A tardigrade survives drying by turning its cell to glass

The job water was doing

A tardigrade dried to a speck runs no metabolism you can measure. Its enzymes have stopped; it holds almost no liquid water; by the chemical tests that tell a living animal from a dead one, it reads as dead. A droplet of water reverses that verdict in minutes, sometimes after years on a dry shelf. What should have killed it is the drying itself. To see why, you have to stop treating water as the place cell chemistry happens and start treating it as a part of the machine.

Water is most of a cell by mass, and it is not idle. It supplies one of the forces that fold a protein, because a chain of amino acids curls up largely to hide its oily residues from the surrounding water; take the water away and the reason for the fold goes with it. It keeps the two oily leaflets of a membrane spaced at the right distance. It sets how fast molecules find one another. In a working cell the macromolecules are packed close, separated by no more than three or four layers of water, so the water is less a bath they float in than a filler holding the whole arrangement at its correct spacing.¹ Pull it out and proteins unfold and clump, membranes press together and leak on rewetting, and concentrated solutes crush what is left. Surviving cold, heat, or vacuum is a matter of not breaking. Surviving the loss of essentially all your water is a matter of building something to do the job the water was doing.

A handful of animals, plants, and microbes do exactly that, and the trick has a name. Anhydrobiosis is life suspended by drying: the organism loses nearly all its water, halts, and waits, chemically inert, until water returns. The organisms that manage it are not close relatives. Tardigrades, brine-shrimp cysts, a few insect larvae, resurrection plants, and bdelloid rotifers arrived at the state separately, which means the solution is convergent and worth reconstructing from the physics up.²

A cytoplasm that sets into glass

The oldest and best-understood answer is to replace the water with a syrup that hardens into glass. The classic ingredient is trehalose, a sugar built from two glucose units that many anhydrobiotes pile up as they dry. As the last water leaves, trehalose does two things. Its hydroxyl groups hydrogen-bond directly to the polar spots on proteins and membrane surfaces that water used to bond, so the surface is chemically propped up as if the water were still there. This is the water-replacement idea, and it was worked out by the Crowe group over decades of drying membranes and enzymes in the lab.²

The second thing is structural. A concentrated sugar solution, dried fast enough, does not crystallize into a lattice. It vitrifies: the molecules jam before they can arrange, freezing mid-flow into an amorphous solid, a glass. The temperature at which such a syrup stops flowing and turns rigid is its glass transition. Below it the cytoplasm is a solid so viscous that nothing inside can move, diffuse, unfold, or react on any relevant timescale. The cell is not paused by a signal. It is set in place.

That the glass itself is what protects, and not merely the dryness, was shown cleanly in the sleeping chironomid Polypedilum vanderplanki, an African midge whose larvae are the largest animals known to dry out completely. The larvae load up on trehalose to roughly a fifth of their dry body mass and enter a glassy state stable up to about 65 degrees Celsius. Push them out of the glass, by gentle heating or by letting them take up a little moisture so the syrup turns rubbery, and survival falls off sharply even though the animal is no drier than before. Staying in the glass is the thing that keeps them alive.³

The tardigrade barely makes the sugar

Here the tidy sugar story breaks on the tardigrade. When researchers measured trehalose across tardigrade species, they found the levels scattered and, in some species, absent. Milnesium tardigradum, a hardy drier, showed no detectable trehalose at all, yet survives desiccation as well as any of its relatives.⁴ A later, precise measurement put the trehalose in the model tardigrade Hypsibius exemplaris at 0.0019 percent of dry weight, against the roughly 20 percent an anhydrobiote like the chironomid carries.⁵ That is a gap of four orders of magnitude between two animals that both survive total drying. Whatever the tardigrade sets its cytoplasm with, it is almost none of it sugar.

For a long time this made the tardigrade an awkward exception. The animal plainly did the same physical trick, replacing water and vitrifying, without the molecule the trick was supposed to run on. The question was what stood in for the sugar.

Proteins that vitrify in the sugar's place

In 2017 the Boothby lab named the substitute. Drying tardigrades strongly switch on a set of their own proteins, the Cytosolic Abundant Heat Soluble family, CAHS for short, along with secreted and mitochondrial cousins. Eleven of seventeen CAHS transcripts rose four to twenty-two fold as the animals dried.⁶ These proteins are intrinsically disordered, and that property is the whole reason they work here.

A normally folded protein has one correct shape and fails when it loses it; a protein with no fixed shape has none to lose. The CAHS proteins exploit that. On drying they do what the sugar did: they vitrify, forming a non-crystalline amorphous solid, and the quality of that glass tracks how well they protect. The lab pinned the connection two ways. Knocking CAHS down cut the tardigrade's own survival of drying. And the tardigrade proteins, moved into organisms that cannot normally dry out, carried the tolerance with them: yeast and bacteria expressing tardigrade disordered proteins survived desiccation far better than their unmodified counterparts.⁶ In a test tube, CAHS shielded the enzyme lactate dehydrogenase through repeated drying and outperformed both a standard protective protein and trehalose itself.⁶ Put it in a new cell and the cell inherits the trick.

