Move Aside Arsenic Bacteria

By Amanda Ruben

                  What is the most indestructible thing on Earth? Some things may come to mind such as nanotubes, diamonds or Rocky Balboa’s jaw. All three of these are good guesses, but if you talk to any biologist the answer would be a water bear. These water bears are also known as tardigrades. Now you may be wondering what conditions are so horrendous or harsh that biologists would put this organism on a pedestal. Space vacuum, solar radiation and extreme temperatures are a few conditions that come to mind, but first where can we find these organisms on Earth and what are they closely related to?
                  Tardigrades are microscopic invertebrates with a well-developed organization. They have a brain, muscles, reproductive organs, osmoregulatory organs and sensory organs. Tardigrades inhabit a variety of environments found worldwide. These habitats can range from aquatic to terrestrial to limno-terrestrial which is an environment that frequently dries out.
                  Tardigrades are most closely related to Arthropoda and Nematoda. There are three classes of Tardigrada: Heterotardigrada, Eutardigrada and the controversial Mesotardigrada. The Mesotardigrada is controversial as it only contains a single species which was isolated from a hot spring in Japan. However, the hot spring or specimens no longer exist due to an earthquake disruption. Tardigrades are thought to have evolved within a marine environment, and the various mechanisms behind adaptive tolerances these organisms withstand have yet to be investigated.
                  As stated above, some of the extreme environmental stresses include a space vacuum, solar radiation and extreme temperatures. These are just a few of the conditions in which tardigrades can survive; nonetheless a combination of these stresses cannot defeat a tardigrade. Previous experiments have shown the amazing survival rates of these organisms and their offspring. A space vacuum is similar to desiccation. One study determined a space vacuum had no significant effect on the egg-laying or hatching as compared to the control organisms that did not undergo desiccation. In order to survive desiccation, the organism closes into a ‘tun’. A tun is when the organism retracts its legs and contracts longitudinally into a ball. During the tun phase, metabolism is at nearly a stand still allowing the organism to survive. Once hydration occurs, the organism expands and extends its legs, and metabolism is restored. Tardigrades have been shown to survive for up to 10 years in anhydrobiosis form, which is extreme dessication and this has been shown to have no effect on the production of viable offspring.

This is an image of a water bear forming a ‘tun’ on the right
side in which extreme dessication results in an ametabolic state.    

                  Solar radiation is also an extreme environmental stress that has not been able to knockout all tardigrades. Some species have lowered survival and fitness, but others are able to produce viable offspring even with a combined treatment of radiation and space vacuum.  Another profound question of radiation and space vacuum exposure illustrates how the configuration of DNA survives.  Experimentation of these conditions may help understand changes in DNA configuration and repair which can be applied to various diseases to humans.

                  Extreme temperature is another environmental stress which is no match for the tardigrades.  They have been proven to withstand temperatures of extreme heat and cold.  For example, tardigrades withstand 151°C and can withstand 1 degree above absolute zero for a few minutes.  The mechanisms behind this rapid adaptation have yet to be understood, but may be very insightful for understanding how these organisms can withstand such temperature fluxes.

                  Tardigrades now belong to an elite group of organisms which had the opportunity to become an astronaut, endure the elements of space and live to tell the tale.  As mechanisms behind these environmental adaptations are further studied, the knowledge gained could have many benefits to the human race and our own health problems and diseases.  Therefore, move aside arsenic bacteria.

References

(1). Jonsson, K. et.al. (2008). Tardigrade survive exposure to space in low Earth orbit. Current Biology                   18 (17): R729-R731.

(2).  Bertoliani, R. (2001). Evolution of the reproductive mechanisms in tardigrades-A Review. Zool. Anz. 240: 247-252.

(3). Mobjerg, N. et. al. (2011). Survival in extreme environments-on the current knowledge of                   adaptations in tardigrades. Acta. Physiol. 202: 409-420.