Giving Up Your Water Without Dying: The Creature That Gets Dried Out

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Dry a living creature out completely and put it away on a shelf; ten years later, let a single drop of water fall on it, and watch it begin to stir. This is not a fairy tale — it is an experiment performed routinely in laboratories. So where does life wait, when it has no water?

In February of seventeen oh two, in the Dutch city of Delft, an elderly cloth merchant scrapes a pinch of dust from the lead gutter on the roof of his house. This is a habit of his. He puts the dust into a glass tube, pours clean water over it, and begins to wait. Less than an hour passes before the inside of the tube comes alive. There are things clinging to the glass, creeping along it, swimming in the water. At the front of their heads they carry crowns of spinning cilia. And yet that dust had been bone dry for months.

Antonie van Leeuwenhoek writes that he could scarcely believe what he was seeing through his own lenses: it had never occurred to him that anything alive could be found inside so thoroughly dried a substance. The creatures he saw were the microscopic animals we now call wheel animals, or rotifers. This was the first time in the history of science that the phenomenon was recorded: drying out without dying. And the question he left behind was unsettling enough to haunt science for three hundred years. Did those animals die and come back, or did they never die at all?

Seventy-one years later, in seventeen seventy-three, a German pastor and naturalist named Goeze, examining a sample taken from the bottom of a puddle, comes across a plump, eight-legged creature walking slowly and heavily, and he calls it the little water bear. Three years later the Italian naturalist Spallanzani names these same animals the slow walkers, after their gait, and notices the same trick in them: dry them out and they shrivel up and stay that way; wet them again and they carry on walking. Today we commonly call these animals water bears, and they are the group of living things with the best known equipment for surviving desiccation.

The drying of a water bear does not look like a sign of life slowly fading out; it looks like an orderly shutdown procedure. As the water begins to withdraw, the animal pulls all eight legs inside its body, contracts along its whole length, loses roughly half its volume, and its outer cuticle turns into a leakproof shell. The resulting shape is so distinctive that the old German naturalists called it the little barrel; in the literature it is still known as the tun state. Inside this barrel, the body's water falls below a few percent of its mass. Measurable metabolism stops; oxygen consumption sinks into the noise level of the instruments. No movement, no digestion, no reproduction, and almost no ageing.

To see why this is scandalous, you have to think clearly about the role of water in the cell. Water is not the empty space in which biochemistry happens; it is one of biochemistry's players. A protein folds because the water-avoiding parts of its chain flee from contact with water; what holds it in shape is, to a large extent, the water around it. The cell membrane is a double layer formed when fat molecules line up because of water. Every enzyme reaction depends on molecules colliding in water. When you take the water away, what you expect is obvious: proteins unfold and stick to one another, membrane layers break and shatter, DNA snaps. Dried egg white never becomes egg white again. So the real question is not how these animals come back to life. The real question is how, with no water inside, nothing breaks.

In nineteen fifty-nine, David Keilin, in the lecture named after Leeuwenhoek that he delivered at the scientific academy in London, the Royal Society, proposed a name for this family of states: cryptobiosis, meaning hidden life. The particular form of it established by the departure of water is called anhydrobiosis, life without water. But naming is not explaining. The explanation would come from inside the cell, and it would take another sixty years to arrive.

The first answer, and for a long time the only one, was a sugar. Trehalose, a simple sugar made of two glucose rings. The idea was elegant: water molecules cling to the surfaces of proteins and membranes by hydrogen bonds; as the water leaves, sugar settles into the space it vacates, forms those same bonds, and holds the molecules in place. Then this ever-thickening syrup stops flowing and turns into a glass. That is exactly what vitrification means: a substance as hard as a solid, but without the ordered arrangement of a crystal. Inside a glass, molecules cannot travel. Molecules that cannot travel cannot find one another, and molecules that cannot find one another cannot react. Damage does not happen, because time has frozen. This model worked beautifully in other animals: in the dry cysts of the brine shrimp, trehalose rises to between thirteen and eighteen percent of dry weight; in some soil worms it is between ten and fifteen percent.

