3 P52220RA Turn over 5. The marine archaeon Geogemma barossii, for example, can survive at a blistering 121 °C, while other microbes have retained metabolic activity at temperatures as low as −20 °C in Siberian permafrost. The bacterium Deinococcus radiodurans remains viable after exposure to 1,000 times the fatal human radiation dose, and the aquatic archaeon Ferroplasma acidarmanus can withstand extremely acidic water, with pH values as low as 0. Such flashy feats have earned these and other microbes the title “extremophiles”—lovers of extreme conditions. But as remarkable as their metabolic capabilities are, calling them “extreme” is a bit human-centric. Because of our own requirement for oxygen and narrow acceptable ranges of temperature, salinity, pressure, pH, and radiation, the survival of other organisms in a wide range of environments seems extreme to us. But for a microbe that has come to depend on the abundant hydrogen ions of acidic hot springs, an air-conditioned suite at the Ritz is a threatening proposition. The wide variety of biochemical modes of existence reflect billions of years of evolution, adaptation, and niche differentiation rather than a standardized characterization of biological fortitude. 6. For the title of “extremophile” to be broadly meaningful, it must refer to a more objective measure of extremeness—an advantageous capability enacted in response to a common challenge. One such challenge, something that all living organisms must face, is the acquisition of chemical energy to drive cellular reactions. Perhaps the ways in which organisms handle this task could separate the truly industrious from the merely viable. The energy of life 7. Energy is the currency of biology. By harvesting electrons from a stunning range of starting materials, Earth’s organisms produce adenosine triphosphate (ATP), which powers biological reactions. In the case of mammals and most eukaryotes, sugars and other organic molecules are common electron sources, the oxidation of which drives ATP production. Bacteria and archaea can use a range of other chemicals, from sulfide to iron to ammonium. 8. Cells take up these electron-rich molecules and capture their electrons, which jump down an electron transport chain in the mitochondrial or cell membrane. As electrons move along the membrane toward a final electron acceptor, protons are pumped from the cell’s interior to the exterior, setting up a chemical gradient. Finally, protons stream back into the cell, releasing the chemical pressure and generating ATP. With each energy-requiring reaction, from flagella construction to cell division and growth, cells draw upon their ATP bank. Courtesy of Jeffrey Marlow Oceanic extremes This experimental setup on Hydrate Ridge off the coast of Oregon samples microbial metabolism in deep-sea methyl seeps, which host a variety of seemingly strange creatures, including some truly extreme archaea and bacteria that cannot survive without each other.