ABSTRACT

Generally, pure cultures of microorganisms grow over a temperature span of25-30 ·c. However, among the thermophiles are several bacteria and archaea which exhibit an extended temperature span for growth of more than 35 ·c (Wiegel 1990, 1992). Methanobacterium thermoautotrophicum exhibits so far the largest span, with growth between 22 ·c and 78 ·c and methane formation between at least 15 ·c and 78 ·c (Wiegel, 1990). Interestingly, all the microorganisms exhibiting such an extended temperature span for growth - as far as examined - exhibit a biphasic relationship between growth temperature and growth rate. The biphasic curve is best illustrated using the Arrhenius plot. Although developed for chemical reactions and generally applied to enzyme kinetics, the Arrhenius plot can also be used to demonstrate elegantly the overall relation between growth temperatures and growth parameters such as doubling times. Usually, if only one reaction (or in a complex system, effectively a combination of overlapping reactions) is rate-limiting, a straight line is observed over the temperature span investigated. For complex systems like whole cells, however, no physical parameters (such as !:Ji for a single chemical reaction) can be associated with the slopes of the gr.aphs. ·However, on obtaining graphs with strong intermediate plateaus and in some cases a change in the slope of the graphs below and above the plateau, one can assume that this indicates a major change in the rate limitation for the growth of an organism. Examples of thermophilic microorganisms with an increased temperature span include a variety of different types and include bacteria such as the aerobic Bacillus stearothermophilus (heterotroph) and B. schlegelii

( chemolithoautotrophic ), and the anaerobic Thermoanaerobacter ethano/icus (glycolytic) and Thermoanaerobacter kivui ( chemolithoautotrophic ), but also archaea such as the anaerobic, thermophilic methanogen Methanobacterium thermoautotrophicum ( chemolithoautotrophic) and the extremely thermophilic Desulforococcus mobilis (sulfur-dependent metabolism) and methanogen Methanonococcus jannaschii ( chemolithoautotrophic ). These few examples represent a wide variety of physiologically and phylogenetically different microorganisms. Some other examples of archaea and bacteria for which a growth span exceeding 35 ·c has been reported are (with Tmin-TDWI and the temperature for the intermediate plateau given in parentheses): Thermodesu/fobacterium commune (45-85 ·c, plateau at 38-43 "C); Calorimator (Clostridium) fervidus (about 40-80 "C); Thermoanaerobacter brockii, T. thermohydrosulfuricus and T. 'sulfurigignens' (35-77/8 ·c, plateau at 55-60 •q; Thermoanaerobacter(Acetogenium) kivui (35-75 "C,plateau 57-60 "C); Methanococcus thermolithotrophicus (30 ·c to above 70 "C); Clostridium (sensu stricto) thermobutyricum (26-61.5 ·c, plateau around 35-38 ·c with a drastic change of the slope above and below a relatively narrow plateau); Thermoanaerobacterium desu/furigenes (35-75 ·c, plateau at 53-85 ·q (but not the closely related Thermoanaerobacterium aotearoense (34-66 "C), T. thermosaccharolyticum, or T. saccharo/yticum (35-65 ·q). Owing to the research interest of the author, most of the examples analysed in more detail belong to the group of anaerobic eubacterial thermophiles. It is speculated that more examples of bacteria and archaea with the wide temperature span and the biphasic curve will be found in groups other than those mentioned above when their species are more closely investigated with respect to their temperature dependence of growth rates.