Archaea were initially thought to be dominant only in habitats extreme with respect
to heat, osmotic stress, or reducing capacity. They were often characterized as
dividing into three main ecological groups: thermophiles, halophiles, and
methanogens. Most of the prokaryotes growing in very hot (>80°C and to >100°C at
depths with sufficient pressure to prevent boiling) or very salty environments are
archaea. All of the methanogens that gain energy by reducing carbon dioxide with
hydrogen, yielding methane as a product, are archaea. They do fix carbon dioxide and
can also assimilate some small-molecular-weight organic compounds such as acetic
acid. The biochemistry of this chemosynthesis is distinct from those modes involving
RuBisCO, including photosynthesis. The metabolic portfolio of Archaea as a group
also includes the oxidation of ammonium, sulfur, and metals. While Archaea were
initially considered to be mostly “extremophiles”, they are now known to exist also in
colder waters below the euphotic zone (DeLong 1992; Fuhrman et al. 1992). They
account for 10–20% of the cells in the deep ocean (Varela et al. 2008) and play a
major part in carbon and nitrogen cycles (Fuhrman & Steele 2008).
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