Natural terrestrial biotopes of HT are mainly hot springs and sulfur-containing solfataric fields (named after Solfatara, Italy), with a wide range of pH values (pH 0-9) and usually low salinity (0.1-0.5%). Solfataric fields consist of soils, mud holes, and surface waters, heated by volcanic exhalations from magma chambers several kilometres below (Figure 7.1). Very often, solfataric fields are found close to active volcanoes and activity increases greatly during eruption phases. The chemical composition of solfataric fields is very variable and site-dependent. Many are rich in iron minerals, like ferric hydroxides and pyrite. Less usual compounds may be enriched at some sites, with magnetite and arsenic minerals like
realgar and auripigment in Caldera Uzon, Kamchatka. Hydrogen may be formed either pyrolytically from water or chemically from FeS and H2S (Drobner et al., 1990). Steam within the solfataric exhalations is responsible mainly for the heat transfer. In addition, sites may contain carbon dioxide, variable amounts of H2, methane, nitrogen, carbon monoxide, and traces of nitrate and ammonia. Active volcanoes may harbour hot-water lakes, which are heated by fumaroles. Usually, those abound in sulfur and are very acidic. Other suitable biotopes for HT are deep, subterranean, geothermally heated oil stratifications, approximately 3500 m below the bottom of the North Sea and the surface of the permafrost soil at the North Slope, Northern Alaska (Stetter et al., 1993). The fluids produced in these regions contained up to 107 viable cells per litre of different species of HT. Artificial biotopes include smouldering coal refuse piles and hot outflows from geothermal and nuclear power plants.
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