Life at High Temperatures

by Thomas D. Brock




The Upper Temperature for Life

© 1994 Yellowstone Association for Natural Science, History & Education, Inc. Yellowstone National Park, Wyoming 82190.
 

When microbiological researchers first began to study the Yellowstone hot springs in the 1960s, one of the biggest surprises was the discovery that procaryotes were thriving even in boiling water. These procaryotes were not obvious, such as those of the microbial mats, but rather lived attached to the rock-like walls of the springs or to pebbles, and sometimes their long intertwined filaments accumulated on the bottom of the channels as in the photo below.

Even if the source pool looks white and sterile, microscopic study usually reveals large numbers of procaryotes. Such procaryotes are found not only in Yellowstone, but in hot springs all over the world, even where springs are at lower altitudes and water therefore boils at higher temperatures. It is amazing that in addition to living in boiling water, these procaryotes are growing surprisingly rapidly; a population can double in as few as two hours.

The presence of procaryotes in boiling water (100 degrees C or 212 degrees F) makes us wonder if there is an upper temperature for life. Temperatures even hotter than 100 degrees C occur in the thermal vents found at the bottoms of the oceans. Because of the high pressure in the ocean depths, temperatures of over 300 degrees C (572 degrees F) are found. Careful study has shown that at such high temperatures, no living organisms are present, but evidence exists of procaryotes living at temperatures as high as 115 degrees C (239 degrees F). In fact, cultures have been obtained that can be easily grown at this temperature in the laboratory.

It is an interesting fact that 115 degrees C (239 degrees F) is near the temperature at which hospital sterilizers are operated, yet here are procaryotes that actually prefer such temperatures!

Procaryotes can grow over the complete range of temperatures in which life is possible, but no one organism can grow over this whole range. The bacteria that cause disease, for which the hospital sterilizer is intended, are completely unable to grow at the high temperatures of hot springs. Likewise, the bacteria living in hot springs are unable to grow at the temperature of the human body.

Thermophiles and Evolution

How is it possible for living organisms to survive at such extremes of temperature? Actually, we are only surprised that life thrives in boiling water because of our anthropocentric orientation. It is true that humans and other animals are very heat-sensitive, but the biological world is much more diverse than we realize from our experience. Life, and especially procaryotic life, is able to adapt to environmental conditions that are deadly to humans.

In fact, many scientists believe that life as we know it might first have arisen three billion or so years ago in high-temperature environments, and that the first organisms on earth might therefore have been thermophiles. Such thermophiles would then have continued to exist on earth in the intervening period, finding refuges in the hot springs that continue to dot the earth. In addition, these thermophiles would have been the forerunners of all other life forms including, eventually, humans.
 
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