Thermophilic Fungi Eukaryotic Life at High Temperature

It is of interest to study the systematics, distribution, and physiological adaptations of organisms which have been successful in colonizing high temperature environments in order to examine the limits to which evolution can be pushed. From an ecological point of view, high temperature environments usually have relatively simple species composition and short food chains, which make a study of productivity, trophodynam-ics, population fluctuation, and species interaction more simple. From the viewpoint of applied ecology, an understanding of the biology of high temperature habitats is essential if we are to predict and control the consequences of thermal pollution by various industrial sources.

Thomas D. Brock

The vast majority of fungi best grow between 20 and 37°C. This temperature range is universally accepted as moderate. The vegetative mycelium of these fungi, called the mesophilic fungi, cannot survive prolonged exposure above 40°C. However, currently some 30 species of fungi are known which show optimum growth between 40 and 50°C, with a few species capable of growth up to 62°C. These species are grouped as thermophilic fungi with their minimum temperature of growth around 20 to 25°C. The temperature range for growth of either the thermophilic or the mesophilic fungi is about 32°C. This means that while the maximum temperature of growth of thermophilic fungi is extended, their minimum temperature of growth too has been raised.

While some species of bacteria and archaebacteria found in hot springs, solfataras or hydrothermal vents grow at temperatures between 80 to 113°C (Brock 1995; Blochl et al., 1997), the thermophilic fungi are the only eukaryotes with the exception of the alga Cyanidium caldarium that thrive between 45 and 60°C—temperatures at which no plant or animal can survive for long (Figure 10.1). Thermophilic fungi are therefore valuable systems for investigations of genetic and biochemical mechanisms that allow higher organisms to adapt and deal with heat stress. They are arbitrarily distinguished from the thermotolerant fungi that have a minimum temperature of growth between 12 and 17°C and a maximum temperature of growth between 50 and 62°C. In general morphology and ultrastructural features, the thermophilic fungi are indistinguishable from the mesophilic or the thermotolerant fungi (e.g., Aspergillus fumigatus, which grows in a temperature range from 12 to about 55°C, http://helios.bto.ed.ac.uk/bto/microbes/thermo.htm).

Upper temperature range of Archaea <

Upper temperature range of Eubacteria<

Upper temperature range of Fungi

Very warm bath ^^ Max. temp. Bangalore, 22 May 1931 —i

Average annual temperature in tropics

Average temperature of earth

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