Tardigrades, also known as "water bears," are microscopic invertebrates that can survive in extreme environments, including high osmotic pressure, freezing temperatures, and complete desiccation.

Scientists have long been fascinated by their ability to enter a tun state, where they retract their limbs and substantially decrease their internal water stores, greatly increasing their ability to survive.

However, the mechanism(s) through which tardigrades initiate tun formation has remained a mystery until now. A new study reveals that tardigrade tun formation is mediated by reactive oxygen species (ROS) and is dependent on reversible cysteine oxidation.

Tardigrades, with more than 1,100 species known, are close relatives of arthropods, measuring just 0.04 inches (~1.01 millimeters) or less in size. They inhabit various environments, from flowering plants and moss to sand, freshwater, and even the ocean. Some tardigrades are plant-eating, while others are predatory carnivores preying on small invertebrates.

The study, led by researchers Amanda L. Smythers, Kara M. Joseph, and their team, dives into the molecular intricacies of tardigrade survival. When faced with adverse conditions such as extreme temperatures, lack of water, or oxygen, tardigrades enter a state known as "tun."

This state involves limb retraction, dehydration, and a slowed metabolism, allowing them to endure harsh environments for extended periods, sometimes years.

Previously, scientists were unsure about the signals that prompt tardigrades to enter or exit this dormant state. The recent study exposes tardigrades to extreme conditions in a lab, including temperatures as low as -112 degrees Fahrenheit (80 degrees Celsius) and high levels of hydrogen peroxide, salt, and sugar.

In response to these stressors, the tardigrades produce reactive oxygen species (ROS) that trigger cysteine oxidation, a key amino acid. This oxidation signals the water bears to go into their dormant state.

The research team used confocal fluorescent microscopy to observe tardigrades exposed to a cysteine-selective fluorescent probe. This visualization revealed that when conditions improved and ROS were no longer present, the tardigrades emerged from dormancy. Importantly, the team found that blocking cysteine in the environment prevented the water bears from detecting ROS, preventing them from entering the dormant state.

Their survival is dependent on reversible cysteine oxidation

The results suggest that cysteine oxidation acts as a crucial sensor, allowing tardigrades to switch dormancy on and off in response to various stressors. This mechanism enables water bears to navigate and survive in ever-changing environments.

Leslie Hicks, a co-author of the study, emphasized the significance of these findings. "Our work reveals that tardigrade survival to stress conditions is dependent on reversible cysteine oxidation, through which reactive oxygen species serve as a sensor to enable tardigrades to respond to external changes," Hicks stated.

The potential implications of this research extend beyond the realm of tardigrades. Future studies could explore whether this mechanism is universally conserved across all tardigrade species and its relevance to aging. Since reactive oxygen species are linked to age-related ailments, understanding tardigrade biology may provide valuable insights into the aging process.

The study was published in PLOS ONE on January 17 and can be found here.