Florida FAST Practice Test 12

Life on Pause — by Tobias Wren

¹Rinse a clump of damp moss from a garden wall into a dish, look through a microscope, and you may meet one of the toughest animals on Earth. Tardigrades—nicknamed "water bears" for their plump bodies and lumbering walk—are rarely longer than half a millimeter. They paddle through the film of water that clings to moss and lichen on eight stubby legs, each tipped with tiny claws.

²That film of water is the tardigrade's whole world, and it is not reliable. Moss dries out in the sun; puddles vanish. Most animals would die when their surroundings dried up. Many tardigrades simply stop. As the water around them disappears, they pull in their heads and legs, curl into a dry, barrel-shaped form called a tun, and lose nearly all of the water in their bodies. Their metabolism slows until it can barely be detected. This state is called cryptobiosis, from Greek words meaning "hidden life."

³In the tun state, tardigrades can endure conditions that would destroy most living things. Laboratory studies have exposed them to temperatures near absolute zero, to heat well above the boiling point of water for short periods, and to pressures greater than those at the bottom of the deepest ocean trench. In 2007, dried tardigrades were carried into orbit aboard an uncrewed spacecraft, and many survived exposure to the vacuum of space. Add water, and within minutes to hours a tun can uncurl and begin to walk again, as if it had merely paused.

⁴How do they do it? For years, scientists assumed the answer was a sugar called trehalose, which helps some other drought-tolerant organisms survive drying. But many tardigrades make little or no trehalose. More recent research points to a family of proteins found only in tardigrades. As the animal dries, these proteins appear to form a glass-like solid inside its cells, holding delicate molecules in place so that they cannot unfold or clump together. When water returns, the "glass" dissolves, and the cell's machinery is released intact.

⁵That discovery has drawn attention far beyond the study of animals in moss. Many vaccines and medicines must be kept cold from the factory to the clinic, a costly requirement in places without reliable electricity. If the tardigrade's protective proteins could be adapted to keep such medicines stable at room temperature, a creature about the size of a grain of sand might one day help deliver medicine to people who now go without it. Researchers caution that such uses remain years away. Still, the water bear has already taught a large lesson: sometimes the best way to survive a crisis is to stop, wait, and begin again.