Archaea

Archaeal Diversity:

Like bacteria, the archaea also has variety of metabolism. They have chemoorganotrophy, chemolithotrophy type of nutrition. Based on the habitat and phylogenic relationship, the archaea are divided into four groups as 1. Extreme halophilic archaea 2. Methanogens 3. Hyper thermophiles 4.Thermoplasma.

1. Extreme halophilic archaea (Euryarchaeota): These archaea survive in extremely higher salt concentrations like Great salt lake (USA). The salt concentration is about 40.1 g/l of Na⁺ and 181 g/l of Cl. These archaea has the optimum salt concentration of 1.5 M to 5.5 M NaCl. These archaea are collectively called as halobacteria.

The genera are Halobacterium, Halorubrum, Halococcus, Natronobacterium, Natronosomonas.

• These bacteria are heterotrophs
• They have aerobic respiration with electron transport chain
• All the organelles were stabilized by K⁺
• They provide purple membrane in the cell membrane, which is used for ATP synthesis.

Bacteriorhodopsin is the light harvesting pigment present in the membrane gives purple colour. These pigment directly produce the proton gradient in the membrane (as that of electron transport chain) but without any electron carriers finally leads to ATP synthesis. This process is known as non-photosynthetic phosphorylation. Protection against salt is due to, (1) Internal accumulation of K ions (2) Acidic amino acids of cell membrane, (3) K stabilized cell organelles are some of the mechanisms by which these archaea protected themselves from salt.

2. Methanogens: They are the obligate anaerobic thermophiles present in fresh water sediments, deep soil, ruman of cattle, paddy fields. (Cattle can produce 50 l of methane per day).

• They are the obligate anaerobes
• They use H₂ and CO₂ as electron and carbon sources for energy generation and anabolism through acetyl CoA pathway
• Some organisms use CO, CH₃OH, HCHO as other substrates for C assimilation
• They use CO₂ as terminal electron acceptor for methane formation
• The role of methanogens in decomposition of organic waste is highly useful in environment and at the same time methane emission from flooded soil is highly harmful to ozone layer of atmosphere.
• Methanobacterium, Methanobrevibacterium, Methanococcus, Methanospirillum, Methanosarcina are some of the important genera present in this group.

3. Hyperthermophiles (Crenarchaeota) They are extreme thermophiles present in hot sulphur rich environments such as hot springs, volcanisms etc.

• They require 80 – 100° C as optimum temperature for growth
• Unusually stable at high temperatures
• They need sulphur as essential nutrients
• Both aerobes and anaerobes are present
• Chemolithotrophy and chemoorganotrophy type nutrition
• Chemolithotrophs reduce the sulphur as terminal electron acceptor, form H₂S and derive energy. Ex. Pyrococcus, Ferroglobus, Archeoglobus
• Chemoorganotrophs use organic compounds and sulphur for energy and carbon source. Ex. Thermoproteus, Thermococcus, Desulfurococcus, Pyrococcus, Sulfolobus
• Sulfolobus was the first hyperthermophile isolated by T.D. Brock from hot spring of Yellowstone National Park.

Note: Thermus aquaticus is a bacterial species (earlier as archaeobacteria) from which the DNA polymerase enzyme isolated, is very much useful in the DNA synthesis in lab condition called as Polymerase chain reaction.

4. Thermoplasma & Picrophilus

They are thermophilic acidophiles. They prefer 55°C as optimum temperature and pH 2.0 as optimum condition for growth. They are present in the coal mine spoils (waste created during coal mining).

The another feature of these archaea is lack of cell wall. They have only membrane and the membrane itself protect the cell from high temperature and acidity.