Members of the Archaea that have been characterized typically thrive in extreme environments that are otherwise devoid of life. These include conditions of high heat, acidity, alkalinity, and salinity. An exception to this attribute is the methanogens, which inhabit anaerobic niches shared with members of the Bacteria. Because of their intimate association with bacteria, the methanogens were discussed earlier in the chapter.
The Archaea fall into two broad phylogenetic groups—the Euryarchaeota and the Crenarchaeota. The Euryarchaeota include all of the known methanogens and extreme halophiles as well as some of the extreme thermophiles, or hyperther-mophiles. The Crenarchaeota include other hyperthermophiles and all of the known thermophilic extreme acidophiles. In addition to the characterized genera, many other archaea have been detected in a variety of environments by using DNA probes that bind to rRNA genes. ■ halophiles, p. 90 ■ hyperthermophiles, p. 87
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