Extremophiles are organisms living beyond the realm of what was once thought to be conducive to life on Earth.
Extremophiles are studied by astrobiologists and other scientists in order to compare the extreme environments on Earth to other planetary bodies which may house habitable environments. Other worlds of particular interest to the study of extremophiles are Mars and the satellites of the outer solar system, Europa, Enceladus, and Titan. These organisms can tolerate extreme desiccation, heat, cold, pH, pressure, UV, nuclear waste, radiation, and many other environments. Some extreme organisms can tolerate combinations of the aforementioned environments and are termed polyextremophiles. The planet Mars is the most feasible planet to find extreme organisms on. A fleet of spacecraft have been sent to Mars for decades which have made many discoveries leading scientists to confirm that Mars was once a habitable environment with all of the resources available for life to exist (See Astrobiology). Any organism found on the Red Planet would be an extremophile by Earth’s standards. Many different types of extremophiles may have existed or may exist now on Mars which is a cold, dry, and high UV light environment.
A psychrophile (cryophile) is an organism that survives in extreme cold conditions. This organism can survive in temperatures ranging from -15o C to 10o C. (Conway et al. 2011, 76) (Scharf 2009, 225) These organisms use specific processes in their metabolic pathways to survive the extreme cold temperatures. Combinations of mechanisms are utilized to enable survival at such low temperatures. These include adjusting lipid ratios and concentration of salts, sugars, and glycerols in such a manner to create an antifreeze effect in the organism. Examples of psychrophiles can be found in many places on Earth such as the microbial mats in the Arctic and Antarctic regions. A common example of a psychrophile is snow algae. These organisms bloom in the summer months in regions where snow and ice are prevalent. They are red in color. These algae and cyanobacteria are also known as “red algae” and have been noted to be growing to a depth of 25 cm into the snow. (Conway et al. 2011, 77) Another example of a cryophilic organism is the Antarctic hair grass, Deschampsia Antarctica a, flowering plant native to Antarctica. This organism responds quickly to temperature fluctuations and deploys its metabolic defenses in order to preserve its life and reproductive processes. (Chew et al. 2012, 829)Many examples of cold tolerant organisms exist that may be able to survive the temperatures on Mars. The temperature ranges from 20o C at the equator on the warmest day of the year (approximately room temperature) to -153o C in the polar regions. (NASA 2013) Even on the warmest day on Mars, the temperature plummets at night due to the very thin atmosphere. However, we have seen that cryophiles have the ability to quickly adjust to temperature fluctuations.
Most life forms on Earth cannot tolerate high doses of radiation which damages the structure of DNA strands. Breaks in DNA may occur when exposed to radiation and cause cells to grow out of control thus becoming tumors. Exceptions to this rule do exist. A radioresistant polyextremophile such as Deinococcus radiodurans is resistant to doses of radiation 500 times more than a human and has been shown to survive doses up to 1500 times larger. This hardy organism is resistant to large doses of radiation by housing several copies of its DNA inside itself. If breaks do occur in the genome, it can easily utilize its spare copies. (Scharf 2009, 225-226) and it has also survived the vacuum of space in laboratory settings. (Abrevaya et al. 2011)
D. radiodurans is also a xerophile. (Conway et al. 2011, 77, 83) Xerophiles survive in severely dry environments, such as Chile’s Atacama Desert. The harsh Atacama Desert is commonly used as an analogue to the Martian environment due to its high radiation exposure and cold, dry environments. Xerophiles can survive for years without water. They have been known to “wake up” after being without moisture for several years. These organisms go into a state of suspended animation. (Conway 79) When the organism is exposed to water it comes back to life. A tardigrade, commonly known as a “water bear”, is a polyextremophile that can withstand desiccation for up to ten years and be brought out of suspended animation with exposure to water. This hardy animal’s ability to survive desiccation also allows it to survive the vacuum of space and exposure to solar radiation. (Jönsson et al. 2008) Scientists believe the ability to survive severely dry conditions are what allow for organisms to survive vacuum and radiation exposure. A xerophile would be an excellent choice for a Martian to be.
