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FEATURE ARTICLE

Tardigrades

These ambling, eight-legged microscopic “bears of the moss” are cute, ubiquitous, all but indestructible and a model organism for education

William R. Miller

Improvise, Adapt and Overcome

2011-09MillerF5.jpgClick to Enlarge ImageTardigrades exhibit distinctly different responses, grouped under the general name of cryptobiosis, to different sources of stress. Anhydrobiosis and cryobiosis lead to the formation of tuns, but they are not equivalent—they are different mechanisms for protection against different environmental assaults.

Anhydrobiosis—metabolic suspension brought on by nearly complete desiccation—is a common state for tardigrades, which they may enter several times a year. To survive the transition, water bears must dry out very slowly. The tun forms as the animal retracts its legs and head and curls into a ball, which minimizes surface area. When nearly all of its internal water has been surrendered, the tardigrade is in anabiosis, a dry state of suspended animation. It is almost as if the animal preserves itself by becoming a powder comprised of the ingredients of life. When rehydrated by dew, rain or melting snow, tardigrades can return to their active state in a few minutes to a few hours.

In cryobiosis, another form of cryptobiosis, the animal undergoes freezing yet can be revived. Any temperature below the cell cytoplasm’s freezing point suppresses molecular mobility and therefore suspends metabolism. Deep-freeze temperatures could be expected to cause additional structural disruptions, yet tardigrades, as noted above, have survived the most drastic chills. It seems likely that survival is conferred by the release or synthesis of cryoprotectants. These agents may manipulate tissue freezing temperature, slowing the process and allowing an orderly transition into cryobiosis, and they may suppress the nucleation of ice crystals, resulting in an ice-crystal form that is favorable for subsequent revival with thawing.

Osmobiosis is a response to extreme salinity, which can cause destructive osmotic swelling. Some tardigrades exhibit strikingly effective osmoregulation, maintaining stasis in the face of steep osmotic gradients. Some others escape via formation of a tun that is impervious to osmotic transfer.

In 2007, tardigrades became the first multicellular animal to survive exposure to the lethal environs of outer space. Researchers in Europe launched an experiment on the European Space Agency’s BIOPAN 6/Foton-M3 mission that exposed cryptobiotic tardigrades directly to solar radiation, heat and the vacuum of space. While the experimental vessel orbited 260 kilometers above the Earth, the researchers triggered the opening of a container with tardigrade tuns inside and exposed them to the Sun. When the tuns were returned to Earth and rehydrated, the animals moved, ate, grew, shed and reproduced. They had survived. In summer of 2011, Project Biokis, sponsored by the Italian Space Agency, ferried tardigrades into space on the U.S. space shuttle Endeavor. Colonies of tardigrades were exposed to different levels of ionizing radiation. The damage is now being assayed to learn more about how cells react to radiation and, perhaps, how tardigrade cells fend off its damage.

My student’s mind is turning.
How far is the nearest solar system? Could cryptobiotic tardigrades make it to Earth? she asks.
I saw a post recently on a listserv that says it might be only about 10 light years away, I answer. So if tardigrade tuns were transported here on a meteor or asteroid at just one tenth the speed of light, they could conceivably make the trip within the known survival capability of the animal. Theoretically. But the likelihood is awfully small. And think of the poignancy of arriving after that great journey but having no way to make it through the fiery descent through an atmosphere. Even tardigrades can’t survive that.

Surviving intense radiation suggests an especially effective DNA repair system in an active organism. Effective osmoregulation in extreme salinity implies a vigorous metabolism—osmoregulation in the face of high environmental salinity is energetically extremely expensive as metabolic transactions go, requiring the pumping of ions against steep osmotic and ionic gradients. Thus, we see in tardigrades two opposing responses to environmental extremes: the passive response of dormancy in the form of cryptobiosis, balanced by the hyperactive responses of impressive DNA repair and high-performance osmoregulation. As practitioners of adaptive evolution, tardigrades are virtuosos.








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