This is the turn from a sugar glass to a protein glass, though it reads too quickly as a straight replacement. The same lab later dried yeast with a single CAHS protein and a small dose of trehalose together and got protection greater than either gave alone; at the natural ratios in which tardigrades hold the two, sugar and protein act in synergy, the trace of sugar tuning the protein glass.⁵ The honest picture is a glass a given organism builds from whatever it has most of, with sugar and protein reinforcing each other.

How the protein glass protects

What does the protein glass physically do? The clearest picture comes from watching CAHS in a tube and in a cell. As a CAHS solution grows concentrated, the proteins link through their tail ends into larger and larger assemblies, and above a threshold near 0.6 millimolar the whole solution gels into a soft solid that melts back to liquid on dilution. The transition is reversible and reads off temperature, salt, and acidity, exactly the variables that swing as a cell dries.⁷ Follow one CAHS protein into a living human cell and the same behavior appears: under osmotic stress it condenses into a mesh of fibers each about 9.5 nanometers across, a temporary internal scaffold that stiffens the cytoplasm, slows the cell's chemistry, and dissolves again when normal conditions return.⁸

1. Water leaves

   CAHS concentration climbs as the cytoplasm loses water.⁷

2. The proteins link

   Above a threshold concentration the disordered chains associate into a network and the solution gels into a soft solid.⁷

3. Motion stops

   The fibrous gel restricts how molecules move, slowing metabolism and holding sensitive structures in place.⁸

4. Water returns

   The gel dissolves, motion resumes, and the cell restarts where it paused.⁸

The mechanism on offer is mechanical. The gel sets the cytoplasm and immobilizes what floats in it, so that a stressed cell is held suspended, its reactions frozen mid-step until water comes back. That reading rules out an intuitive wrong guess. One would expect a drying-protectant protein to hold on to a shell of water. The Boothby lab tested that directly and found CAHS retains no more water than ordinary proteins.⁹

Two cautions keep this from hardening into a finished story. The founders of the glass hypothesis argued from the start that vitrification is necessary but not sufficient: a glass alone does not stabilize a dried protein unless something also hydrogen-bonds to its surface the way water did, so replacement and vitrification are two jobs, not one.² And whether CAHS gelation is truly the mechanism inside a drying tardigrade, rather than a behavior seen in tubes and transplanted cells, is still open. The gel-to-biostasis chain is the current best account, argued and partly demonstrated, not yet settled.

From transplanted genes to open space

Vitrification answers for the proteins and membranes of the cell, but not for its DNA. Drying and radiation both spawn hydroxyl radicals that snap the strands, and here the tardigrade carries a dedicated part. Sequencing the genome of Ramazzottius varieornatus, one of the toughest species, turned up a small, highly charged, unstructured nuclear protein its discoverers named Dsup, for damage suppressor, which binds directly to DNA. Expressed in cultured human cells, Dsup cut the DNA breakage from a dose of X-rays by about 40 percent.¹⁰ A second, independent lab then showed how: Dsup binds the nucleosome, the spool of protein that packages DNA, and coats the chromatin closely enough to shield it from the hydroxyl radicals, through a stretch of sequence that resembles a known human nucleosome-binding protein.¹¹ The division of labor is now explicit. A glass sets the cytoplasm and holds the proteins and membranes; a separate protein lies over the genome and takes the radical hits.

The same transplant that carried CAHS into yeast carries Dsup into a human cell, and the human cell holds its DNA together better under X-rays than it otherwise would. These are working parts, moved between kingdoms and still functioning, not a folk tale about a hardy bug.

The extreme test was run in orbit. In 2007 the European Space Agency's TARDIS experiment flew dried adults of two tardigrade species on a satellite and exposed them, outside the hull, to the open vacuum and radiation of low Earth orbit for about ten days. Back on the ground and rehydrated, most of the animals that had faced raw vacuum revived, and some went on to lay viable eggs; the tardigrades that also took the full unfiltered solar ultraviolet fared far worse, and few of those came back. Vacuum, the condition that boils the water out of anything exposed to it, was survivable for an animal that had already given its water up. The limiting factor was the ultraviolet, not the vacuum.¹²

Strip the wonder back to its parts and what is left is a cell that answers a physical problem in kind. Water held the cell's structures at their working spacing, so the tardigrade builds a solid to hold them when the water is gone: a glass poured from sugar where a sugar is on hand and from its own disordered proteins where it is not, with a charged little protein thrown over the DNA against the radicals that drying and radiation both throw off. How much of the tardigrade's glass is sugar and how much is protein, and whether the gel seen in a tube is the gel that forms in a drying animal, are questions the field is still measuring. Being alive, it turns out, is in part a problem of holding a structure in place — and the tardigrade's surprise is that a cell can go on solving it without staying wet.