The study of extremophilic microorganisms, in short extremophiles, has increased drastically over the last few years. An illustration for this increased interest is the establishment of the new International Society for Extremophiles and the recently introduced journal Extremophiles. Microorganisms are named extremophiles, when they are well adapted to and grow optimally at environmental and physicochemical parameters unsuitable for the typical and widely studied, mesophilic microorganisms such as Escherichia coli, Bacillus subtilis and Neurospura crassa, to name a few.

Despite the acceleration of descriptions of novel species, most of the described extremophiles are characterized only by one distinctive 'extreme'. In this chapter, we discuss a subgroup of 'multi-extremophiles' coined the alkalithermophiles (also referred to as thermoalkaliphiles). They are of interest to the scope of his book for two reasons: 1) ancestral alkalithermophiles could have been one of the earliest forms of life as some geochemical models and geological evidence suggest that the ocean of Early Earth has been alkaline in nature and capable of supporting primitive alkaliphilic microorganisms, and 2) alkalithermophiles can be regarded as one type of model organism for the study of possible extraterrestrial life. We believe alkalithermophiIic microorganisms are one of the possible types of organisms that could have evolved on Mars, if life ever arose there (see below). Based on reasoning as discussed elsewhere (Wiegel and Adams, 1998), the authors believe that life probably originated not in hyperthermobiotic environments but on mineral surfaces in moderate thermobiotic (e.g., 60-85°C range), relatively shallow pools at the edges of the early Earth's oceans. The drastic changes of physico-chemical parameters over space and time in such environment would have provided the necessary dynamic conditions for frequent association and dissociation of prebiotic and biotic structures and thus changing selection pressures to lead to superior surviving combinations (Shock et al., 1998; Baross, 1998; Miller and Lazcano, 1998). These assumed selection conditions proposedly lead to a 'bush-like origin' of life as suggested by Kandier (1998) and thus is different from the frequently assumed quasi monophylogenetic progenote. Thus, some form of alkalithermophiles can be proposed as logical descendants of hypothetical early life forms.

Alkalithermophiles are alkaliphilic thermophiles (thermophilic alkaliphiles). A simplified definition for thermophiles and alkaliphiles is given in Table 1. Different definitions have been proposed for both extremes (Wiegel 1986, 1998a 2002 and literature cited…