Tardigrades survive extreme dehydration by turning into glass

By Jessica Hall

Just when we thought we’d seen all the tricks waterbears keep up their tiny sleeves, they’re back to surprise us again. Scientists have revealed one of the tardigrade’s best-kept secrets: how they protect themselves from harm under extreme dehydration. They do it by turning into glass. Now we know how.
The trouble with falling from heights isn’t the fall itself; it’s the sudden stop at the end. In much the same way, the trick isn’t necessarily that the tardigrade can turn into glass, but that it can come back from that state and be fine. To accomplish that remarkable feat, the animals rely on bespoke proteins called tardigrade-specific intrinsically disordered proteins (TDPs).
Proteins have a three-dimensional structure in space, and it’s often different depending on what the protein is “in” — whether it be water, methane, carbon dioxide, or a vacuum. TDPs are no exception. In water, many proteins become hydrated, and they sort of loosen up and act like kelp, with waving, frondlike arms instead of a tightly folded structure. But when they start to dry out, TDPs do something special. They form a kind of bioglass, keeping the waterbears’ innards from harm.
“When the animal completely desiccates, the TDPs vitrify, turning the cytoplasmic fluid of cells into glass,” said lead author Thomas Boothby, of UNC Chapel Hill. “We think this glassy mixture is trapping [other] desiccation-sensitive proteins and other biological molecules and locking them in place, physically preventing them from unfolding, breaking apart or aggregating together.”
This forced me to re-evaluate my entire understanding of the notion of glass, much like this recent story changed my understanding of wetting — which has more to do with the tight contact between two things than it does with water.

A glass is an amorphous solid that doesn’t have a crystalline structure. Even some things that have a crystalline form can also have a glass form. For example, when you heat and cool some crystalline sugars, they go through a noncrystalline glass phase. That’s how hard candies stay crisp and transparent, instead of melting into a pile of goo: sugar glassing.

In a culinary setting, sugar glassing requires temperatures in excess of 300 F, but in the tardigrade these TDPs seem to grab onto sugar molecules and glass them at much lower temperatures. That’s another cool power of proteins: being able to conduct chemical reactions far from equilibrium. Being a glass requires “long-range atomic disorder,” and the mishmosh of proteins and sugars seems to supply that requirement for chaos.

Freezing and bioglassing both have something in common with drying, in that when you dry something out, you’re pulling all the water molecules out of solution and sending them into the ambient air. When water freezes, what’s actually happening is that it’s becoming crystals; that has the effect of pulling the water molecules out of aqueous solution and depositing them into the crystal matrix. That’s why freeze-drying is a food preservation tactic.

The formation of water ice crystals is why frostbite is so damaging, and it’s also why frozen fruits and veggies are sort of mushy and leaky when they thaw. Those needlelike crystals puncture individual cells and let the cytoplasm leak out. But tardigrades can survive being freeze-dried in the vacuum of space, and now we’re starting to understand why. Doubtless these continuing breakthroughs will help us to tie together concepts and plunder the tardigrade’s survival skillset for our own gain.