“MASTER of survival. Can withstand pressures six times greater than those at
the bottom of the ocean and endure temperatures ranging from more than 100
°C down to absolute zero. Can shrug off lethal radiation, survive in a
vacuum and go without water for more than a century.”
It sounds like the résumé of a superhero. But these traits
belong to little-known animals less than a millimetre long that probably live on
your rooftop. Like superheroes, the animals conceal their powers behind a
mild-mannered everyday appearance. Known as tardigrades, their cylindrical
bodies, stumpy legs and tiny claws have earned them the nicknames of “water
bears” and “moss piglets”.
Tardigrades are by far the toughest animals on Earth. They have also
steadfastly defied our attempts to define their limits. What the few biologists
who study them have discovered, however, is that the secret of their survival is
the ability to shut down their metabolism completely while maintaining their
cellular structure—a trick that may give us a tip or two for extending our
own longevity.
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The ability to switch off all living processes and then start them up again
has fascinated researchers for nearly 300 years. Anton van Leeuwenhoek started
it all when, early on 3 September 1702, he sprinkled a pinch of dust into a
glass tube, poured water over it and peered down his microscope to watch what
happened. The 70-year-old Dutch scientist had taken the dust from the gutter on
the roof of his house and dried it for two days. The water he added had been
boiled to kill anything living in it and then allowed to cool. Within half an
hour, however, the glass tube was swimming with life.
With its overtones of spontaneous generation, van Leeuwenhoek’s experiment
entertained Europe’s chattering classes during the latter half of the 18th
century. Among them was the Italian scientist Lazzaro Spallanzani, who recreated
van Leeuwenhoek’s work in 1776 and named some of the animals that he saw
“tardigrades” because of their ponderous gait. He also showed that the animals
could survive freezing, describing them as “so much privileged by nature as to
enjoy the advantage of real resurrection after death”.
Following in Spallanzani’s footsteps, physiologists tried to outdo each other
in testing the animals’ capacity to withstand hardship. In 1842, the French
naturalist Louis Doyère showed that they could survive heating to 125
°C for several minutes. Going to the other extreme, Paul Becquerel watched
the animals revive after cooling them to a fraction of a degree above absolute
zero in 1950. Fourteen years later, Raoul-Michel May and his colleagues zapped
them with a dose of X-rays 250 times stronger than that required to kill
mammals—and they crawled back for more. Last year, Kunihiro Seki and his
colleagues at Kanagawa University in Japan subjected tardigrades to a pressure
of 6000 atmospheres, and still they refused to die.
Tardigrades can even survive having their picture taken by an electron
microscope, which involves putting them in a vacuum and bombarding them with
electrons. “When we take them out, we can put water on them and they will climb
around,” says Reinhardt Kristensen of the University of Copenhagen, who has
studied the animals for 26 years.
Roughly 700 species of tardigrades have been found, in habitats ranging from
the tops of the Himalayas to the depths of the Pacific. So how do they withstand
such extreme conditions? When the going gets tough, many tardigrades curl up
into a barrel-like shape called a “tun”, pulling in their legs and generally
battening down the hatches. Curling up reduces their surface area and slows the
rate of water loss from their bodies. This gives them a chance to make the
biochemical preparations for shutting down their metabolism. “They have to have
a little time to prepare,” says Kristensen.
Once they have shut themselves down, he says, there is no respiration. Their
metabolism stops totally. Of course, any animal can stop its
metabolism—the process is generally known as “death”. The problem is
starting it up again, which requires protecting and preserving essential cell
structures such as membranes.
To do this, tardigrades have come up with a sweet solution. In 1975, John
Crowe of the University of California in Davis showed that tardigrades in the
tun state contain a sugar called trehalose, which is peculiar to animals that
can survive desiccation. In a follow-up experiment in 1991, Kristensen’s PhD
student Hans Raml v and chemist Peter Westh of the University of Copenhagen
measured a rapid increase in levels of trehalose in tardigrades as they entered
the tun state. Crowe suggests that tardigrades protect the vital components of
their cells by replacing the water in membranes with the sugar.
Cell membranes are built from two layers of phospholipids, molecules with
water-loving heads and water-hating tails. Water molecules play an important
role in maintaining the fluidity of membranes by separating the head groups and
keeping them apart. Take the water away and the spacing between the heads
collapses, turning the membrane into a useless gel. “The sugar sits between the
head groups and pushes them apart as if it were water,” says Crowe.
Substituting trehalose for the water in membranes allows tardigrades to
survive for long periods without any water at all. The record is 120 years, held
by tardigrades taken from dried-out moss kept in a museum in Italy. Once their
membranes are safely cosseted by trehalose, the animals can shut down their
metabolism and survive without oxygen or food.
Surviving extreme cold requires yet another clever trick. When they freeze,
the animals have to protect themselves against damage caused as ice crystals
grow. Westh has identified large proteins in an Arctic tardigrade that raise the
freezing point of the animal to ensure that it freezes quickly. This creates
lots of tiny crystals that are less likely than larger ones to damage cells,
enabling some species to survive freezing in liquid helium.
With such an arsenal of adaptations for survival, tardigrades appear to be
over-engineered. “They can tolerate outer space, no doubt about it,” says
Kristensen. So did they arrive on Earth after a long journey from the planet
Tardigron? It’s unlikely. Diane Nelson of East Tennessee State University in
Johnson City has compared the ribosomal RNA of tardigrades with that of other
animals and deduced that they are an early offshoot of the lineage that gave
rise to insects and crustaceans. “We think that they originally came from the
sea and went from that into brackish water and then into freshwater,” she
says.
The motivation behind their move stems from their only weakness. “Tardigrades
are not very well adapted for competition with other animals,” says Kristensen.
“They had to go to the extreme environments.” The animals acquired and honed
their survival skills on the way, as not all members of the group share the same
abilities. Marine tardigrades, for example, cannot survive drying out and do not
form the tun state.
Moss pigs might fly
But some terrestrial species can pull off one of the most impressive stunts
of all—they can move around the planet through the stratosphere. “Just as
we get dust particles here in Tennessee from the Sahara desert, we can get
tardigrades that flow over on those air currents from Africa,” says Nelson.
Curled in their tun state, the animals would have encountered temperatures lower
than –100°C during their high-altitude journey.
What can we learn from these survival experts? Harnessing the tardigrades’
powers of self-preservation, Seki and his team have used trehalose to store rat
hearts for 10 days before reviving them (Â鶹´«Ă˝, 7 November
1998, p 7). The goal is to develop long-term organ banks for transplant
operations. “We believe that this method can be effectively applied to prolong
the storage periods of other organs as well,” he says.
If brains can follow the hearts, then the preservation and resuscitation of
whole bodies might not be far behind. At present, only single mammalian cells
such as blood and sperm can be reliably stored for long periods, but tardigrades
have around 40 000 cells, including nerves. Such suspended animation has long
been the staple fare of science fiction, allowing humans to endure the tedium of
crossing vast interstellar distances. So while tardigrades might not have come
from the planet Tardigron, they might one day lead us there.