and lichens.

Their probability of dying increases in proportion as they are exposed to highly challenging circumstances, so, unlike classic extremophiles, tardigrades are evidently adapted to what human beings, at least, consider moderate circumstances. However, they are extraordinary in their ability to survive when their environments become extreme. Not only that, but whereas typical extremophiles specialise in going about their lives along one axis of environmental extremity – extreme heat or cold, one or another heavy metal, and so forth – tardigrades can survive when things get dicey along many different and seemingly independent dimensions, simultaneously and come what may. You can boil them, freeze them, dry them, drown them, float them unprotected in space, expose them to radiation, even deprive them of nourishment – to which they respond by shrinking in size. These creatures, also known as water bears, are featured on appealing T-shirts with the slogan ‘Live Tiny, Die Never’ and in the delightful rap song that describes their indifference to extreme situations, entitled Water Bear Don’t Care.

Tardigrades might be the toughest creatures on Earth. You can put them in a laboratory freezer at -80 degrees Celsius, leave them for several years, then thaw them out, and just 20 minutes later they’ll be dancing about as though nothing had happened. They can even be cooled to just a few degrees above absolute zero, at which atoms virtually stop moving. Once thawed out, they move around just fine. (Admittedly, they aren’t speed demons; the word ‘tardigrade’ means ‘slow walker’.) Exposed to superheated steam – 140 degrees Celsius – they shrug it off and keep on living. Not only are tardigrades remarkably resistant to a wide range of what ecologists term environmental ‘insults’ (heat, cold, pressure, radiation, etc), they also have a special trick up their sleeves: when things get really challenging – especially if dry or cold – they convert into a spore-like form known as a ‘tun’. A tun can live, if you call their unique form of suspended animation ‘living’, for decades, possibly even centuries, and thereby survive pretty much anything that nature might throw at them. In this state, their metabolism slows to less than 0.01 per cent of normal. (...)

Given that tardigrades possess the kind of powers we otherwise associate with comic-book superheroes, it might seem that they are creatures out of science fiction, but maybe it’s the other way around. Liu Cixin’s novel The Three-Body Problem (2010), a Chinese blockbuster that broke all records for sci-fi literature in its home country, became the first book not originally published in English to win the coveted Hugo Award for best science-fiction novel in 2015. It describes extraterrestrials known as Trisolarans, whose planet is associated with three suns, the real-life interactions of which – as physicists and mathematicians understand – would generate chaotically unstable conditions.

Trisolarans, therefore, are unpredictably subjected to extreme environments depending on the temporary orientation of their planet relative to its chaotically interacting stars: sometimes lethally hot, other times cold, sometimes unbearably dry and bright, other times dark, and so forth. As a result, these imagined extremophiles have evolved the ability to desiccate themselves, rolling up like dried parchment, only to be reconstituted when conditions become more favourable.

I don’t know if Liu was aware of real-life, Earth-inhabiting tardigrades when he invented his fictional Trisolarans, but the convergence is striking. (In the interest of scientific open-mindedness, it should perhaps also be considered that maybe tardigrades are real Trisolarans, refugees from a planet that was chronically exposed to intense environmental perturbations. This would explain the puzzling fact that tardigrades appear hyper-adapted, able to survive extremes that greatly exceed what they experience here on Earth.)

by David P. Barash, Aeon |  Read more:
Image: Water bear (Paramacrobiotus craterlaki) in moss. Photo by Eye of Science/Science Photo Library
[ed. Plus, Tardigrades on the Moon! (Ars Technica).]