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Chapter 26 : Life at the Extremes of Temperature
Category: Microbial Genetics and Molecular Biology
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Bacteria are classified into three main categories as a function of the temperature of their usual environment: the thermophiles, living at temperatures exceeding 50⁰C; the mesophiles, that thrive at temperatures around room temperature; and the psychrophiles (from the Greek psychros, meaning cold), able to live at temperatures around 0⁰C or below. In this chapter the term psychrophiles is used to designate microorganisms living in permanently cold environments. The main challenge of thermophiles is to secure an appropriate stability of their cellular components; this mainly concerns the cell membrane and its selectivity, the nucleic acids, and the proteins. In the case of psychrophiles, although the stability problem cannot be completely excluded because of the possibility of cold denaturation, their main problem is to achieve appropriate reaction rates despite the low temperature of the environment. The selective membrane permeability to protons is essential for the production of cellular ATP from the proton motive force and membrane ATP synthases. Low temperatures probably induce the reinforcement of secondary, tertiary, and quaternary structures of nucleic acids through a strengthening of hydrogen bonds and this increase in stability can alter some crucial function such as transcription and translation. Up to now, no specific characteristics have been described apart, maybe, from some specific posttranscriptional modifications of RNAs such as incorporation in tRNAs of higher proportions of dihydrouridine thought to improve the conformational plasticity of these nucleic acids at low temperatures.
Small-subunit rRNA-based phylogenetic tree of the hyperthermophiles (in bold) and their relation with the other members of the three domains of life. (Reproduced, with permission, from Stetter, 2007 .)
Schematic structure of lipids from bacteria (A) and tetraether lipids from archaea (B) showing the methyl branched phytanyl chains forming a C40 monolayer.
Thermodynamic stability curves of a-amylases from psychrophilic, mesophilic, and thermophilic counterparts. (Reproduced, with permission, from D’Amico et al., 2003 .)
Specific activities of α-amylases from the psychrophile Pseudoalteromonas haloplanktis (in bold) and of the thermophilic counterpart from Bacillus amyloliquefaciens. (Reproduced, with permission, from Feller et al., 1992 .)
Characteristics of some thermophilic and hyperthermophilic microorganisms a
Use of H+ or Na+ cycle in hyperthermophilic bacteria and archaea