When first discovered in 1977, these microscopic single-celled organisms were classified as Bacteria. However, later genetic and biochemical analyses showed that they were more closely related to the Eukaryotes (which include humans). The researchers who discovered this, Woese and his colleagues, proposed the addition of the “domain” as a taxonomic level above that of kingdom, with all life classified within one of three domains, now called Bacteria, Eukaryotes, and Archaea. As of 2010 more than 250 species of Archaea had been described, most fitting into one of two groups, Euryarchaeota and Crenarchaeota. Many of these live in extreme environments – places where we once believed no life could exist.
Description of Archaebacteria
Apparently nearly anywhere and eveywhere on Earth.
Because Archaea can thrive in extreme environments, they occur in places we didn’t think were habitats – very low (4 °C) and very high (120 °C) temperatures of polar seas, hydrothermal vents and hot sulfur springs; high and very low pH; hypersaline water, and anaerobic environments like human colons and termite guts. They also live in moderate habitats in the oceans, soils, freshwater (Kimball, 2010).
Evolution and Systematics
Systematics and Taxonomy
PHYLA / DIVISIONS
Euryarchaeota: methanogens, halophiles, thermophiles
Crenarchaeota: hyperthermophiles, cool-water marine plankton, nitrifying soil/sediment microbes
Korarchaeota (proposed): thermophiles
Thaumarchaeota (proposed): chemolithoautotrophic ammonia-oxidizers
(Hohn et al., 2002; Huber et al., 2002; Brochier-Armanet et al., 2008; Kimball, 2010; Spang et al., 2010; Auchtung et al., 2011)
Proteins of halophilic archea resist the denaturing effect of highly saline environments via tightly folded configurations and small accessible surface areas.
"Proteins from halophilic organisms, which live in extreme saline conditions, have evolved to remain folded at very high ionic strengths. The surfaces of halophilic proteins show a biased amino acid composition with a high prevalence of aspartic and glutamic acids, a low frequency of lysine, and a high occurrence of amino acids with a low hydrophobic character. Using extensive mutational studies on the protein surfaces, we show that it is possible to decrease the salt dependence of a typical halophilic protein to the level of a mesophilic form and engineer a protein from a mesophilic organism into an obligate halophilic form. NMR studies demonstrate complete preservation of the three-dimensional structure of extreme mutants and confirm that salt dependency is conferred exclusively by surface residues. In spite of the statistically established fact that most halophilic proteins are strongly acidic, analysis of a very large number of mutants showed that the effect of salt on protein stability is largely independent of the total protein charge. Conversely, we quantitatively demonstrate that halophilicity is directly related to a decrease in the accessible surface area." (Tadeo et al. 2009:e1000257)
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The metabolism of ANME-1 and ANME-2 Archaea allows survival in anoxic environments via methane oxidation, and are often found in association with sulfate-reducing bacteria.
"Evidence supporting a key role for anaerobic methane oxidation in the global methane cycle is reviewed. Emphasis is on recent microbiological advances. The driving force for research on this process continues to be the fact that microbial communities intercept and consume methane from anoxic environments, methane that would otherwise enter the atmosphere. Anaerobic methane oxidation is biogeochemically important because methane is a potent greenhouse gas in the atmosphere and is abundant in anoxic environments. Geochemical evidence for this process has been observed in numerous marine sediments along the continental margins, in methane seeps and vents, around methane hydrate deposits, and in anoxic waters. The anaerobic oxidation of methane is performed by at least two phylogenetically distinct groups of archaea, the ANME-1 and ANME-2. These archaea are frequently observed as consortia with sulfate-reducing bacteria, and the metabolism of these consortia presumably involves a syntrophic association based on interspecies electron transfer. The archaeal member of a consortium apparently oxidizes methane and shuttles reduced compounds to the sulfate-reducing bacteria. Despite recent advances in understanding anaerobic methane oxidation, uncertainties still remain regarding the nature and necessity of the syntrophic association, the biochemical pathway of methane oxidation, and the interaction of the process with the local chemical and physical environment. This review will consider the microbial ecology and biogeochemistry of anaerobic methane oxidation with a special emphasis on the interactions between the responsible organisms and their environment." (Valentine 2002:271)
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