Sulfate-methane transition zone
The presence of AOM marks the transition from dissimilatory sulfate reduction to methanogenesis as the main metabolism utilized by organisms.
[1] The SMTZ is a global feature that can occur at depths that range anywhere from a few millimeters to hundreds of meters below the sediment surface.
[4] It was previously believed that methane and sulfate could not coexist due to the established hierarchy of metabolisms in sediments.
[3] However, it was discovered that sulfate reduction and methanogenesis could occur simultaneously in marine sediment in 1977 by Ronald S. Oremland and Barrie F.
After oxygen, nitrate, manganeses, and iron are depleted, sulfate is the main electron acceptor used in anaerobic respiration.
The metabolism associated with this is dissimilatory sulfate reduction (DSR) and is carried out by sulfur-reducing bacteria, which are widely distributed in anoxic environments.
AOM uses sulfate to oxidize methane into bicarbonate and forms hydrogen sulfide as a byproduct, and is described by the following equation:
The rate of AOM is pretty slow, with turnover times for the coexisting sulfate and methane in the oceans ranging from weeks to years.
This metabolism take sulfate and methane in a 1:1 ratio and produces certain carbon species (mainly bicarbonate) and sulfide.
[1] Sulfate-methane transition zones have various signatures besides the sudden increase of methane at nearly depleted sulfate concentrations.
At the SMTZ, there are expected rises in pH, alkalinity, phosphate, and carbonate precipitation rates.
A very significant marker of the SMTZ is an elevated concentration of barium ion (Ba2+), which is caused by the dissolution of sedimentary barite, BaSO4.
This drives the accelerated depletion of oxygen and other substrates used for respiration before sulfate towards the top of the sediment column.
This would lead sulfate reduction and methanogenesis to occur higher up in the sediment column, bringing up the SMTZ.
However, a direct correlation between organic matter deposition rates and SMTZ depth has yet to be established.
[3] Geochemical profiles of sulfate around the SMTZ, in particular, have been greatly affected by sampling artifacts, like seawater contamination.
Additionally, it has been proposed that AOM cannot account for all of the carbon budget and isotopic variations found in the SMTZ and perhaps.
[1] Some of the first organisms found that perform AOM were sulfide-oxidizing bacteria, which surrounded aggregates of methanogenic archaeal cells.
[11] AOM is now loosely characterized by the presence of the sulfate-reducing bacteria, Desulfosarcinales, and methane-eating archaea, anaerobic methanotroph (ANME-2), consortia.
The Desulfosarcinales and ANME-2 consortia has now been observed in several locations like along the coast of California, suggesting a significant partnership between the microbial groups.
[7] Other common microbial groups that could potentially define a global signature include Planctomycetes, candidate division JS1, Actinobacteria, Crenarchaeota MBGB.
Green non-sulfur bacteria are prevalent, along with the archaeal and bacterial groups found within the SMTZ.
[1] It is still difficult to broadly name microbial communities found in all SMTZs because dominant groups are determined by ecological and chemical factors.
[6] The production and consumption of methane leads to archaeal and bacterial highly depleted in 13C biomarkers, specifically lipids.
