Baltic Gas

Methanogenesis

Methanogenic microorganisms, the methanogens, belong to the domain Archaea. These methane producing microorganisms are responsible for the final stage of organic matter decay in environments where oxidants for respiration (oxygen, nitrate, manganese and iron oxides, and sulphate) are absent or remain in very low concentrations. In marine sediments, bacterial sulphate reduction typically predominates in the upper meters and methanogenesis only takes over in the deeper sediment where sulphate has been exhausted.Methanogenesis is a strictly anaerobic process (i.e. functions without oxygen) and oxygen completely inhibits growth of methanogens. Methanogens can use only a narrow spectrum of fermentation products for their energy metabolism and growth. The two main pathways of methanogenesis are reduction of carbon dioxide with molecular hydrogen:

CO2 + 4H2 → CH4 + 2H2O

or the conversion of acetate (CH3COO-):

CH3COO- + H+ → CH4 + CO2

Anaerobic oxidation of methane (AOM)

Most of the methane formed in marine sediments is continuously degraded sub-surface by the process of anaerobic oxidation of methane (AOM). According to our current understanding, the process is carried out in syntrophy by methane oxidizing archaea and sulphate reducing bacteria. The archaea convert methane to a – yet unknown – intermediate electron carrier and the sulphate reducing bacteria oxidize that electron carrier using sulphate as an oxidant. When operating in close concert, the two types of organisms are able to turn the reversed methanogenesis into an exergonic net reaction from which they can draw energy and make a living. The metabolic pathway of the archaea seems to be largely a reversal of methanogenesis.

The net reaction of anaerobic oxidation of methane with concurrent sulphate reduction to hydrogen sulphide is:

CH₄ + SO₄^2- + 2H⁺ → CO₂ + H₂S + 2H₂O

Sulphate reduction

Sulphate reducing bacteria are responsible for up to half of the total mineralization of deposited organic matter in ocean margin sediments. The bacteria are active throughout the sulphate zone of several meters depth and convert the products of organic fermentation, such as acetate and other short chain fatty acids, into carbon dioxide and hydrogen sulphide:

CH₃COO^- + SO₄^2- + 3H⁺ → 2CO₂ + H₂S + 2H₂O

They may also use H₂:

4H₂ + SO₄^2- + 2H⁺ → H₂S + 4H₂O

Thus, the sulphate reducing bacteria compete with methanogenic bacteria for both acetate and hydrogen, however much more efficient. This explains why methanogenesis only takes over in deeper sediments where sulphate has been exhausted or remains at very low concentration.

In the sulphate-methane transition, hydrogen sulphide is produced in amounts equal to the amount of methane oxidized

CH₄ + SO₄^2- + 2H⁺ → CO₂ + H₂S + 2H₂O

The methane reacts with buried iron minerals and partly precipitates as iron mono sulphide (FeS, colouring the sediment black) and pyrite (FeS₂) or accumulates in the pore water. Hydrogen sulphide dissolved in the pore water diffuses up towards the sediment surface. Here the hydrogen sulphide is oxidized back to sulphate, by bacteria or by inorganic reactions, thereby closing the sulphur cycle. The oxidation of hydrogen sulphide may, directly or indirectly, consume up to 50% of the total oxygen uptake of the sea floor. Only a minute fraction of the hydrogen sulphide escapes from the sea floor, for example carried up through the ebullition of methane.