Within each microbial group, thermophiles that have a wide growth temperature range are known as eurythermal, while those with more restricted range are stenothermal. Generally, thermophiles have been considered as those organisms able to grow in the temperature range of 55°C and above. This range is selected for ecological reasons. For instance, while temperatures below 50°C are common on earth surfaces, associated with sun heated habitats, temperatures above 55°C are rare as biological/ natural habitats. Additionally, 60°C is the maximum for most eukaryotic life. However, a temperature continuum exists within and between groups, making sharp delineation impracticable (Wolf and Sharp, 1981). Suutari and Laakso (1994) adopted a liberal delineation of thermophiles, in which thermotolerant organisms were defined as those with optimum growth temperature <45 °C and Tmax >45 °C, and thermophiles as those with Topt >50°C and Tmax >60°C. They also defined as extreme thermophiles or caldoactive, those organisms with Topt >65°C and Tmax >90°C, and hyperthermophiles as the more exotic isolates able to grow at 100°C and above.
Thermophiles are mostly heterotrophic, nutritionally versatile, and capable of utilising a wide variety of organic carbon sources, including simple sugars, alcohols, organic acids, and polysaccharides. Many are able to utilise more recalcitrant and exotic compounds such as phenols, cresols, benzoates and hydrocarbons (Mutzel et al., 1996). Some may grow prototrophically, or exhibit a requirement for growth supplements such as vitamins, amino acids or complex organic mixtures (Sundaram, 1986). Oxygen is an important nutrient for the growth of aerobic thermophiles, but the low solubility of oxygen at high temperatures, makes supply difficult during large scale cultivation of thermophiles, and may impose severe limitations on the attainable cell density (Krahe et al., 1996). Thermophiles grow optimally in either acidic environment of pH 1.5-4.0 (thermoacidophiles), or in neutral to moderately alkaline pH of 5.8 and above (neutrophiles). There are also thermophilic alkalophiles, although these appear to be more restricted in abundance. The ability of thermophiles to play important roles in the process of valorisation of waste biomass (particularly of the genre of TAD) derives from some of their intrinsic biochemical features that make bioprocessing with them attractive. Some of these interesting features of thermophiles that enhance their appeal in bioprocessing are highlighted briefly below.
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