Archaea are single-celled microbes with a reputation for living in tough environments like salt lakes, deep sea vents or boiling acid. One strain can grow at temperatures as high as 121 degrees Celsius (249.8 degrees Fahrenheit), a heat that kills most organisms; others thrive at the seriously acidic pH of zero.
They are not restricted to life at the fringes, however. As we have learned how to detect them, archaea have turned up all over the place. One survey estimated that they account for as much as 20 percent of all microbial cells in the ocean, and they’ve been discovered living in soil, swamps, streams and lakes, sediments at the bottom of the ocean, and so on. They are also routinely found in the bowels of the Earth — and the bowels of animals, including humans, cows and termites, where they produce methane. Indeed, the archaeon known as Methanobrevibacter smithii may account for as much as 10 percent of all the microbial cells living in your gut.
But here’s the thing. The tree of life falls into three big lineages, or realms of life. (Confession: the technical term is “domains,” not “realms,” but I’m taking poetic license.) The most familiar realm comprises the eukaryotes — which is the blanket term for most of the organisms we are familiar with, be they mushrooms, water lilies, tsetse flies, humans or the single-celled beasties that cause malaria. Eukaryotes have many distinguishing features, including the fact that they keep their genes in a special compartment known as the cell nucleus.
The second member of the trinity is made up of bacteria. We tend to associate bacteria with disease — for they can cause a range of nasty infections, including pneumonia, syphilis, leprosy, tuberculosis and the like. But in fact, most bacteria lead blameless lives (some of which I have written about in previous columns). There are many differences between eukaryotes and bacteria; but one of the most obvious is that bacteria do not sequester their DNA in a cell nucleus.
The third great lineage of living beings is the archaea.