Phylum Crenarchaea

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Taxonomy

  • Phylum Crenarchaeota
    • Class Thermoprotei
      • Order Thermoproteales
        • Family Thermoproteaceae (e.g. Thermoproteus, Pyrobaculum)
        • Family Thermofilaceae (Thermofilum)
      • Order Caldisphaerales
        • Family Caldisphaeraceae (Caldisphaera)
      • Order Desulfurococcales
        • Family Desulforococcaceae (e.g. Desulfurococcus, Aeropyrum)
        • Family Pyrodictiaceae (Pyrodictium, Pyrolobus, Hyperthermus)
      • Order Sulfolobales
        • Family Sulfolobaceae (e.g.Sulfolobus, Acidianus, Desulfurolobus)
    • Incertae sedis Group I marine Archaea (e.g. Cenarchaeum symbiosum)

About this phylum

Diversity

Cultivated crenarchaea are all relatively closely-related and primitive (at least in terms of their ssu-rRNA branch sequences). Crenarchaeal sequences from environmental surveys are much more diverse, and are often abundant in non-thermophilic environments from which no cultivated crenarchaea are known. A large phylogenetic group of cultivated crenarchaea known as the “marine group I Archaea” was first seen in marine water surveys but since found in many other non-thermophilic environments,

Metabolism

Cultivated crenarchaeal generally oxidize and/or reduce sulfur and sulfur compounds (at least facultatively) by one of three biochemical processes:

1. Sulfur reduction

Sulfur + H2 ---> H2S + protons

These organisms are autotrophic anaerobes that fix carbon from CO2. Hydrogen is the electron donor for electron transport and elemental sulfur (or sulfur compounds such as thiosulfate) is the terminal electron acceptor.

2. Sulfur respiration

Sulfur + organics ---> CO2 + H2S

These organisms are heterotrophic anaerobes. Both carbon and energy are from organic compounds. Organics are the electron donor for electron transport and sulfur (or sulfur compounds) is the terminal electron acceptor. This process is much like aerobic respiration, except that sulfur compounds take the place of O2. In fact, many organisms that grow by sulfur respiration can also grow by aerobic respiration.

3. Sulfur oxidation

Sulfur + O2 ---> H2SO4

These organisms can usually grow heterotrophically, getting fixed carbon from low concentrations of organics in the medium. Most can also be grown autotrophically, fixing carbon from CO2 via the reverse TCA cycle. All are aerobes, of course, since it is the terminal electron acceptor (sulfur is the electron donor) for electron transport.

In most cases reduced sulfur compounds such as thiosulfate or sulfite are also usable in place of elemental sulfur. Cultivated crenarchaea are all thermophilic, and most are extremely thermophilic, with optimal growth temperatures above 80°C. As a group, these are the most thermophilic organisms known. Many are also acidophilic and autotrophic. Because this phenotype is shared by the most primitive and deepest branches of the Euryarchaea, it is probably the primitive phenotype of the Archaea.

At least some of the “marine group I” crenarchaea are anaerobic ammonia oxidizers.

Morphology

Cellular morphology of crenarchaea generally follow the phylogenetic subgroups: most Thermoproteales are rod-shaped or filamentous, most Desulfurococcales are flattened ovoids, an most Sulfolobales are irregular cocci.

Habitat

Cultivated crenarchaea are common in hydrothermal environments, especially acidic hot springs, solfataras, and marine hydrothermal vents. Uncultivated crenarchaea are apparently very common in some non-thermophilic environments, including ocean water, soil, and cave rock surfaces. One uncultivated specie, Cenaerchaeum symbiosum, is a symbiont of a marine sponge, but no other crenarchaeal symbionts or parasites of plants or animals are known.

thePit
"The Pit", and acidic hot spring in Yellowstone National Park, an environment rich in Crenarchaea : James W. Brown

Example species

Thermoproteus tenax

Thermoproteus
Thermoproteus tenax : The Prokaryotes, pp679 Robert Huber

Thermoproteus tenax, like other Thermoproteales, are strict anaerobes that grow best (at least in cultivation) by sulfur respiration. It is an extreme thermophile, and so anaerobiosis is not surprizing; the solubility of oxygen in water at temperatures above 85°C is extremely low. Although it grows best in culture by sulfur respiration, the more usual growth conditions for these organisms in the wild is probably autotrophic sulfur reduction. So, T. tenax can either respire heterotrophically or reduce sulfur autotrophically for a living, and the switch between growth modes is a distinct developmental process.

T. tenax is a long rod-shaped organism that reproduces by branch formation; the branch bud forms near the end of a cell and grows into a new individual cell. It is a common solfatara inhabitant, with an optimal growth temperature of 85°C. T. tenax is motile, but this is not usually seen microscopically; after all, the room temperature of a microscope slide is 65°C below the optimal growth temperature of the organism!

Pyrodictium occultum

Pyrodictium
Pyrodictium occultum : The Prokaryotes, pp682 Robert Huber

Pyrodictium is a marine organism common in deep sea hydrothermal vents. It is a flat irregular coccus (think of the body of a prickly-pear cactus) with a network of tubular fibriles that connect cells together. The optimal growth temperature is 105°C, and cultures grow well at temperatures up to 115°C, making it one of the most thermophilic species known. They are also one the most primitive organisms known; this is a general rule, that thermophiles, and especially extreme thermophiles, are primitive, at least in terms of their ssu-rRNA sequences. P. occultum can grow either by sulfur respiration or sulfur reduction.

Sulfolobus solfataricus

Sulfolobus
Sulfolobus solfataricus : Agta Gambacorta & Barbara Nicolaus, Naploi, IT : http://www.fi.cnr.it/r&f/n6/ingrand.htm

S. solfataricus is a lobed coccus; the lobes seem to be budding scars from reproduction, and often these buds can be almost like appendages that hold the cell to the sulfur granules they're growing on. S. solfataricus and its relatives are common, even predominant organisms of solfataras and boiling mud pots.

S. solfataricus is an obligate aerobe or microaerophile, capable of autotrophic sulfur oxidation, chemolithoheterotrophy (using sulfur oxidation for energy but requiring organic carbon for growth), or heterotrophy by oxidative respiration. S. solfataricus is an obligate thermoacidophile, requiring a pH of ca. 4.5 and temperature of 87°C for optimal growth. Like most acidophiles, they are sensitive to fatty acid toxicity; fatty acids are protonated and so uncharged at these environmental pH’s, diffuse readily through the cytoplasmic membrane, then ionize at the more moderate cytoplasmic pH. This results in an uncoupling of the proton gradient and acidification of the cytoplasm.