The water bears broke the model. Even in the best documented measurement, the amount of trehalose does not exceed two point three percent of dry weight, and there is only a single family that increases its sugar production as it dries; in many species the sugar is almost entirely absent. So in the water bear, trehalose is at best a supporting player. Something else had to be doing the heavy lifting.

That something was proteins. Boil down an extract of a water bear, and most of its proteins curdle like the white of a boiling egg and sink to the bottom; but one group of proteins stays in solution. That is how they got their name: proteins abundant in the cell fluid that remain soluble in heat — cytoplasmic abundant heat soluble proteins, CAHS for short. The odd thing about them is that in water they have no particular three-dimensional shape. They don't have the lock-and-key geometry the textbooks describe; they sit there like loose, aimless chains. They have relatives in the same family too: one group secreted outside the cell, another that sits inside the mitochondria. Every compartment of the cell carries its own protector.

What these proteins actually do has become clear over the past decade. As water begins to withdraw, the aimless chains latch onto one another and build long fibres. Inside cultured cells placed under osmotic pressure, they have been imaged weaving a fibrous network, much like a cytoskeleton; in a test tube, they turn to gel. Most importantly, the process is reversible: when water returns, the network comes apart and the protein falls back into its old loose state. So as the water pressure that gives the cell its shape disappears, a protein scaffold takes its place. Instead of collapsing, the cell stiffens. A study in two thousand twenty-two showed that sugar and protein together protect better than either one alone; one fills the gaps at the molecular scale, the other holds up the roof at the scale of the cell.

Here we need to be honest: whether the dried cell fluid really is a glass built by CAHS is still a matter of debate. There are researchers who say the glass-like behaviour that has been measured could just as well be explained by the evaporation of the last remaining water molecules. This is not a closed question. The soundest picture we have at the moment is not a single trick but a layered defence: sugar, a protein network, a stiffening fluid — and on top of all of that, one more thing.

Because neither a glass nor a scaffold solves one particular problem. As water withdraws, free radicals build up in the cell, the hydroxyl radical above all, and these cut through the DNA strand like scissors. In twenty sixteen, when the Japanese team led by Takuma Hashimoto and Takekazu Kunieda read the genome of a land-dwelling species extraordinarily resistant to drying, they found a protein with no counterpart in any living thing other than water bears, and named it Dsup, for damage suppressor. This protein, too, is a chain with no fixed shape and a high electrical charge. In the cell nucleus it grips the spools that DNA winds around — the nucleosomes; a study in twenty nineteen confirmed this. What it does is almost physical: it spreads itself over the DNA and places itself between the DNA and the radicals. When the team transferred this gene into a culture of human kidney cells, DNA breaks in cells exposed to X-rays fell by roughly forty percent, and the cells survived better.

There is an inference here that often gets overlooked. No natural environment on Earth delivers to an animal the kind of radiation doses handed out in a laboratory. So the water bear's resistance to radiation is not an ability it was selected for. It is a by-product of equipment built against drying out. A vitrified fluid, a pinned-down strand of DNA and proteins that never unfold also reduce, all at once, the harm done by radiation, by vacuum, by pressure and by heat. The tun state is a defence built against one single thing, the loss of water; it just happens to answer a great many things at once. And precisely for that reason, it has sharp limits.

The first limit is time. This trick has to be done slowly. Water bears can only turn into a proper tun under conditions where humidity is high and drying proceeds gradually. Left in dry air, the tun doesn't even form; the animal collapses in a ragged, flattened shape and never opens again. The threshold varies between species: the lethal relative humidity limit for that first drying has been measured at between fifty-three and seventy-eight percent. That makes sense, because weaving the protein network, producing the sugar, drawing the legs inward are each a piece of work, and every piece of work takes time. A water bear caught suddenly by a hot, dry wind dies like any other living thing.

The second limit is that the abilities just described belong to the tun, not to the animal. An active water bear, swimming in water, is extremely fragile. In measurements on one species, under conditions of twenty-four hours of exposure, half of the active animals die at thirty-seven point one degrees Celsius; when the animals are first given time to acclimatise in a warm environment, that threshold rises only to thirty-seven point six. That is an ordinary summer's heat. The dried form of the same species withstands up to eighty-two point seven degrees over one hour of exposure, but stretch the exposure to twenty-four hours and the threshold drops to sixty-three point one degrees. Headlines of the "it survives at one hundred fifty degrees" variety present moments of exposure measured in seconds or minutes as though they were a way of life.

