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.

(Woese et al., 1990; Kimball, 2010; UCMP, accessed 2011)

Also referred to as Archaea, the smaller set on non-nucleated cellular life that used to be referred to as bacteria or prokaroytes. Considered one of the three major types of cellular life. They differ biochemically in the arrangement of the bases in their ribosomal RNA and in the composition of their plasma membranes and cell walls from the Eubacteria. Often regarded as extremophiles, with tendencies to methanogens, halophiles, and thermophiles. The methanogens are anaerobic bacteria that produce methane. They are found in sewage treatment plants, bogs, and the intestinal tracts of ruminants. Ancient methanogens are the source of natural gas. Halophiles are bacteria that thrive in high salt concentrations such as those found in salt lakes or pools of sea water. Thermophiles are the heat-loving bacteria found near hydrothermal vents and hot springs. Many thermophiles are chemosynthetic using dissolved sulfur or other elements as their energy source and iron as a means of respiration. Archaebacteria emerged at least 3.5 billion years ago and live in environments that resemble conditions existing when the earth was young. Arachae have pseudopeptidoglycan cell walls, lipids are branched chain hydrocarbons linked to glycerol molecules by ether linkages - fatty acid components are not found in archeal lipids, DNA in a single circular molecule but with extrachromosal plasmids, histone-like DNA binding proteins, complex (up to 14 subunits) RNA polymerases, high internal salt concentrations.

Apparently nearly anywhere and eveywhere on Earth.

Archaea are generally less than one micron long, may or may not have flagella, and cell-shapes include spherical, box-like, triangular, and slender filamentous forms (UCMP, accessed 2011).

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).

PHYLA / DIVISIONS

Euryarchaeota: methanogens, halophiles, thermophiles

Crenarchaeota: hyperthermophiles, cool-water marine plankton, nitrifying soil/sediment microbes

Nanoarchaeota: thermophiles

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 function in saline environments: archea
 

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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Microbes consume methane: ANME-1 and ANME-2 Archaea
 

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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