Some water bears eat microscopic animals, while others consume algae.
Credit: Daiki Horikawa, NASA Ames
Astrobiologists work at the cutting edge of scientific research, investigating the possibility of life elsewhere in our universe. They are, however, plagued by a single, potentially critical problem: a lack of samples. Studying alien organisms is naturally difficult when none have been discovered.
Some scientists have taken a novel approach to circumventing this issue. Life on Earth is abundant, and often very hardy. Extremophile creatures exist in places we would consider as exceptionally hostile, such as deep in the ocean floor or areas where even a drop of water is almost impossible to find. Similarly, some of the potential places for life elsewhere in our solar system, such as the planet Mars, Jupiter's moon Europa, or Saturn's moon Titan, have conditions that are viewed as extremely hostile but still potentially survivable. So extremophile organisms are studied in lieu of genuine extraterrestrial samples, to see if they could survive the rigors of life beyond the Earth.
Dr. Daiki Horikawa from NASA Ames Research Center has been examining strange creatures called tardigrades for the last seven years. He explains, "tardigrades are small invertebrate animals, 0.004 to 0.04 inches (0.1 to 1.0 mm) in body length, that live in terrestrial mosses, soil, or lichens. They also inhabit ocean and polar regions." Often known by their nickname, "water bears," studies of these bizarre creatures have surprised scientists.
Recent research has shown that water bears can survive the dangerous conditions of space. Previously, the only organisms that have been exposed to the radiation and vacuum of space and lived to tell the tale are certain types of bacteria and lichen. That simple animals like tardigrades also can survive gives more credence to the theory of panspermia, which claims that organisms could move from world to world after travelling though space.
Perhaps the most interesting feature of tardigrades is their ability to enter a suspended animation-like state when they cannot find enough water. In this kind of survival mode they become very resistant to harsh environments. When they encounter water they awaken and become active again.
Horikawa realised that if he wanted to produce worthwhile, reproducible studies, he would have to raise samples of water bears in laboratory conditions. Those collected from their natural habitat can be quite different to one another because of climate variations, nutritional differences and other environmental considerations. So Horikawa raised a collection on agar plates with green algae for food. Although labor-intensive, this produced uniform water bears which were then separated into groups and exposed to various stresses to see how tough they really are.
A barrage of tests
First they were heated up to sizzling temperatures of 194 F (90 C). Then a group was frozen at -321 F (-196 C). The next batch was given a dose of radiation similar to what they would receive in space around 4,000 times stronger than that which would make humans ill. The last selection was covered in a dissolving chemical (99.8% acetonitrile, a chemical which may be present in Titan's atmosphere).
The results, published in a recent issue of the journal Astrobiology, were that water bears in their suspended animation state survived everything that was thrown at them. Active creatures were less fortunate, but some did tolerate the extreme conditions. Now their tolerances to specific stresses are known, plans are underway to expose them to many harsh environments simultaneously.
If extraterrestrial life is as tough and resilient as water bears, then other worlds could be inhabited. "It's a possibility that water bear-like creatures could survive and thrive on other planets despite harsh environments," says Horiwaka. "Judging from data of the planets in our solar system, there could even be some in their suspended state on Mars." There is a chance that Earth-based water bears could be transported to other worlds via meteorite impacts, but more research is required before this is known for sure.
Horikawa plans on continuing his research, particularly looking at the ability of water bears to repair DNA after being exposed to high levels of radiation. As he says, "the most fascinating feature of tardigrades for me is their ability to survive complete dehydration of the body and high tolerance to radiation exposure which must cause critical DNA damages."
Horikawa's method of rearing water bears in the laboratory could lead to many more studies of these amazing animals, and may help astrobiologists in their search for genuine extraterrestrial life.
Knowing the survival tricks of water bears also could one day be a key component in enabling the human exploration of the universe.