Methanogens

Methanogens:

Methanogens are the unique group of widely distributed archaebacteria that are found in anaerobic environments, as also in the intestinal tracts of animals, freshwater and marine water, and sewage. Methanogens nave capability of producing methane from a limited number of substrates, including hydrogen, carbon dioxide, acetate, and methylamines; an important source of natural gas.They are obligate anaerobic bacteria having slow growth rate. They play a major role in the breakdown of the substrate into gas forms. They are only the organisms which can anaerobically catabolise acetate and hydrogen to gaseous products in the absence of exogenous electron acceptors other than carbon dioxide or light energy. These are microscopic organisms which produces methane as a byproduct of their metabolism. These organisms belong to a group of organisms called archaea and these are obligate anaerobes. In morphology, they are different in shapes like either they are Cocci, Spirilla, Bacilli or Sarcina.

Characters of Methanogens:

1. Methanogens are strictly anaerobic or obligatory anaerobic and most of them have a cell wall and helps for cell support and its rigid structure protects them.
2. These group of organisms can be both Gram's positive and Gram's negative.
3. They are both motile and non-motile.
4. These organisms belong to a fascinating group of organisms which are known as extremophiles.
5. These organisms consume carbon dioxide and hydrogen and release methane with the help of microorganisms.
6. Methanogens morphologically are of two shapes: coccus, which is a circle-shape, and bacillus , which is a rod-shaped.
7. Methanogen depends on upon syntrophy, or when one organism lives off of the products of another organism. Mostly these organisms live closely with certain bacteria that produce molecules that the methanogens can consume.
8. Most of the methanogens can survive in high salt concentrations, acidic conditions, and extreme temperatures.
9. These group of organisms belongs to the archaea domain, which is one of three domains and other two domains are bacteria and eukaryotes.

Fig : Three domains of life: Archea includes Methanogens.

Classification of Methanogens :

Batch et al(1979) has classified Methanogens in three orders as Methanobacteriales, Methanococcales and Methanomicrobiales with their consecutive family.

Some of the methanogens are:

Fig: Methanosarcina acetivorans

Fig:Methanosarcina

Fig : Flow diagram for Methane production

Role of Methanogens in Methane Production / Methanogenesis :

This is the actual phase of methane gas production and the final phase of anaerobic digestion. During this stage, the different substrates like low molecular weight acids, alcohols, CO₂ and H₂, etc. are further converted to methane gas by a group of highly specialized group of bacteria commonly referred as methanogens. These are also called methane producing bacteria (Archeobacteria). The different species of methanogens are involved in the production of methane and may act on the different substrate. Acetate and (CO₂+H₂) are the two common substrates for methanogenic bacteria.

Biochemistry of Methane gas production:

A methanogenesis is a form of anaerobic respiration in which the carbon acts as terminal electron acceptor. The carbon may be in the form of CO2 or it can occur in organic compounds. Thus, the oxidation of organic compounds with the reduction of carbon, as methanogens can anaerobically catabolise acetate, formate, ethanol, methanol, CO₂+H₂. The different chemical reactions during methane gas production from different substrates by different methanogens include:

1. CH₃COOH → CH₄+ CO₂ (Aceticlastic methanogens like Methanosarcina) (in absence of H₂ )
2. CO₂+ 4H₂ ­→CH₄+ 2H₂O (H₂- oxidizing methanogens like Methanobacterium, Methanobrevibacter, Methanospirillum) (in presence of H₂ )
3. 2CO + 2H₂O→2CO₄ + 4H₂­→ CH₄ + 2H₂O ( Methanosarcina barkeri can produce CH₄ from CO)
4. 4CH₃OH → 3CH₄ + CO₂ +H₂O
5. CH₃CH₂OH → CH₄ + CO₂

Mechanism of Methane gas production :

Methane gas is second most important greenhouse gas after carbon dioxide with a global warming potential 25 times more than CO₂ . The methane molecule gets trap by infrared radiation after receiving and absorbing a portion of solar radiation by the earth emitted toward the atmosphere. Otherwise, the infrared radiation could escape to space. As , methane molecules are energized and then begin to emit heat in all directions including back towards the Earth. The mechanism of methane formation is not well understood as it is complex and several coenzymes like CoM, Methanopterin (MP), Methanofuran (MF- CO₂ reducing factor), etc. are part of the reaction.

Fig: Energy-Yielding Reactions of Methanogens (Source: Thauer, et al., (1977)

Role of Methanogens in biogas production:

1. The biogas is only produced if only when Methanogens acts on the substrate like low molecular weight acids, alcohols, carbon dioxide, hydrogen, etc undergoing biochemical reaction.
2. Methanogens are obligatory or strictly anaerobic microorganisms which facilitate in catabolization of acetate and hydrogen to gaseous products anaerobically in the absence of exogenous electron acceptors other than carbon dioxide or light energy.
3. Methanogens are capable of converting almost all types of polymeric materials to methane and carbon dioxide under anaerobic conditions.
4. Methanogens play an effective role in the reduction of pollution rather than conventional aerobic methods.
5. Methanogens consume carbon dioxide and hydrogen and release methane gas as the byproduct.

Refrence :

1. Alexander, M. Introduction to Soil Microbiology. Academic Press, 1961.
2. Rangaswami, G and Bagyaraja PT. Agricultural Microbiology. 2nd. Prentice Hall of India, 1993.
3. RM, Atlas and R Bartha. Microbial Ecology : Fundamental and Applications. The Benjamin Cummins Publication Co. Inc., 1998.