Temperature classifications of microorganisms:
  • Psychrophiles - cold-loving bacteria (15oF/-10oC to about 75oF/25oC)
  • Mesophiles - moderate temperature bacteria (70oF/20oC to about 115oF/45oC)
  • Thermophiles - heat-loving bacteria (minimum 70oF/20oC to 160oF/50oC or more, optimum temperature around 125oF/50oC)
  • Hyperthermophiles - very high-temperature loving bacteria (optimum temperature at or above 170oF/75oC - for example, Pyrodictium, found in geothermal vents on the sea floor has a temperature minimum of 180oF/82oC, optimum of 221oF/105oC and growth maximum of 230oF/110oC)

Geothermal vents and hot springs are the most common source of many of the thermophilic and hyperthermophilic bacteria. The largest concentration of these environments can be found in Yellowstone National Park. The other main environment for these organisms is in decaying organic material. The process of composting is a complex interplay of temperature gradations and in highly dependant on succeeding populations of both thermophilic bacteria and fungi. The following is a simplified timeline of the typical composting process:

  • Initial rapid growth - The most readily available sugars and amino acids are metabolized by mesophilic organisms, which generate heat in the process to the point of actually suppressing their own growth.
  • Thermophilic colonization - Thermophilic fungi such as Rhizomucor pusillus and several types of thermophilic bacteria such as Bacillus stearothermophilus raise the temperature as high as 175oF/80oC within just a few days. This inactivates all mesophilic organisms, and the majority of the thermophilic fungi.
  • Prolonged high-temperature phase - The temperature remains high (usually around 105oF/40oC to 140oF/60oC) for a prolonged period of around three weeks, allowing several other thermophilic bacterial species to further metabolize the available organic compounds. A second group of thermophilic fungi colonize, including Chaetomium thermophile, Humicola species, Thermoascus aurantiacus, Aspergillus fumigatus, and several others. These fungi break down much of the cellulose and similar coumpounds in the compost, often nearly dropping the dry weight of the compost by half. I have personally nearly burned my hand in a pile of composting grass clipping after two days!
  • Mesophilic recolonization - Eventually the mesophiles recolonize, displacing most of the thermophiles. Aspergillus fumigatus can continue to grow. This organism, interestingly, can also thrive on the hydrocarbons in jet fuel, and can infect humans via inhalation. Aspergillosis is the growth of this fungus in the lung tissues, and can be dangerous or deadly.

  • helios.bto.ed.ac.uk/bto/microbes/thermo.htm