Prokaryota – Archaea
1. Morphology
Archaea are unicellular microscopic organisms with a striking variety of cell shapes and unique geometric forms Many are rod-like (referred to as bacilli and or spherical (referred to as cocci while the heat-loving (thermophiles) are highly irregular cocci. By contrast others have both a long rod shape (filamentous) with sheaths that surround adjoining cells. Additionally, some archaea form clusters, while others form many irregular shapes. Some species are even square-shaped, triangles or flat discs.
2. Taxonomy
Archaea, the third domain of life are split into 3 phyla, the Euryarchaeota, the Crenarchaeota and the Thaumarchaeota. The Euryarchaeota are physiologically the most diverse, with a number of methane- producing orders (methanogens); The Crenarchaeota are almost all extremophiles, and are primarily involved in sulphur or iron metabolism. The Thaumarchaeota contain most of the isolated mesophilic archaea, which are associated with aerobic ammonia oxidation (nitrification).
3. Microorganisms and the environment
Some microorganisms, including the archaea, are able to modify their shape or size in response to environmental conditions − this is also known as pleomorphism. Organisms that exist only in moderate temperatures, typically between 20°C and 45°C, are referred to as mesophiles.
By contrast, extremophiles are organisms that thrive in extreme environmental conditions. It is possible to have different classes of extremophiles, depending on the environmental factors:
- thermophile: an organism that loves high temperature;
- psychrophile: an organism that loves low temperature;
- alkaliphile: an organism that loves high pH values;
- acidophile: an organism that loves low pH values;
- halophile: an organism that loves high salt concentration.
4. The discovery of archaea altered our understanding of evolution, and recent research suggests that humans may actually be derived from archaea. Archaea live in the widest range of environmental conditions of any organisms, from pH 0 to pH 12, 0˚C to 120˚C, and up to 35 % salinity. Hyperthermophilic archaea survive at temperatures greater than 90°C by having a thin membrane, made up of double-headed lipids, that insulates the cell interior from the heat. In acid or salty environments, this sort of membrane acts as a barrier to water molecules and other ions. The halophilic archaeon, is box shaped and forms large fragile flat sheets in the environment. Archaea do not have a nucleus.
5. Microhabitat
Euryarchaeota, in particular methanogens, dominate waterlogged soils. Six of the seven methanogen orders can be found in different soil types, either free-living or associated with other organisms, such as ciliates and termites. Methanogens can also be found in dry and aerated soils. Members of the Halobacteriales order are often found in high salinity soils, and many use light as an energy source. Archaea in soils under termperate climates are dominated by the Thaumarchaeota, a group that was previously linked to the Crenarchaeota. Many Thaumarchaeota are able to convert ammonia to nitrite (amonia-oxidisers). In low pH soils, and under low ammonia and low oxygen conditions, these archaea are more important than their bacterial counterparts. The extremophilic Crenarchaeota are primarily found in harsh soils, such as hot volcanic soils, rich in sulphur and iron compounds. And in high acidic, deeply weathered, red soils (Ferralsols) in China.
6. Diversity, abundance and biomass
Over 300 archaeal species have been described, primarily found in extreme environments and many more species have been detected in the environment. Soils contain between 105 and 1010 microbial cells in each gramme (0.04 ounces), and all contain archaea. Generally,up to 10 % of microbial cells in temperate soils may be archaea (mesophilic species), while in conditions of high temperature, high salinity or at high or low pH, archaea (extremophilic species) can be the dominant members of the microbial community
Ref: A Global Atlas of Soil Biodiversity p 33
