Although a number of Archaea species have been known for many years, until recently they were considered bacteria and not recognized as belonging to a separate major domain. Three kingdoms of Archaea (Table 10.8) are now recognized, and with the exception of the methane producers (methanogens), most of the known species are extremo-philes (high temperature, high or low pH, and/or high salinity). They include both aerobes and anaerobes, chemoorganotrophs and chemolithotrophs, heterotrophs and auto-trophs. Some are flagellated, others are nonmotile. None are known to be pathogenic. However, it is still too early in the investigation of these organisms to tell how diverse and widespread the group may truly be.

10.6.1 Korarchaeota

Members of the Korarchaeota have been observed in hot springs but have only recently been cultured. For this reason, little is yet known about them other than that the ones observed are hyperthermophiles. They are of particular interest to microbiologists because they may be the closest living relatives to the earliest life-forms to develop on Earth.

10.6.2 Crenarchaeota

Most of the known Crenarchaeota are hyperthermophiles, including Pyrolobus, the organism with the highest known growth temperatures (minimum, 90°C; optimum, 106°C; maximum, 113°C). Such organisms growing at temperatures above 100°C are found in

TABLE 10.8 The Archaea: A Proposed Phylogeny Including Some Representative, Interesting, and Environmentally Important Generaa

Kingdom 1. Kingdom 2. Class 1. Order 1.

Order 2.

Order 3.

Kingdom 3. Class 1.

Class 2. Order 1.

Korarchaeota Crenarchaeota Thermoprotei Thermoproteales Pyrobaculum Desulfurococcales Pyrodictium Pyrolobus Sulfolobales

Sulfolobus Euryarchaeota Methanobacteria

Methanobacterium Methanococci Methanococcales Methanococcus

Order 2. Methanomicrobiales Methanomicrobium Methanospirillum Order 3. Methanosarcinales Methanosarcina Class 3. Halobacteria

Halobacterium Natronococcus Class 4. Thermoplasmata Picrophilus Thermoplasma Class 5. Thermococci Pyrococcus Thermococcus Class 6. Archaeoglobi

Archaeoglobus Ferroglobus Class 7. Methanopyri

Methanopyrus aAll classes and orders are listed if more than one.

marine hydrothermal vents, areas of volcanic activity on the ocean floor (where high pressures raise the boiling point of water). Pyrolobus is a chemolithotrophic autotroph that grows on hydrogen, utilizing nitrate (which is reduced to ammonium), thiosulfate, or oxygen as an electron acceptor. Pyrodictium can grow at 110°C (optimum, 105°C), utilizing hydrogen or organic material, with elemental sulfur as the electron acceptor. Other species (such as Pyrobaculum) can also utilize ferric iron (Fe3+) as an electron acceptor.

Other Crenarchaeota grow in hot springs. Sulfolobus, for example, grows aerobically, oxidizing hydrogen sulfide to sulfuric acid and ferrous iron (Fe2+) to ferric form. Its maximum temperature is "only" 87°C, but it is also an acidophile.

Not all Crenarchaeota are thermophilic. They have now been found to be widespread in the marine environment, including in Antarctic waters, where temperatures are typically below 0°C.

10.6.3 Euryarchaeota (Including Methanogens)

The Euryarchaeota include thermophiles and hyperthermophiles, hyperhalophiles, and methanogens. Among the thermophiles are Thermoplasma (optimum, 55 to 60°C), an acidophile (optimum, pH 1 to 2) found in hot springs and in coal refuse piles (large mounds of waste soil and rock from coal mining operations). Interestingly, Thermoplasma (like the mycoplasmas) lacks a cell wall, despite the harsh environments in which it thrives. Picrophilus (which does have a cell wall) can grow at a pH below zero.

Coal refuse piles represent an interesting ecosystem. They contain residual coal, iron pyrite (FeS2), and other organic and inorganic compounds. Aerobic chemolithotrophs oxidize the pyrites, leading to highly acidic conditions (see acid mine drainage, Section 13.4.3), while oxidation of organics leads to self-heating (see composting, Section 16.2.3) and further depletion of oxygen. The elevated temperatures are believed to effect a partial chemical breakdown of high-molecular-weight organics present in the coal into smaller, more readily biodegradable compounds. This sets the stage for Thermoplasma to utilize the organics aerobically or through sulfur respiration.

Hyperthermophilic Euryarchaeota include Thermococcus, which grows on organic matter using elemental sulfur as its electron acceptor, and Archaeoglobus, a sulfate reducer. Ferroglobus can oxidize ferrous iron to ferric form utilizing nitrate.

The hyperhalophiles, such as Halobacterium, are found in very salty waters, such as the Dead Sea (Israel/Jordan) and Great Salt Lake (Utah), as well as in salt drying ponds and on salted fish. Others, including Natronococcus, thrive in highly alkaline soda lakes, such as those of the African Rift Valley. Of particular interest to some biochemists is that several of these organisms contain pigments that allow them to obtain energy (produce ATP) from light through a nonphotosynthetic pathway.

Methanogens are strict anaerobes that produce methane. Most commonly this is done by the reduction of carbon dioxide used as an electron acceptor during growth on hydrogen, but some methanogens can reduce methanol or other methyl compounds, cleave acetate (to methane and carbon dioxide), or carry out a very small number of related reactions. Methanogens are found in a variety of natural habitats, such as freshwater sediments and flooded soils (producing "swamp gas'') and the digestive tracts of animals ranging from termites to humans. One genus found at hydrothermal vents, Methanopyrus, is hyperther-mophilic (optimum 100° C, maximum 110° C).

The methanogens are the Archaea of greatest interest to environmental engineers and scientists. In addition to their critical role in the carbon cycle in anaerobic environments, their activities are widely utilized in such anaerobic organic waste treatment processes as anaerobic digestion of sewage sludge. Without the final conversion of anaerobic metabolic products to methane (and the potential for energy recovery), such anaerobic processes would be of very limited usefulness. Methanogens are also responsible for the production of methane at sanitary landfills.

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