While learning about enzymes we learnt that different organisms have different optimum temperatures, at which they can function, they can range from 37C, (humans), to 105C.
Microorganisms can be grouped into broad (but not very precise) categories, according to their temperature ranges for growth.
- Psychrophiles (cold-loving) can grow at 0oC, and some even as low as -10oC; their upper limit is often about 25oC.
- Mesophiles grow in the moderate temperature range, from about 20oC (or lower) to 45oC.
- Thermophiles are heat-loving, with an optimum growth temperature of 50o or more, a maximum of up to 70oC or more, and a minimum of about 20oC.
- Hyperthermophiles have an optimum above 75oC and thus can grow at the highest temperatures tolerated by any organism. An extreme example is the genusPyrodictium, found on geothermally heated areas of the seabed. It has a temperature minimum of 82o, optimum of 105o and growth maximum of 110oC.
It must be stressed that the temperature ranges for the groupings above are only approximate. For example, we would use different criteria to classify prokaryotes and eukaryotes. The upper temperature limit for growth of any thermophilic eukaryotic organism is about 62-65oC. And the upper limit for any photosynthetic eukaryote is about 57o – for the red alga Cyanidium caldarium, which grows around hot springs and has a temperature optimum of 45oC. In contrast to this, some unicellular cyanobacteria can grow at up to 75oC, and some non-photosynthetic prokaryotes can grow at 100oC or more.
Below, we consider two major types of thermophile – the microbes that grow in geothermal sites, and those that grow in “self-heating” materials such as composts. However, some very recent reports suggest that these different types of environment can share some common organisms.
Many of the prokaryotes that grow in the most extreme environments are archaea – a group that is clearly distinguishable from both the present-day bacteria and the eukaryotes. There is little doubt that many of them still remain to be discovered and described, but this is a difficult field of research because of the problem of reproducing their natural growth conditions in a laboratory environment. Members of the genus Sulfolobus (archaea) are among the best-studied hyperthermophiles. They are commonly found in geothermal environments, with a maximum growth temperature of about 85-90o, optimum of about 80o and minimum of about 60oC. They also have a low pH optimum (pH 2-3) so they are termed thermoacidophiles. Sulfolobus species gain their energy by oxidising the sulphur granules around hot springs, generating sulphuric acid and thereby lowering the pH.
The study of extreme environments has considerable biotechnological potential. For example, the two thermophilic species Thermus aquaticus and Thermococcus litoralis are used as sources of the enzyme DNA polymerase, for the polymerase chain reaction (PCR) in DNA fingerprinting, etc. The enzymes from these organisms are stable at relatively high temperatures, which is necessary for the PCR process which involves cycles of heating to break the hydrogen bonds in DNA and leave single strands that can be copied repeatedly. Another thermophile, Bacillus stearothermophilus (temperature maximum 75oC) has been grown commercially to obtain the enzymes used in ‘biological’ washing powders.