Water Bears: Extremely Lovable Aliens
A newly discovered microbe may help identify if there is life on other frozen moons or planets. It was found in Ace Lake, Antarctica, having attached itself to ice by a kind of grappling hook made of large antifreeze proteins.
Ace Lake, which was initially a freshwater lake and then a salty body of water that freezes over for several months of the year, is situated in the Vestfold Hills at the east coast of Antarctica. It is a truly inhospitable environment, but it still harbors life—microbes, thriving in its frigid waters.
In order to survive in the planet’s coldest places they produce antifreeze proteins—molecules that latch onto small ice crystals and stop them from growing bigger. By releasing these proteins into the surrounding space them, the microbes can lower the freezing point of water and create a network of liquid in which they swim and feed.
Among the hundreds of cold-loving microbes scooped up from Ace Lake, graduate student Jack Gilbert discovered one, named Marinomonas Primoryensis, that produced an “uncharacteristically powerful” antifreeze. Its glyco protein was a titan of a molecule—between 30 and 50 times bigger than the average protein, and almost half as long as the microbe that makes it.
The research team, led by Professor Peter Davies, kept studying the giant protein. They soon realized that only one tip of the long molecule can latch onto ice; the other stays stuck to the bacterium’s body. It seemed that Marinomonas was using the protein like a grappling hook, to grab onto ice rather than stop it from forming.
Davies suspects that the microscopic grappling hook is shaped so that it organizes the water molecules around it. These line up in an orderly lattice—a bit like they would in ice, but slightly more fluid. They’re now halfway between liquid and solid, between water and ice. The water at the very surface of ice sheets is in a similar state, and when these two liquid-ish layers meet, they mingle and freeze. The tip of the protein is then embedded within the ice, and the bacterium is firmly fastened.
Glyco (antifreeze) proteins are produced by many cold-acclimatized species, including fish, plants, and insects. A microscopic organism called Tardigrade, better known as water bear, which is very common throughout the world, has also been found in Antarctica. They are some of the most durable creatures known to man. They can even handle hard UV and the vacuum of space, which the ESA found out in 2007 when they stuck aluminum thimbles full of dried tardigrades and tardigrade eggs to a satellite.
They don't seem to care what temperature it is either. In the 1920s, a Benedictine friar named Gilbert Franz Rahm brought tardigrades back to life after heating them to 151 °C for 15 minutes. Rahm also tested them in the cold. He immersed them in liquid air at -200 °C for 21 months, in liquid nitrogen at -253 °C for 26 hours, and in liquid helium at -272 °C for 8 hours. Afterwards the tardigrades sprang back to life as soon as they came into contact with water.
We now know that some tardigrades can tolerate being frozen to -272.8 °C, just above absolute zero. However, antifreeze proteins haven't been found in tardigrades. Instead it seems tardigrades can actually tolerate ice forming within their cells. Either they can protect themselves from the damage caused by ice crystals, or they can repair it. Tardigrades may produce chemicals called ice nucleating agents. These encourage ice crystals to form outside their cells rather than inside, protecting the vital molecules.
Water bears are also excellent at repairing their DNA. Research into the absurd durability of these little creatures continues; scientists are currently working to figure out their DNA repair mechanisms, in hopes of copying their tactics for human health applications.