Life at High Temperatures
by Thomas D. Brock
Temperature is one of the most important environmental factors and organisms differ strikingly in their ability to adapt to high temperatures. Biologists recognize three major categories of living organisms, called Eucarya, Archaea, and Bacteria. Higher organisms (plants and animals) are all Eucarya, so called because their cells have true nuclei and undergo cell division by mitosis. We call these organisms with true nuclei eucaryotic. Archaea and Bacteria are both much simpler organisms, seldom occurring as multicellular forms, and lacking true nuclei and mitosis. They are called procaryotic.
The table below shows the upper temperature limits for growth of various types of living organisms. Note that the eucaryotes are unable to adapt to high temperatures, their upper limit being about 60-62 degrees C (140-144 degrees F). The upper temperature limit for plants and animals is even lower, less than 50 degrees C (120 degrees F). It is important to note that only a very few eucaryotes are able to adapt to these upper limits, the majority being restricted to much lower temperatures.
At temperatures above 60-62 degrees C (140-144 degrees F) the only organisms present are procaryotes. The photosynthetic bacteria have upper temperature limits lower than those of nonphotosynthetic bacteria. The upper limit of photosynthetic bacteria, defined by the "V" shown in the photo below and on the previous page, is about 70-73 degrees C (158-163 degrees F). At higher temperatures, only nonphotosynthetic bacteria are able to grow. At the highest temperatures, over 100 degrees C (212 degrees F), the only bacteria found are a few unusually heat-adapted Archaea called hyperthermophiles. Water boils in Yellowstone at about 92 degrees C (198 degrees F). These bacteria are not just surviving, they are thriving in the boiling water!
Upper Temperature Limits for Growth of Various Organisms
|Group||Upper temperature limits (degrees C)||Upper temperature limits (degrees F)|
|Cyanobacteria (oxygen-producing photosynthetic bacteria)||70-73||158-163|
|Other photosynthetic bacteria (do not produce oxygen)||70-73||158-163|
|Heterotrophic bacteria (use organic nutrients)||90||194|
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