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"Extreme" is a relative word. An extreme environment can be characterized by conditions that are far outside the boundaries in which we humans dwell comfortably in these categories: pH (measure of acidity), pressure, temperature, salinity, radiation, desiccation (measure of dryness), and oxygen level. An organism that thrives in an extreme environment is called an extremophile. There are extremophiles from all three taxonomic domains: the Archea, the Eubacteria, and the Eukaryota (which include the plants and animals), so when one thinks of extremophiles, one shouldn’t think only of microorganisms. But, in the words of NAI scientist Lynn Rothschild, extreme is “in the eye of the beholder.” We humans and many other creatures live in an environment that many other organisms would find toxic and potentially fatal, so to them, our oxygen rich and relatively low temperature and pressure environment seems extreme! There are organisms out there called hyper-thermophiles, who live in high temperatures that would melt our cellular membranes - >80 degrees Celsius! Psychrophiles, on the other hand, love the cold, and cannot grow above 15 degrees Celsius. Acidophiles can thrive in a pH as low as 1, while alkaliphiles can survive a pH as high as 11 (the pH of pure water is 7). While we humans would suffocate and die without oxygen, an obligate anaerobe would find the slightest hint of oxygen deadly. The most “extreme” extremophile could possibly be the organisms who live happily in near-boiling sulfuric acid. It’s a microorganism from the domain Archea that metabolizes complex carbon sources for energy. There are also the acidophilic hyperthermophiles which live in mud pots in Yellowstone National Park at a pH between 2-3 and temperatures exceeding 65 degrees Celsius. There are a whole raft of species called chemotrophs who metabolize sulfur. There are lots of other examples of extreme organisms living at the bottom of the ocean and in ice and without water and in salt crystals and so on. They all seem extreme from our relatively restricted comfort zone, but they are all quite at home in their extreme environments.

Select 56K or T1 NAI Astrobiologist Jack Farmer talks about an unusual extreme environment... Underwater in mono lake in eastern California Jack Farmer samples from a hotspring in Yellowstone National Park A hypersaline pond Exploring the extreme environment of the Antarctic at the Ross Ice Shelf