Some organisms use metabolic pathways known as autotrophic pathways in order to survive. They make use of CO2 to produce organic compounds. We know that Mars has plenty of CO2 for such an organism if it were to exist on the planet. (Scharf 2009, 225) Lithoautotrophs are organisms that use such pathways. They are known as “rock eaters” which utilize minerals, such as, pyrite, which has been found on Mars, to metabolize. (Zolotov et al. 2005) Lithoautotrophs and other extremophiles known as chemotrophs use the energy gradient of minerals and clays in order to survive harsh conditions. The Curiosity Rover has discovered such gradients in Gale Crater on the Martian surface. (JPL/NASA. 2013) NASA scientist Michael Meyer has stated, "A fundamental question for this mission [Curiosity] is whether Mars could have supported a habitable environment. From what we know now, the answer is yes." With such unequivocal evidence that Mars was once habitable, the planet is an ideal location for lithoautotrophs to exist or to have existed in the past.
Many other extremophiles exist on Earth. Everywhere astrobiologists look on Earth, from the highest mountains to the depths of the ocean, from extreme cold and heat, and from acidity to alkalinity life is found. According to Mars Society President, Dr. Robert Zubrin, “Across our own globe, in the most extreme environments imaginable, scientists have discovered life tenaciously hanging on making do with scant resources.” (Zubrin 2011, 35) The evidence is quickly mounting and pointing to the ability for life to exist on other planetary bodies such as Mars.
Conway, Andrew and Gilmour, Lain and Jones, Barrie W., Patel, Manish R., Rothery, David A., Sephton, Mark A., Zarnecki, John C. 2011. An Introduction to Astrobiology, New York: Cambridge University Press. (Conway et al. 2011, 76) (Conway et al. 2011, 77, 83)
Scharf, Caleb A. 2009. Extrasolar Plantets and Astrobiology. United States of America: University Science Books. (Scharf 2009, 225)
Chew, Orinda and Lelean, Suzanne and John, Ulrik P., Spangenberg, German C. 2012. “Cold acclimation induces rapid and dynamic changes in freeze tolerance mechanisms in the cryophile Deschampsia antarctica E. Desv.” Plant, Cell & Environment. 35: 829-837. Accessed December 27, 2013. doi: 10.1111/j.1365-3040.2011.02456.x. (Chew et al. 2012, 829)456 Quest.NASA.gov. “Mars Facts.” Accessed December 27, 2013. http://quest.nasa.gov/aero/planetary/mars.html. (NASA 2013)
Abrevaya, Ximena C., Paulino-Lima, Ivan G., Galante, Douglas and Rodrigues, Fabio and Mauas, Pablo J.D., Cortón, Eduardo and de Alencar Santos Lage, Claudia. 2011. “Comparative Survival Analysis of Deinococcus radiodurans and the Haloarchaea Natrialba magadii and Haloferax volcanii Exposed to Vacuum Ultraviolet Irradiation.” Astrobiology. 11(10): 1034-1040. Accessed December 27, 2013. doi:10.1089/ast.2011.0607. (Abrevaya et al. 2011)
Jönsson, K. Ingemar and Rabbow, Elke and Schill, Ralph O., Harms-Ringdahl, Mats and Rettberg, Petra. 2008. “Tardigrades survive exposure to space in low Earth orbit.” Current Biology. 18(17):R729-R731. Accessed December 27, 2013. doi:10.1016/j.cub.2008.06.048 (Jönsson et al. 2008)
Zolotov, Mikhail Y. and Shock, Everett L. 2005. “Formation of jarosite-bearing deposits through aqueous oxidation of pyrite at Meridiani Planum, Mars.” Geophysical Research Letters. Vol. 32 L21203. doi:10.1029/2005GL024253,2005. (Zolotov et al. 2005)
JPL/NASA.gov. “NASA Rover Finds Conditions Once Suited For Ancient Life On Mars.” March 12, 2013. Accessed December 27, 2013. http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1438 (JPL/NASA. 2013)
Zubrin, Robert. 2011. The Case for Mars. New York: Free Press. (Zubrin 2011, 35)