Archaea





The Archaea are one of the three major groups of            Archaea
living organisms, together with the bacteria and   Scientific classification
eukaryotes. They are prokaryotes like the
bacteria, and were originally included among them. Domain*: Archaea
Their separate identity was discovered in the late
1970s by Dr. Carl Woese at the University of           Phyla* & classes
Illinois by genetic comparison. Originally they    Crenarchaeota
were termed the Archaebacteria, and the other      Euryarchaeota
prokaryotes the Eubacteria, but now there is a     ΚΚΚ Halobacteria
growing tendency to restrict the term bacteria to  ΚΚΚ Methanobacteria
the latter and the names have adjusted             ΚΚΚ Methanococci
accordingly. The Archaea may be treated as a       ΚΚΚ Methanopyri
single kingdom or as a domain, in which case the   ΚΚΚ Archeoglobi
subgroups may be ranked as kingdoms.               ΚΚΚ Thermoplasmata
                                                   ΚΚΚ Thermococci
Archaea differ from the true bacteria in many      Korarchaeota
important respects, as well as from the            Nanoarchaeota
eukaryotes. These differences include:             * Or kingdom

   * wall structures and chemistry (lack of peptidoglycan and gram staining)
   * lipidic membrane structure (their lipid bilayers consist of branched
     chain hydrocarbons linked by ether linkages to glycerol
   * metabolism (methanogens, sulfate reducers...)

Many Archaea live in extreme environments, including water whose temperature
exceeds that of boiling water, like geysers, very salty, acid or alkaline
water or black smokers. They are very diverse, both in morphology and
physiology. Some are single-celled, while others form filaments or
aggregates. They may be spherical, rod-shaped, spiral, lobed. Their size
varies in diameter from 0.1 to over than 15 ΅m (filaments up to 200 ΅m).

They show a great diversity in multiplication modes, which may be by binary
fission, budding or fragmentation. For a nutrional point of view, they range
from being chemolithoautotrophic to organotrophic. Physiologically, they can
be aerobic, facultatively anaerobic, or stricly anaerobic. Some are
mesophiles, others hyperthermophiles (may live over 100‘C). Though most of
them live in high-temperature, anaerobic, hypersaline environment, some have
also been found in cold places. They are mostly found in aquatic and
terrestrial habitats, but a few have been found in animal digestive systems.
The environmental conditions archaea prefer and their unusual biochemistry
make them usually harmless to organisms belonging to the other two domains.
No case of infection of a human with archaea has been reported so far.

There are two main groups of Archaea, the Crenarchaeota and Euryarchaeota.
The Korarchaeota have been described from DNA samples, but the actual
organisms remain unknown, and the Nanoarchaeota are known from a single
species discovered in 2002, Nanoarchaeum equitum. Some work suggests that
the Euryarchaeota may be closer to the eukaryotes than the Crenarchaeota, in
which case the domain Archaea would be abandoned. Microbiologists who
consider the Bacteria to be paraphyletic also argue that the Archaea are not
sufficiently different to be considered a separate group.