Extreme Environments
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EXTREME ENVIRONMENTS
Extreme Environments Geomycology Hydrothermal Environments, Terrestrial Mat-Related Sedimentary Structures Microbial Biomineralization Microbial Mats Stromatolites
EXTREME ENVIRONMENTS Volker Thiel University of Göttingen, Göttingen, Germany
Definition The word “extreme” comes from the Latin word “extremus,” the superlative of “exter” (= on the outside) (Rothschild and Mancinelli, 2001). Whereas there is no general agreement on how to define an extreme environment, the term is commonly used for any setting that exhibits life conditions detrimental or fatal to higher organisms with respect to its physicochemical properties. Thus, extreme environments differ in one or more aspects from those which humans consider as “normal,” moderate conditions with circumneutral pH, temperatures between 20 C and 35 C, pressures around 0.1 MPA (1 atm), and adequate concentrations of nutrient and saline. It should be considered, however, that such definition represents an anthropocentric view and that what is extreme and what is normal from a microbial perspective remains questionable. If not completely uninhabitable to life, extreme environments typically harbor specially adapted organisms, the so-called extremophiles. Most extremophiles are unicellular organisms, that is, protists, bacteria, and archaea. As a rule, extreme environments show a low diversity of multicellular organisms and only few animals are able to withstand the harsh conditions of particular extreme environments. Terms describing extremophiles usually combine an environment-specific prefix with the suffix “-phile” (Greek word for “-loving”). Replacing the suffix “-phile” by “-tolerant” implies that an organism tolerates rather than requires the respective conditions, and actually has its optimum at more moderate conditions. Prefixes may be combined for organisms that thrive in more than one extreme (e.g., “thermoacidophiles”). Such organisms are termed polyextremophiles. Extreme environments can be categorized into several classes. Acidic environments Environments below pH 5, including, for example, sulfuric pools, geysers, and areas polluted by acid mine drainage are called acidic environments. Organisms dependent on acidic conditions below pH 5 (pH 7 = neutral) are called acidophiles. Examples are the bacterial genus Acidithiobacillus (used in biomining, see entry), Ferroplasma acidophilum, a sulfuric-acid producing
archaeon involved in acid mine drainage (see entry Acid Rock Drainage), the red alga Cyanidium caldarium (the most heat and acid tolerant alga known), the green alga Dunaliella acidophila, the fungus Trichosporon cerebriae, and the archaeum Picrophilus (Rothschild and Mancinelli, 2001). The major challenge for these organisms is to maintain their internal cellular pH at a constant, circumneutral level, that is, around pH 7. Individual strategies encompass, inter alia, (i) reinforcement of the cell membrane, (ii) secretion of extracellular polymeric substances (EPS) (see entry “Extracellular Polymeric Substances (EPS)”) to l
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