FROM EXTREMOPHILESTO ASTROBIOLOGY

1. Introduction to Extremophiles The 3.5 - 3 .8 billion years during which Prokaryotes have lived on Earth have been sufficient time to evolve the characteristics necessary to colonize every habitat compatible with the stability of biomolecules. It has now been well established that diverse communities of microorganisms are able to thrive in very extreme niches on and below the global surface, habitats hostile to most other forms of life. Nearly all the extremophiles are Prokaryotes, small-sized microorganisms that have a very high rate of reproduction and a high degree of physiological flexibility. These characteristics enable their fast adaptation to various extreme habitats. Extremophiles grow at the various frontiers of life: at extreme levels of temperature (thermophiles and psychrophiles), pH (acidophiles and alkaliphiles), pressure (barophiles) , dryness (xerophiles), hypersaline environments (halophiles) and high levels of radiation ; many can live in the absence of molecular oxygen. These extremophiles provide us with excellent models for the study of biodiversity on Earth, for the formulation of theories on the origin of life, and for speculations on the possible existence of extraterrestrial microbes. More information on the extremophiles may be found in Horikoshi and Grant (1998), Madigan (in Seckbach 2000), Ollivier et aI. (in Seckbach 2000), Oren (in Seckbach 2000), Rainey and Ward­ Rainey (in Seckbach 2000), Seckbach (1997, 2000) and Seckbach and Oren (in Seckbach 2000). Below are a few examples of microorganisms living on the edge of life.