Introduction

Various microorganisms grow fastest (“optimally”) at temperatures above 40 °C. In an anthropocentric kind of view (the human body temperature is only 37 °C) these are designated as thermophiles (Brock 1986; Kristjansson and Stetter 1992). These “heat lovers” flourish within natural and anthropogenic biotopes like hot waters, sun-heated soils, geothermal areas, self-heated waste dumps, and industrial cooling waters. Depending on the isolates, most thermophiles exhibit an upper temperature limit of growth between 50 and 70 °C. On the other hand, these moderate thermophiles are still able to grow (with reduced propagation rates, however) at 25–40 °C within the mesophilic temperature range. Thermophiles are widespread within many different taxonomic groups of pro- and eukaryotic microorganisms like Bacilli, Clostridia, Streptomycetes, Cyanobacteria, fungi, algae, and protozoa, which harbor mainly mesophilic species. Due to their close phylogenetic relationship to mesophiles and their modest thermophily, moderate thermophiles may represent secondary adaptation to heat in the history of life.

The upper temperature border of life is represented by hyperthermophilic prokaryotes with optimal growth temperatures between 80 and 113 °C (under slight overpressure). In contrast to moderate thermophiles, as a rule, hyperthermophiles are unable to grow below 60 °C. The most extreme hyperthermophile, Pyrolobus fumarii, is unable to propagate even at 90 °C since this temperature is still too low to support its growth (Blöchl et al. 1997).