One of them can survive in temperatures as high as 250 degrees Fahrenheit. Another lives deep below the surface of Arctic ice. And still another is so tough, so resistant to everything nature and humans can possibly throw at it, that some scientists have nicknamed it “Conan the Bacterium.”

All of these microscopic organisms are known as “extremophiles,” with “phile” derived from the Greek word “philos,” or “loving.” Literally, this translates to “lovers of extremes.” And indeed they are. Some extremophiles can thrive in extreme temperature, others in extraordinarily high acidity, alkalinity, or salt.  Still more can be pummeled with 2,000 times the amount of radiation that would kill a human – and enjoy it. Well, maybe “enjoy” is too strong a word for a bacterium, but the equivalent of cliff diving 24/7 does keep these tiny creatures alive and multiplying.

Extremophiles live in every sort of harsh environment from deep-sea vents, to geysers, to nuclear waste, and they’ve attracted the attention of scientists eager to learn about life’s origins on Earth as well as the possibility of life on other planets. The Mars Viking did not find life as we know it, but some speculate that extremophiles could be living in deep aquifers miles below the surface. Here on Earth, scientists know that most of the planet’s bacteria live underground with no need of sunlight to exist and reproduce.

Extremophiles also interest scientists working in other fields, including medicine, biotechnology, and bioenergy. Bacteria that can survive extreme heat, for example, are sought after for breaking down biomass and for use in fermentation tanks during biofuel production.

Here is a compendium of some of the most extreme extremophiles:

Mesophile (“Moderation loving”)

Deinococcus radiodurans, otherwise known as Conan the Bacterium, is registered in the Guinness Book of World Records as the world’s toughest bacterium. Despite an affinity for moderate temperatures, this miniscule monster can withstand acid baths, high and low temperatures, dehydration, living in a vacuum, and most especially, radiation.

This one was discovered when researchers tried to sterilize meat with a dose of radiation that would normally kill all known life forms. But d. radiodurans survived, and the meat spoiled. Although NASA Science News describes its “humble beginnings” in cow pies and elephant dung, it may be the only one of its kind memorialized in a comic book cover: “Conan the Bacterium in the Lair of the Gamma Ray.” In a more practical use, it is now being engineered to help clean up radioactive sites left over from the Cold War.

Psychrofile (“Cold loving”)

One strain of the microbe Psychrobacter arcticus was found in a Siberian permafrost core – permanently frozen ground -- that is more than 20,000 years old. It was still growing and dividing…very, very slowly. In 2013 a new cold-loving microbe was discovered in the Canadian high Arctic, also growing in permafrost. Planococcus halocryophilus OR 1 was found on Ellesmere Island and thrives at 5 degrees Fahrenheit. The bacterium may offer clues to adaptations that might be essential for life on Mars or on Saturn's moon Enceladus, where temperatures drop to well below freezing.

Thermophile (“Heat loving”)

A recently discovered microbe known only as Strain 121 because it can survive in temperatures of 121 degrees Celsius (250 degrees Fahrenheit) is believed to be the world’s hottest organism.  (Technically, it’s a hyperthermophile, or uber-heat-loving organism.) This strain was isolated from an underwater chimney called a “black smoker” two miles below the ocean’s surface about 250 miles off the coast of Washington state. This bug, in a class of extremophiles called thermophiles, uses iron to metabolize food and may one day be helpful in generating electricity.

Acidophile (“Acid loving”)

Found in water near boiling geysers in Yellowstone National Park, Cyanidium caldarium is an algae that loves hot acid. The genus Acidithiobacillus produces fungi and bacteria that live in limestone caves in an environment of sulfuric acid and may become an important model for studying bacterial ecology and evolution.

Halophile (“Salt loving”)

Halophiles are extremophiles that thrive in areas with high salt concentrations. They can be found anywhere with a salt concentration at least five times greater than that of the ocean; this includes the Great Salt Lake, Owens Lake in California, the Dead Sea, and evaporation ponds. Where normal organisms would desiccate, halophiles survive because they actually exclude salt from their cytoplasm. One extreme halophile is Haloarchaea, which is always found in extraordinarily salty bodies of water – about 36 percent water to 64 percent salt. The deep red color that is sometimes seen in their habitat is due to carotenoids (organic pigments) that are part of their makeup.

Alkaliphile (“Alkaline loving”)

What about microbes that flourish in a place that’s both salty and alkaline? One example is Spirochaeta americana, a bacterium that lives in the mud deposits of California's naturally saline Mono Lake. Mark Twain once described Mono Lake as "nearly pure lye" and added that it had "nothing, in fact, that goes to make life desirable-" – except, perhaps, to scientists and microbes like S. Americana.

Extremophiles in our midst

While researchers are finding many practical applications for extremophiles, perhaps the “Conan” microbe and astonishing abilities is the most remarkable.  D. radiodurans could help NASA build better space suits or spacecraft. Researchers are also studying this remarkable microbe for clues to awakening dead cells. Not only does it withstand forces that would make an average extremophile quake, it can also survive radiation that breaks its DNA into pieces. It then sheds the broken DNA parts and uses an enzyme to attach the still-good DNA to the rest of the operating DNA – in essence, bringing itself back to life.



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