Tardigrades, commonly known as water bears or moss piglets, are microscopic invertebrates renowned for their ability to survive conditions that would instantly kill most other forms of life. These eight-legged animals possess a unique biological strategy to endure severe environmental stress by entering a state called cryptobiosis. Cryptobiosis is a reversible suspension of metabolism, essentially life on pause, which allows them to survive without water, food, or oxygen for extended periods. The sophisticated and coordinated preparation process the tardigrade undergoes is key to achieving this near-death state.
Environmental Triggers and Physical Transformation
Preparation for life suspension is typically initiated by a decline in the availability of free water in the tardigrade’s immediate environment. This primary trigger, known as anhydrobiosis, forces the animal to respond rapidly to impending severe dehydration. Other conditions, such as extreme cold (cryobiosis) or a lack of oxygen (anoxybiosis), can also prompt a similar preparatory response.
As the surrounding water film evaporates, the tardigrade must begin a physical transformation to minimize its surface area and water loss. The animal retracts its four pairs of stout legs and head, curling its body into a compact, barrel-shaped structure called a “tun”.
The formation of the tun is a metabolically active process that buys the tardigrade time to synthesize the necessary internal protective molecules. During this contraction, the tardigrade expels nearly all of the free water from its body, reducing its internal water content to a mere one to three percent of its normal level. This physical condensation is the first line of defense, creating a durable, desiccated pellet that can withstand external physical forces and temperature fluctuations.
Molecular Shielding
Once the physical tun is formed, the tardigrade’s internal chemistry takes over to prevent the collapse and damage of cellular structures caused by water loss. In many desiccation-tolerant organisms, a sugar called trehalose is produced to replace the structural role of water molecules inside the cell. While some tardigrade species utilize trehalose, it is often present at low or undetectable levels, suggesting they rely on a more specialized system.
Instead of a sugar-based defense, tardigrades generate vast amounts of Tardigrade-specific Intrinsically Disordered Proteins (TDPs), such as Cytosolic Abundant Heat Soluble (CAHS) proteins. These unique proteins perform the protective function primarily by a process known as vitrification. As the water leaves the cell, these hydrophilic proteins form a non-crystalline, amorphous, glass-like matrix that permeates the cytoplasm.
This molecular glass locks the internal machinery of the cell—including proteins, enzymes, and organelles—in a stable, fixed position, preventing them from unfolding or aggregating as they dry. The glassy matrix ensures that the cellular components retain their correct three-dimensional structure until rehydration. Heat Shock Proteins (HSPs) also assist in this process by binding to other proteins to maintain their proper folding and stability during drying.
Safeguarding Genetic Material and Cell Integrity
The final preparation involves stabilizing the cell membranes and protecting the genetic blueprint, the DNA. Cell membranes, composed of delicate lipid bilayers, are highly susceptible to damage and fusion when water is withdrawn. To counter this, specialized intrinsically disordered proteins are synthesized to associate with the membranes, helping to maintain their structural integrity as the cell shrinks.
The DNA, which is particularly vulnerable to breakage from desiccation and radiation, is shielded by a protein known as Damage Suppressor (Dsup). This unique, highly positively charged protein migrates to the cell nucleus and physically wraps itself around the DNA. Dsup functions as a protective cloud, shielding the genetic material from the damaging effects of desiccation-induced stress and reactive oxygen species.
The tardigrade also prepares for the oxidative stress that occurs upon rehydration. It upregulates the production of various antioxidant enzymes and enhances DNA repair mechanisms while still metabolically active. This foresight manages the harmful reactive oxygen species (ROS) generated when metabolism suddenly restarts, ensuring the cell has the tools to repair any genetic damage accumulated during its dormant state.