Sulfate-methane transition zone

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.[6] DSR oxidizes organic carbon using sulfate, and is described by the following equation:

${\displaystyle {\ce {SO4^2- +2CH2O -> H2S +2HCO3^-}}}$.[6]

The main metabolism is anaerobic oxidation of methane (AOM). AOM uses sulfate to oxidize methane into bicarbonate and forms hydrogen sulfide as a byproduct, and is described by the following equation:

${\displaystyle {\ce {SO4^2- + CH4 -> HS^- +HCO3^- +H2O}}}$.

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 inefficiency can be a result of the small change in free energy associated with the reaction. Highest rates of AOM usually over methane gas seeps.[3] The maximum rates of AOM generally overlap with the maximum rates of sulfate reduction.[2] It has also been proposed that methanogens can also oxidize methane into acetate or carbon dioxide, and not just bicarbonate.[7]

Below the SMTZ, methanogenesis is the main metabolism after AOM. Methanogens are organisms who produce methane and take a carbon source, either carbon dioxide or organic matter, and reduce it to methane through the following reaction:

${\displaystyle {\ce {4H2 +CO2->CH4 +2H2O}}}$.[8]

It is this reaction that leads to the sharp increase in methane concentrations below the SMTZ.

In most cases, sulfate tends to linearly decrease depth, which mostly reflects the diffusion of sulfate downwards.[2] This diffusion is the main source of sulfate to the SMTZ. The sharper decrease in the sulfate that occurs further down is the result of microorganisms using dissimilatory sulfate reduction, which consumes sulfate.

^{Sulfate and methane concentration profiles as observed in the Black Sea by B.B. Jorgensen et al. (2001). Profile on the left crosses an SMT at around 150 cm, while the one on the right crosses at around 250 cm.}

Here, sulfate diffusing down and methane diffusing up coincide, resulting in anaerobic oxidation of methane (AOM). This metabolism take sulfate and methane in a 1:1 ratio and produces certain carbon species (mainly bicarbonate) and sulfide. It is through AOM that sulfate and methane concentrations remain relatively low within the SMTZ.[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.[9] The SMTZ is also partially controlled by the amount of organic matter in the sediments. Higher organic deposition rates tends to push the SMTZ up higher, since a community of organisms will respire more rapidly due to the influx of nutrients provided by organic matter. 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.[2]

There is a sharp increase in methane concentrations due to methanogenesis. This microbial metabolism reduces carbon dioxide or organic matter into methane. This region is the source of methane that then diffuses up.[3]

Geochemical profiles of sulfate around the SMTZ, in particular, have been greatly affected by sampling artifacts, like seawater contamination.[10] This is a difficult challenge that has yet to be overcome. Additionally, it has been proposed that AOM cannot account for all of the carbon budget and isotopic variations found in the SMTZ and perhaps. Instead, processes like organic carbon remineralization, where organic carbon is converted into smaller organic compounds or inorganic compounds, could account for some of the missing carbon budget.[1]

DSR is the predominant metabolism, so sulfate reducing bacteria are abundant above the SMTZ. Examples of sulfate-reducing bacteria are green non-sulfur bacteria which are part of the Planctomycetes phylum, Gammaproteobacteria, Betaproteobacteria. The archaeal community is also involved in sulfate reduction above the SMTZ and consists mainly of members of Euryarchaeotal Marine Benthic Group D.[1]

A group of Deltaproteobacteria that reduces sulfate makes up the majority of the bacterial community.[1] The methane oxidizing archaea (ANMEs) found belong to two out of the three phylogenetic groups, ANME-1 and ANME-2.[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. These organisms have a syntrophic interaction. Other related organisms are ANME-1, which are also anaerobic methanotrophs, but from a different archaeal lineage. Both ANME-1 and ANME-2 are members of the order Methnosarcinales. Sulfate reducing bacteria use a carbon source, like carbon dioxide, and hydrogen excreted by the methanogenic archaea. The bacteria partners are not as specific as the archaea. Desulfosarcinales are more globally widespread so it is still unknown as to whether there is a specific sulfate-reducing bacterial group associated with AOM. 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.[1]

Methanogens, which mainly belong to the Archaea domain, are abundant under the SMTZ. Green non-sulfur bacteria are prevalent, along with the archaeal and bacterial groups found within the SMTZ. There has yet to be a significant difference between the microbial diversity within and under 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. However, it has been observed that the richness in species is relatively similar across SMTZ horizons, especially within the Deltaproteobacteria. The diversity of archaea and bacteria in the SMTZ vary with depth, but bacteria tend to have richer diversity than the archaea.[1]