The third limit is the difference between surviving and carrying on. In a study using gamma rays, the median lethal dose came out at five thousand gray in hydrated animals and four thousand four hundred gray in dried ones. Note the detail: the dry state is not the better one here. What's more, doses above one thousand gray sterilised the animals; those that were irradiated laid almost no eggs at all. An animal that lives but leaves no offspring is, in evolutionary terms, a dead end.

The fourth limit showed itself in space. In two thousand seven, in an experiment backed by the European Space Agency, dried water bears prepared by the Swedish researcher Ingemar Jönsson were placed on the outside of a Russian Foton capsule and left exposed to open space for ten days at an altitude of two hundred and seventy kilometres. Most of the samples subjected only to vacuum and cosmic rays came back, and they reproduced. But when the full ultraviolet light of the Sun was allowed through onto the package — more than a thousand times the dose that reaches the ground — the picture changed: of the genus most widely studied in laboratories, only three individuals survived. Vacuum is something you can get through; unfiltered sunlight very nearly is not.

The fifth limit is a legend in its own right. A report published in Italy in nineteen forty-eight announced that a water bear had been revived from a moss sample that had been dry since eighteen twenty-eight; from this came the sentence "water bears can stay dry for a hundred and twenty years," and it is still in circulation. A review by Jönsson and Bertolani in two thousand and one set out plainly where that claim stands: it is the only record that passes a century, what was seen in the animals amounted to no more than a few twitches of the legs, and no one has ever reproduced it. The solid record is a different one. A moss sample collected in Antarctica in November of nineteen eighty-three and kept at minus twenty degrees was thawed thirty point five years later, and out of it came one animal and one juvenile hatched from an egg; both went on to reproduce. But here the thing doing the preserving is freezing, not dryness. Endurance times documented in the dry state are measured in years, not centuries.

The sixth limit sits on the Moon. In two thousand and nineteen, Israel's Beresheet spacecraft — the name means Genesis — crashed into the lunar surface. Among its cargo was a Lunar Library, built from nickel plates etched at microscopic resolution and bonded together with epoxy resin, and thousands of dried water bears had been placed inside it. Headlines around the world announced that water bears had settled on the Moon. Impact experiments carried out afterwards put the ceiling for these animals' tolerance of shock pressure at around zero point nine kilometres per second and one point one four gigapascals — which is precisely the threshold of the craft's reported final speed. So we do not know whether they came through intact. But the more important point is this: even if they did, there is no liquid water there. A tun without water is no different from a seed sitting on bare rock. No living colony was founded on the Moon, and there is no route to founding one.

There is also a case where the field corrected itself. In two thousand and fifteen it was proposed that roughly one sixth of a water bear genome had been acquired from other organisms through horizontal gene transfer, and this was offered as the explanation for the animal's toughness. Within a year another team showed that the great majority of these foreign sequences came from bacterial contamination that had got into the sample. The water bear's equipment is not borrowed; it is built from its own genes.

There is a price, too. Every cycle of drying and rewetting leaves damage behind that has to be repaired; the first hours after meeting water are a full-scale repair operation. There are measurements showing that in the tun state the clock of ageing largely stops, but that does not mean drying out is a free bank account of time.

Knowing these limits does not shrink the story; it makes it usable. What is being taken from the water bear today is not a legend of toughness, but molecules. Boothby's laboratory mixed factor eight — the human blood-clotting factor used to treat haemophilia — with water bear proteins, dried it, then rewetted it, and did this six times over; the drug did not lose its potency. In the researchers' own words, this is not a product but a proof of principle. But the goal is clear: vaccines and medicines that need no cold chain and can sit dry on a shelf — treatment, in other words, in places where no refrigerator can be installed. What the water bear teaches is not indestructibility. It is that the fragility of a living thing is really the fragility of water, and that one sugar and a few shapeless proteins can, for a while, take over water's job.

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