Details for Methanococci

A large number of Euryarchaeota produce methane (CH₄) as an integral part of their energy metabolism. Such organisms are called methanogens and the process of methane formation methanogenisis.

Physiologically, methanogens are obligate anaerobes, and strict anoxic techniques are necessary to culture them. Depending on the species, cultures of methanogens can be established in a mineral salts medium under an atmosphere of H₂ plus CO₂ (in the ratio 4:1), or in complex media. Most known methanogens are mesophilic, although "extremophilic" species growing optimally at very high temperature have been described.

At least 11 substrates have been shown to be converted to methane by pure cultures of methanogens. Interestingly, these substrates do not include such common compounds as glucose and organic or fatty acids (other than acetate and pyruvate). This is not to say that a compound like glucose can never be converted to methane. Cooperative reactions involving methanogens and other anaerobic bacteria can virtually convert any organic compound to methane and CO₂.

Three classes of compounds make up the list of 10 methanogenic substrates; these include CO₂-type substrates, methyl substrates, and acetotrophic substrates. The first class includes the important substrate CO₂ which is reduced to methane using H₂ as electron donor:



Other substrates here include formate (which is simply CO₂ + H₂ in combined form) and CO, carbon monoxide.

The second class of methanogenic substrates are methyl group substances. Using CH₃OH as a model methyl substrate here, the formation of CH₄ can occur in either of two ways. First, CH₃OH can be reduced using an external electron donor such as H₂:



Alternatively, in the absense of H₂, some CH₃OH can be oxidized to CO₂ in order to generate the electrons needed to reduce other molecules of CH₃OH to CH₄:



The final methanogenic process is the cleavage of acetate to CO₂ plus CH₄, called the acetotrophic reaction:



As can be seen by inspection of each of the above reactions, they are all exergonic and can thus be used to synthesize ATP. The process is coupled to proton motive force formation and ATP synthesis by normal chemiosmotic mechanisms. Substrate-level phosoporylation, typical of fermentative bacteria, apparently does not occur in methanogens.

Methanocaldococcus jannaschii has been considered a model of the methanogens. Click here for more information.