Limits for Life

The two Solar System bodies beyond Earth that have elicited the most interest as potential habitats for life are Mars and Europa because both have clearly been impacted by aqueous processes. Considering the expanse of time subsequent to the Solar System s origin, conditions conducive to the occur- 

The cold, dry surface of Mars today is not an ideal habitat for life. Nevertheless, evidence for a warmer, wetter early Mars has stimulated considerable speculation about the prospects for life on Mars (McKay and Stoker, 1989 Klein et al., 1992 McKay et al., 1992 McKay et al., 1996 Gibson et al., 1997 Shock, 1997 Jakosky and Shock, 1998 Fisk and Giovannoni, 1999 Max and Clifford, 2000 Cabrol et al., 2001 Kargel, 2004). 

By generally clement Earth standards, the habitats for life on Mars and Europa are likely to be extreme environments. On the other hand, extrater- 

There are three environments on Earth where microbes have been identified with temperature tolerances in a range of 100°C to 121 °C, namely, submarine hydrothermal vents, the subterranean deep biosphere, and terrestrial hot springs (Table 4.1). The highest temperature tolerances (110-121 °C) are found in microbes from marine hydrothermal vents and the subterranean deep biosphere high pressures prevent these waters from boiling at 100 °C, the normal boiling point of water at 1.01 bar (1 atm) pressure. From terrestrial hot springs, microbes have been isolated that can tolerate temperatures up to 103°C (Table 4.1). 

Hyperthermophiles are invariably either bacteria or archaea. Eukaryotes have an upper temperature range of 50-60°C (Madigan and Marrs, 1997 Nealson, 1997 Nealson and Conrad, 1999 Rothschild and Mancinelli, 2001). Until recently, Pyrolohus fumarii (an archaea) had the highest known temperature tolerance of 113°C (Blochl et al., 1997) this organism has a minimum temperature for growth of 90 °C and an optimum temperature of 106 °C and is a strict hyperthermophile (Stetter, 1999). Recently, an archaea was isolated 

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Kashefi, K. and D. R. Lovley, 2003, Extending the upper temperature limit for life. Science 301, 934. 

D. Hafenbradl, H. W. Jannasch, and K. O. Stetter, I yrolohus fumarii, gen. and sp. nov., represents a novel group of archaea, extending the upper temperature limit for life to 113°C, Extremophiles 1997,... 

The properties of water have such pervasive influences on living systems that some aspect of the physics and chemistry of water is almost certain to trickle into each chapter of this volume. As a prefatory note to the following chapter on temperature effects, it is useful to review the thermal properties of water, and see how these relate to the thermal relationships of organisms. This brief review of the effects of temperature on water will serve to introduce the key roles played by water in establishing the thermal sensitivities of organisms and in setting the temperature limits for life. 

The role of water in governing the upper thermal limits for life also is based on covalent transformations in which water is a reactant. As emphasized earlier in this chapter, the removal of a molecule of water from reactants is common in diverse biosynthetic reactions, including the polymerization of amino acids into proteins and nucleotide triphosphates into nucleic acids. The breakdown of biomolecules often involves hydrolysis, and increased temperatures generally enhance these hydrolytic reactions. The thermal stabilities of many biomolecules, for instance, certain amino acids and ATP, become limiting at high temperatures. Calculations suggest that ATP hydrolysis becomes a critical limiting factor for life at temperatures between 110°C and 140°C (Leibrock et al., 1995 Jaenicke, 2000). Thus, at temperatures near 110°C, both the covalent and the noncovalent chemistries of water that are so critical for life are altered to the extent that life based on an abundance of liquid water ceases to be possible. 

Examples of barophUes include Moritella and Shewanella species that laboratory experiments suggest can grow at 70 MPa, but not at pressures of 50 MPa [28], suggesting they are true barophiles. Similar bacteria have been obtained from the Mariana Trench. More remarkable evidence has been presented for growth using a diamond anvil apparatus. Shewanella oneidensis and Escherichia coli were reported to remain physiologically active at pressures up to 1,680 MPa for up to 30 h [29]. As for low temperatures, the pressure limits for life remain unknown. 

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