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August 4, 2019

Life is tough: What extremophiles can teach us

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Above, an arresting color scanning electron micrograph of a tardigrade, or water bear (Paramacrobiotus craterlaki) in moss.

It stopped me in my tracks when I happened on it leading this essay.

You couldn't make an organism that looks like this up.

From the piece:

Extremophiles tell us that everything we think we know about the fragility of life is wrong.

Life is indeed extraordinary, not to mention precious and deserving of reverence — but not in any sense miraculous.

My personal favourite extremophiles are tardigrades.

These multicellular creatures are rarely more than one millimetre in length and often invisible to the unaided eye.

They have four legs along each side, each outfitted with tiny claws.

They also have a clearly discernible mouth, and are impossibly adorable.

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 are also known as water bears.

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 percent of normal.

Compared with them, a deeply hibernating mammal is living at lightning speed.

Tardigrades have two more arrows in their extremophile quiver.

In 2017, a team of cell biologists discovered that tardigrades possess genes that produce a peculiar array of constituent chemicals, known as "intrinsically disordered proteins," which help induce a solid internal state in these animals when they experience desiccation.

In addition, some scientists claim that fully one-sixth of tardigrade DNA consists of genetic material from other species, although this finding has been disputed.

It has long been known that some cross-species (horizontal) transfer of DNA takes place among many species, but not on the scale claimed for water bears.

Most animals sport less than 1 percent foreign DNA.

Tardigrades might have this additional trick: when they dehydrate in response to environmental exigency, their DNA breaks into fragments, after which it is reconstituted upon rehydration.

As tardigrade genes reconstitute themselves from the tun state, their cell and nuclear membranes become leaky, whereupon they could have acquired the habit of incorporating various pieces of non-tardigrade DNA that are inevitably floating around.

All species possess some capacity to repair errors in their genome.

Individuals and hence species lacking this ability will have been selected against, simply given the unavoidable tendency of DNA to mutate, even without the extra challenge of adjusting to extreme conditions.

Although presumably the stitching in of extra-specific DNA would be initially random, it is possible that some of these genomic additives contributed to their possessor's fitness.

If the early reports of unusually large amounts of extra-specific DNA in tardigrades turn out to be accurate, it will be yet another example of their charm, whereby in so many wonderful ways the extreme adaptations of these aptly named extremophiles italicise the potency of natural selection.

In his book What Is Life? (1947), the mathematical biologist J. B. S. Haldane wrote: "The Universe is not only queerer than we imagine but queerer than we can imagine."

Extremophiles also demonstrate that life is more resilient.

In a world in which nature has been subjected to ever more pain and stress, they represent a kind of cosmic optimism, evidence of the strength and durability, if not of human beings, then at least of life itself.

August 4, 2019 at 04:01 PM | Permalink


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