Archaea

The archaea (formerly known as archaebacteria, from the Greek archaios, ancient, and bakterion, a small rod), are one of the three domains of life, apart from the eubacteria and eukaryotes. They were discovered in the late 1970s by Dr. Carl Woese at the University of Illinois by genetic comparison.

Archaea are prokaryotes who differ in many important respects from the eubacteria. 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.

Archaeobacteria differ from both eubacteria and eucaryotes. These differences include :

• 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...)

The archaeabacteria 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 µm (filaments up to 200 µm).

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.

They are divided into three major groups

• methanogenic archaeobacteria
• extreme halophiles
• extremely thermophilic

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.

See also: extremophile-- phylogeny -- ARNr