1967

Study of Extremophiles

Thomas Brock (b. 1926)

Astrobiology is the study of the origin, evolution, and distribution of life and habitable environments in the universe. It is perhaps a unique discipline in that it has only one data point with which to conclusively justify its existence. So far, we know of only one example of life in the universe—that is, life on earth—all of which is fundamentally similar, based on similar RNA, DNA, and other carbon-based organic molecules.

The search for life elsewhere is more than just the search for complex life forms like us, however. It is a search for other planetary environments that could be suitable for the most dominant form of life on our own planet—bacteria and other “simple” life forms. The best place to start a search for those conditions is right here on our own planet, where significant advances in our understanding of habitability have been made in the past 50 years.

In 1967, the American microbiologist Thomas Brock wrote a landmark paper describing heat-tolerant bacteria (hyperthermophiles) that flourished within hot springs at Yellowstone National Park. He challenged the prevailing wisdom that the chemistry of life requires moderate temperatures to operate. Brock’s work helped to spur the study of extremophiles—life forms that survive and even thrive in harsh environments.

Hyperthermophilic bacteria have since been identified in very hot water near deep sea hydrothermal vents as well; on the opposite extreme, psychrophiles have been found that live and thrive in near- or below-freezing temperatures. Life forms have also been found that exist over extremes of salinity (halophiles), acidity (acidophiles and alkaliphiles), high pressure (piezophiles), low humidity (xerophiles), and even high levels of UV or nuclear radiation (radioresistant).

The message for astrobiologists from the history of life on our planet is clear: life can thrive in an enormous range of environments. Thus, searching for evidence of past or present extremophiles or their habitable environments in extreme places such as Mars, the deep oceans of Europa and Ganymede, or the frigid, organic-rich surface of Titan is not as crazy an idea as it used to be.

Morning Glory Pool, a hot spring in Yellowstone National Park, Wyoming. The colors along the spring’s outer edges are from a variety of hyperthermophilic bacteria that can survive and thrive even at the high temperatures of the spring (above 176°F [80°C]).