Methanogenesis

Acetogenesis

Sulfate reduction

Anaerobic Oxidation of Methane (AOM)

Methanogenesis

Methane-producing microorganisms dominate the final stages of organic matter decay in environments where oxygen, nitrate, manganese and iron oxides, and sulfate are absent or remain in very low concentrations. In marine sediments, bacterial sulfate-reduction initially dominates and methanogenesis only takes over in deeper sediments where sulfate has been exhausted. Microbial methanogenesis produces enormous amounts of CH₄ in both marine sediments and terrestrial wetlands. Marine gas hydrate deposits alone contain ~8 x 10¹⁸ m³ CH₄ and wetlands produce ~25% of the 550 million tons of CH₄ released annually to the atmosphere^{1 2}. The microorganisms and microbial pathways responsible for the production of this potent greenhouse gas (~23 times more effective per molecule than CO₂) are thought to be fundamentally different between fresh water and marine systems, although the cause of this difference is unclear. This situation restricts our ability to understand the environmental mechanisms and controls on CH₄ production and, hence, the cause of its increasing atmospheric concentration. In addition, the lack of understanding of the pathways of methane production and consumption in sediments of continental margins reduces our ability to accurately quantify global CH₄ emissions from stable isotope mass balance calculations, as the isotopic composition of CH₄ is controlled by both its formation and consumption pathways.

Literature:

1) Kvenvolden, K.A., G. Ginsburg and V. Soloviev (1993) World-wide distribution of subaquatic gas hydrates. Geo-Mar. Lett. 13: 32-40.
2) Cicerone, R.J. and R.S. Oremland (1988) Biogeochmiecal aspects of atmosphereric methane. Global Biogeochem. Cycles : 299-327.
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Acetogenesis

The role of autotrophic acetogenic bacteria, which produce acetate from H₂ and CO₂ in freshwater and marine sediments, is very poorly understood. Such microorganisms can affect the terminal steps of carbon flow in anaerobic environments through substrate competition (e.g. for H₂) and production (e.g. for acetate)³. Moreover, autotrophic acetogens have significant isotope discrimination effects associated with their metabolism that can indirectly exert a strong influence on the stable carbon and hydrogen isotope composition of CH₄ and thus affect the interpretation of isotope data. Sulfate reduction in marine sediments can deplete pore waters of acetate, which forces methanogens at greater depths to use the H₂/CO₂ pathway. Acetate could become available for methanogens when sulfate reduction becomes naturally limited by sulfate depletion, for example by methane oxidation. There is evidence for the occurrence of acetogenesis from CO₂ in anoxic marine sediments that are ordinarily dominated by sulfate reduction and methanogenesis. Depletion of pore water sulfate can result in elevated H₂ concentrations that make acetogenic CO₂ reduction thermodynamically favorable. The maintenance of elevated but constant H₂ concentrations immediately following sulfate depletion likely reflects control by acetogenic bacteria, suggesting they can be the dominant consumers of H₂⁴.

Literature:

3) Dolfing, J. (1988) Acetogenensis. In: A.J.B. Zehnder (ed.) Biology of Anaerobic Mircroorganisms. Wiley. New York. 872 pp.
4) Hoehler, T.M., M.J. Alperin, D.B. Albert and C.S. Martens (1998) Thermodynamic control on hydrogen concentrations in anoxic sediments. Geochim. Cosmochim. Acta 62: 1745-1756.
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Sulfate reduction

Sulfate reducing bacteria are responsible for up to half of the total mineralization of deposited organic matter in shelf sediments⁵. The bacteria are active throughout the sulfate zone of several meters depth and convert the products of organic fermentation into CO₂ and hydrogen sulfide. At the methane-sulfate transition, the H₂S is produced in amounts equal to the amount of methane oxidized. The H₂S accumulates in the pore water and partly precipitates as pyrite. However, most of it migrates up towards the sediment surface where it is reoxidized to sulfate, thereby closing the sulfur cycle. The reoxidation of H₂S may consume up to 50% of the total oxygen uptake of the sea floor.

At methane seeps, large quantities of the deeply formed H₂S are carried up to the sediment surface together with the methane where they provide the energy basis for chemoautotrophic bacteria. These, in turn, nourish rich communities of invertebrates which host the bacteria as symbionts. Thus, the sulfur cycle is closely linked to the transformations of methane.

Literature:


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