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.
Nearly indestructible
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.
Self-repairing DNA
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.
advertisement
