Call them what you will—moss piglets, water bears, or by their real name, tardigrade—but these intriguing tiny creatures can come back from the brink of death. They can survive boiling, deep freezing, UV radiation, completely drying out, and even a trip to space—without the benefit of being in a spacecraft.
Tardigrades are microscopic animals, about 1 mm long, that live just about everywhere on this planet. Water species live in fresh or saltwater, and terrestrial species live in the damp interior of moss, lichen, leaf litter, and soil. They are renowned for their tolerance of extreme environments.
New research, published in the journal Molecular Cell, reveals that these unusual creatures use a process to survive desiccation—a state of extreme drying created by removal of water—that is just as unusual.
Tardigrade have their own unique set of proteins that the researchers named tardigrade-specific intrinsically disordered proteins. They are a type of protein that are amorphous—they don't have a defined or predictable 3D shape that proteins usually have. The flexibility of disordered proteins may help them assume different shapes needed for different functions, such as binding to other proteins.
Desiccation should be an irreversible state, but in a few organisms, it isn't. Brine shrimp (aka Sea-Monkeys) and yeast are two organisms that are commonly found in dry form, but come to life when water is added.
The integrity of proteins within a cell is vital to maintain during the process of dehydration. If the protein clumps or folds, it can't return to its original structure and cannot function. Think of the process as cooking an egg. Once the proteins are heated, they denature, changing form and losing structure. You can't uncook the egg and return it to its previous state. The cells in the egg can never "live" again.
Those two organisms above survive desiccation because a sugar they produce, called trehelose, helps proteins survive denaturing. There are two hypotheses about how trehalose accomplishes this.
One theory is that trehalose uses vitrification, a process where a glass-like matrix forms inside cells and physically prevents protein from clumping and folding in response to dehydration. The other theory is that trehalose protects proteins against dehydration by bonding to the dried protein and preventing the destruction of the protein during dehydration.
Tardigrades have been known to have survived desiccation for decades, so clearly this process is reversible in these tiny animals, as well.
And scientists had thought tardigrade probably used trehalose in the process, too, until they discovered that tardigrades make little to none of the sugar and don't have the gene required to make the enzyme needed to synthesize it.
The study authors speculated that the new disordered protein protects tardigrade through vitrification. Although the proteins themselves don't have shape when they are in the presence of water, when they dry out, then enter a glass-like matrix phase. At this point, the protein matrix acts as a protective covering with pores that trap proteins that are sensitive to desiccation, preventing them from denaturing and clumping. And that process renders desiccation reversible in tardigrade.
These proteins may have a number of uses, according to Thomas Boothby, first author on the study, from the departments of Biology and Chemistry at the University of North Carolina at Chapel Hill. They could eventually prove useful in protecting crops from drought and safe-guarding medications sensitive to drying out for use in countries without adequate or readily-available refrigeration.
The big takeaway from our study is that tardigrades have evolved unique genes that allow them to survive drying out. In addition, the proteins that these genes encode can be used to protect other biological material—like bacteria, yeast, and certain enzymes—from desiccation.
We are just beginning to peel back the layers of mystery that shroud tardigrade. We've found the key to one of their survival mechanisms. Who knows what secrets they will reveal next?