Extreme Environments and Their Inhabitants 931 Extremophiles and Extremotolerants

It is beyond the scope of this review to decorticate in detail extremophiles and their natural environments. Table 9.1 summarises general features of ex-tremophilic and extremotolerant organisms: their classification as a function of the physicochemical parameter considered, their most outstanding molecular adaptations, as well as some examples of species or groups of extremophiles and the habitats they colonise. Figures 9.3 to 9.9 illustrate different extreme environments. Temperature, pH, salinity (osmotic pressure), pressure, dryness (water availability) and exposure to radiation are parameters whose values usually configure the extreme nature of a particular environment (Table 9.1). Some authors also consider vacuum, gravity, and partial oxygen pressure (Rothschild and Mancinelli, 2001). Nevertheless, no organism is known to live under hypo-or hypergravity and, although many organisms are able to resist vacuum conditions, they do it in an inactive state (dormant or resistance forms), that is, they

Fig. 9.2. Schematic diagram showing the tree of life based on 16S/18S rRNA sequences. Each line represents a sequence (in turn representing the corresponding organism); each triangle represents a group of closely related sequences. Blue triangles correspond to environmental lineages for which no cultivated members are known

Fig. 9.3. Sampling at a hot mud pool in a hydrothermal field at the Azores archipelago (Photo courtesy of P. Forterre and E. Marguet)

do not depend on those conditions for a living. Oxygen induces the formation of free radicals that can damage cells. Indeed, most organisms have mechanisms to neutralise oxygen-derived deleterious effects. In this sense, aerobic organisms like us could be considered extremophiles. However, a vast diversity of organisms belonging to the three life domains is able to live optimally at all oxygen partial pressures found on Earth: from complete absence to atmospheric levels and even more, for instance in oxygen-producing photosynthetic bacterial mats. This indicates that life has adapted to all possible terrestrial niches with regard to oxygen concentration, without it being a general limiting factor. Therefore, anaerobic organisms cannot be considered extremophiles by this feature.

For some of the parameters mentioned above, we find only extremotolerant organisms. Thus, there are radiotolerant micro-organisms, but none is "radiophile". In addition to the parameters listed in Table 9.1, oligotrophic conditions, or extremely low nutrient concentrations, which very often characterise some extreme environments, constitute a major limiting factor for life.

Table 9.1. Major features of extreme environments and associated biota

Physico-

Type of

Definition/

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