the methane barrier
|Some of the most important microbiological processes in the context of METROL are:
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 atmosphere1 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.
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.
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.