Anaerobic Oxidation of Methane AOM

Below the sulfate zone, methanogenesis is the main terminal pathway of organic carbon mineralization. Methane is produced exelusively by anaerobic archaea that utilize a narrow speetrum of substrates for the process (Whitman et al. 

Methanogenic bacteria or eukaryotes are not known to exist. The primary sourees of methane formation in marine sediments are from the splitting of acetate (CH COO) and from the reduction of CO by hydrogen (Eq. 8.6 and 8.7)  

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Anaerobic CH4 oxidation, now referred to as anaerobic oxidation of methane (AOM), has received renewed attention with introduction of new organic geochemical and molecular techniques. Hinrichs et al. (1999) applied compound-specific isotope analyses of lipid biomarker molecules associated with specific archea and culture-independent techniques involving 16S rRNA identification studies to samples collected from an Eel River Basin seep. This work showed that the biomarker compounds were so strongly depleted in that CH4 must be the source rather... 

Fig. 14.22 Schematic illustration of gas hydrate deposits and biogeochemical reactions in near-surface sediments on southern Hydrate Ridge. High gradients in pore water sulfate and methane are typical of methane hydrate-rich environment close to sulfate-rich seawater. At the sulfate-methane interface (also named sulphate-methane transition in earlier chapters of the book) a microbial consortium of methanothrophic archaea and sulfate-reducing bacteria (Boetius et al. 2000) perform anaerobic oxidation of methane (AOM) leading to carbonate precipitation. AOM rates influence hydrogen sulfide fluxes and gradients, which are reflected on the seafloor by the distribution of vent communities around active gas seeps and gas hydrate exposures (Sahling et al. 2002).

Microorganisms able to perform anaerobic oxidation of methane were only recently discovered through intensive studies, not of the normal subsurface SMT, but of unique marine environments in which AOM dominates the carbon and sulfur cycles, such as sediments associated with gas hydrates and methane seeps. (Gas hydrates are ice-like solids - generally composed of water and methane - which occur... 

C enrichment and indicated that the associated sulfate reducers were also involved in the AOM process (Hinrichs et al. 2000 Elvert et al. 2000 Pancost et al. 2000). Final evidence that the aggregates carried out anaerobic oxidation of methane was provided by a recently developed SIMS technique (secondary ion mass spectrometry) by which carbon isotopic analysis could be done on individual microscopic AOM aggregates and thereby confirm that their cell material contained the depleted isotope signal of methane (Orphan et al. 2001). 

The metabolic coupling involved in AOM, produces sulfide and dissolved inoiganic carboa Both methane and sulfate needed for AOM, are available in large amounts where methane vents are present at the seafloor. In the case of Hydrate Ridge, gas hydrates provide an almost inexhaustible supply of methane and the ocean water constitutes a large sulfate reservoir. Here the anaerobic methane oxidation rate is large because of the conti-nuous supply of methane from deeper sediments. 

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