Tardigrades Survive Freezing via "Pause" Button

Tardigrades, often called water bears, are famous for their ability to survive conditions that would kill almost any other living thing. Recent research has finally pinpointed the biological mechanism behind this resilience. It turns out these microscopic animals use unique proteins to turn their interiors into a protective “bioglass,” effectively hitting a pause button on life until conditions improve.

The Secret of the "Tun" State

When a tardigrade senses its environment drying up, it enters a dormant state known as anhydrobiosis. During this process, the animal expels more than 95% of the water in its body. It pulls its eight legs inside its body, curls into a ball, and shrinks significantly. This dried husk is called a “tun.”

For decades, scientists understood the physical changes of the tun state, but the molecular chemistry remained a mystery. In most organisms, removing water causes cells to collapse or allows sharp ice crystals to form, which shred internal tissues. Tardigrades avoid this damage completely. In the tun state, they can survive:

• Temperatures near absolute zero (-459°F).
• Heat up to 300°F.
• The vacuum of outer space.
• Crushing pressures found in the deepest ocean trenches.
• Massive doses of radiation.

The key to this survival is not a hard outer shell, but a transformation of the liquid inside their cells into a solid, glass-like matrix.

CAHS Proteins: The Magic Ingredient

Researchers at the University of North Carolina at Chapel Hill, led by Silvia Sanchez-Martinez and Thomas Boothby, identified a specific family of proteins responsible for this glass transformation. These are called Cytosolic Abundant Heat Soluble (CAHS) proteins.

Unlike most proteins, which have a rigid and fixed 3D structure (like a key fitting into a lock), CAHS proteins are “intrinsically disordered.” This means that under normal conditions, they are shapeless and flexible, floating freely in the tardigrade’s cellular fluid.

However, the moment the tardigrade begins to dry out or freeze, these proteins trigger a phase change:

1. Concentration: As water leaves the cell, the concentration of CAHS proteins increases.
2. Gelation: The proteins begin to link together, forming a web-like gel network.
3. Vitrification: As desiccation continues, this gel hardens into a solid, non-crystalline structure resembling glass.

This bioglass acts like a microscopic scaffold. It physically supports the cell walls and traps sensitive molecules in place, preventing them from unfolding, breaking, or fusing together. It stops the chemistry of life without destroying the machinery required to restart it.

Reversibility: Melting the Glass

The most remarkable aspect of the CAHS mechanism is its reversibility. The “pause” button works both ways. When water is reintroduced to the environment, the tardigrade takes it in.

As water molecules re-enter the cells, the glass matrix dissolves. The CAHS filaments detach from one another and return to their disordered, liquid state. The cell machinery, which had been frozen in time, picks up exactly where it left off.

This process is incredibly fast. A tardigrade can go from a dormant glass tun to an active, swimming animal in a matter of minutes to hours, depending on how long it has been dormant. This cycle can be repeated multiple times throughout the animal’s life.

Beyond Sugar: A New Understanding

For a long time, the scientific consensus was that a sugar called trehalose was responsible for tardigrade survival. Trehalose is found in brine shrimp (Sea Monkeys) and helps them survive drying by replacing water in cells.

However, studies revealed that not all tardigrade species produce trehalose, yet they still survive extreme drying. The discovery of CAHS proteins provides the missing piece of the puzzle. While trehalose may play a role in some species, the protein-based glass formation is the primary driver for the water bear’s extreme durability.

Why This Matters for Humans

Understanding how tardigrades turn into glass has massive implications for medical science and logistics. The team at UNC-Chapel Hill has already demonstrated that CAHS proteins can be isolated and used to stabilize other biological materials.

Eliminating the “Cold Chain”

Currently, many life-saving medicines, vaccines, and blood products must be kept refrigerated or frozen from the moment they are manufactured until they are used. This logicistical hurdle is known as the “cold chain.” It makes distributing vaccines to remote or developing regions incredibly difficult and expensive.

By using CAHS proteins, scientists hope to develop storage methods that allow these biologic drugs to be dried and stored at room temperature. They would essentially be “turned to glass” just like the tardigrade, and then reconstituted with water when needed.

Anti-Aging and Cell Preservation

The research also has implications for cell preservation. In lab experiments, introducing CAHS proteins into human cells and bacteria increased their resistance to drying. This suggests that the proteins function independently of the tardigrade’s specific physiology. This could lead to breakthroughs in organ preservation for transplants or therapies that arrest cellular degradation.

Frequently Asked Questions

What exactly is a tardigrade? A tardigrade is a microscopic invertebrate, often between 0.5mm and 1mm in length. They are aquatic animals found in moss, lichens, and soil. They have four pairs of legs and are colloquially known as “water bears” or “moss piglets.”

How long can a tardigrade stay in the glass state? Tardigrades have been revived after spending 30 years in a frozen, desiccated state. While they do not live forever (their active lifespan is only a few months), the tun state stops their biological clock, allowing them to exist in suspension for decades.

Do the proteins kill the cell when they harden? No. Unlike ice crystals, which are sharp and expand (causing cells to burst), the CAHS protein structure is amorphous. This means it hardens without forming sharp edges or expanding significantly, protecting the cell’s integrity rather than damaging it.

Can humans use these proteins to hibernate? Not currently. While the proteins improve the resilience of individual human cells in a petri dish, applying this to a complex, multicellular organism like a human is significantly more complicated. We cannot simply dry out a human body and expect it to survive, as our organs and circulatory systems are far too complex for this specific mechanism alone.