Most ecosystems are complicated. A member of any given species has to find other species to eat and avoid those species that want to eat it. If it’s a parasite, it needs a host; if it’s a plant, it may need bacteria to help it process nitrogen or bees to pollinate its flowers.
Not so at the bottom of an almost two-mile-deep South African gold mine. There, Candidatus Desulforudis audaxviator is all there is. This species of bacteria, one of the deepest ever found, lives at about 140 degrees Fahrenheit, fixes its own nitrogen, and eats sulfate—all in complete isolation.
7. The Galapagos Islands
Sure, they’re famous for inspiring Darwin’s theory of evolution by natural selection. But the reason it’s easy (well, in retrospect) to observe evolution on these islands is that they’re almost entirely inhospitable to life. They emerged in the middle of the Pacific Ocean as the tops of still-active volcanoes. They were heat-sterilized and 600 miles from land.
Everything that lives there now flew in on the wind (most plants there have airborne seeds), rode a freak current (including Galapagos penguins, the only species of its kind to live at the equator), or floated on a raft of vegetation (like the giant tortoises). (That is, aside from the species humans have introduced more recently.) Colonization happened rarely and most species stayed where they landed, so relatively simple ecosystems grew up, with enough differences among islands to make them a showcase of evolutionary principles.
6. Acidic Mine Drainage (and Runners-Up)
California’s Iron Mountain was mined starting in the 1800s for gold, silver, copper and other minerals. The minerals originated in the roots of a volcano and were deposited with a lot of sulfide—a compound that turns to sulfuric acid in the presence of water. Mining exposed the sulfides and eventually made the tailings as acidic as battery acid and full of heavy metals such as arsenic.
But plenty of microbes live in the mine. They float on a lake of acid in a pink slick called a biofilm that is made by certain bacteria in the microbial community. Some of the archaea in the mine eat iron and make the already acidic conditions even more acidic by actively converting sulfide into sulfuric acid. The acid eats away pyrite (fool’s gold) and other minerals in the cave, adding more metals into the toxic soup.
This habitat barely edged out other harsh conditions for microbes: extreme heat or cold, intense pressure, and even radiation from a nuclear reactor. Three Mile Island was no Chernobyl, but a 1979 accident there caused the partial meltdown of a reactor and released radioactive gas into the atmosphere. It took many years to clean up the mess, mostly with robots and remotely operated cranes overseen through video cameras. Much to the clean-up crew’s surprise, the coolant water near the core was cloudy: microorganisms were thriving in it despite high levels of radioactivity.
As for pressure, the greatest that any bacteria have ever withstood is 16,000 times greater than the atmospheric pressure we experience at sea level. In experiments at the Carnegie Institution in Washington, D.C., Robert Hazen and his colleagues “subjected a strain of the familiar intestinal bacterium Escherichia coli to the ridiculous pressure of 16,000 atmospheres — a value obtained accidentally by overzealous tightening of a diamond anvil pressure cell.” Oops! But when they examined the bacteria later, a few had survived this pressure—which is greater than any pressure at any potentially life-sustaining depth (that is, any depth that is not hotter than the theoretical heat limit for life of 302 degrees Fahrenheit) on the planet.