Sulfate-methane transition zone

The sulfate-methane transition zone (SMTZ) is a zone in oceans, lakes, and rivers found below the sediment surface in which sulfate and methane coexist. The SMTZ forms because sulfate is more abundant at the top of the sediment column, while methane is more abundant at depth. As sulfate diffuses down the sediments, methane also diffuses up through the sediments and their diffusion profiles meet in this narrow zone. At the SMTZ, sulfate and methane react can react with one another, which allows the SMTZ to harbor a unique microbial community whose main form of metabolism is anaerobic oxidation of methane (AOM). The SMTZ also marks the transition from dissimilatory sulfate reduction to methanogenesis as the main metabolism utilized by organisms.

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. It tends to span several centimeters, but can also reach widths up to a whole meter. It is characterized by low concentrations of sulfate and methane because the anaerobic oxidation of methane consumes both molecules.

History

It was previously believed that methane and sulfate could not coexist due to the established hierarchy of metabolisms in sediments. In well-oxygenated sediments, oxygen is the main electron acceptor in aerobic respiration. Once all of the oxygen is consumed, organisms begin using substrates like nitrate, manganese oxides, and iron oxides as the electron acceptor in anaerobic respiration. However, these substrates tend to be low in concentrations throughout sediments. Sulfate, on the other hand, is relatively high in abundance in comparison, so sulfate reduction is the main form of respiration after oxygen is consumed. Methanogenesis is the next form of metabolism after sulfate reduction, but was thought to begin only when all the sulfate in the sediments was reduced. However, it was discovered that sulfate reduction and methanogenesis could occur simultaneously in marine sediment in 1977 by Ronald S. Oremland and Barrie F. Taylor. Following this discovery, non-zero concentration of sulfate and methane were found the same in zone in ocean setting, leading Niels Iverson and Bo Barker Jorgenson to investigate the methane oxidation rates in the so-called "sulfate-methane transition" in 1985. Since then, many studies have been conducted to trace the sulfate and methane profiles above, in, and below the SMTZ.

Geochemistry

Above SMTZ

In most cases, sulfate tends to linearly decrease depth, which mostly reflects the diffusion of sulfate downwards. This diffusion is the main source of sulfate to the SMTZ. The sharper decrease in the sulfate is the result of microorganisms using dissimilatory sulfate reduction, which takes sulfate as the electron acceptor during respiration.

^{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.}

Within SMTZ

Here, the sulfate diffusing down and methane diffusing up coincide and results 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.

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. 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.

After SMTZ

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.

Geochemical profiles of sulfate around the SMTZ, in particular, have been greatly affected by sampling artifacts, like seawater contamination. This is a difficult challenge that has yet to be overcome. Additionally, it has been proposed that anaerobic oxidation of methane can not account for all of the carbon budget and isotopic variations found in the SMTZ and perhaps processes like organic carbon remineralization could fill in the gaps.

Metabolic Processes

Above the SMT, dissimilatory sulfate reduction is carried out by sulfur-reducing bacteria. Sulfate is used as the main electron acceptor for bacterial respiration after oxygen, nitrate, manganese, and iron are absent or depleted. Below the SMT, methanogenesis is the main metabolism and produces methane. Methanogens take carbon dioxide or organic matter and reduce it to methane.

Within the SMTZ, the main metabolism is anaerobic oxidation of methane (AOM). AOM uses sulfate is used to oxidize methane into bicarbonate and forms hydrogen sulfide:

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

AOM is actually 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. The maximum rates of AOM generally overlap with the maximum rates of sulfates reduction. Highest rates of AOM usually over methane gas seeps. However, it has also been proposed that methanogens actually oxidize methane into acetate or carbon dioxide. The possibility of anaerobic oxidation of methane first proposed in 1977 by Oremland et al and has since then been verified and extensively studied.

Microbiology

Above SMTZ

In bodies of water like the Santa Barbara Basin, the dominant bacterial groups are green non-sulfur bacteria which are part of the Planctomycetes phylum. Species like Gammaproteobacteria and Betaproteobacteria have been found only above the SMTZ boundary. The archaeal community consists mainly of members of the euryarchaeotal marine benthic group D.

Within SMTZ

A group of Deltaproteobacteria that reduces sulfate makes up the majority of the bacterial community. The methane oxidizing archaea (ANMEs) generally found are ANME-1 and ANME-2. Some of the first organisms found that catalyze AOM were sulfide-oxidizing bacteria, which surrounded aggregates of methanogenic archaeal cells. 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, suggesting a significant partnership between the microbial groups.

Other common microbial groups that could potentially define a global signature include Planctomycetes, candidate division JS1, Actinobacteria, Crenarchaeota MBGB.

Under SMTZ

In places like the Santa Barbara basin, green non-sulfur bacteria was again 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.

It is, however, 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.

Impacts on Global Carbon Cycle

The SMTZ is actual a major sink for methane because AOM consumes mostly all of the methane diffusing upwards, such that none or very little methane is released  into the atmosphere. It has been shown that AOM takes up over 90 percent of all the methane produced in the ocean.

Isotopes

Isotopic mass balances calculations have implied that sulfate reduction and anaerobic oxidation of methane strongly fractionate sulfur. The production and consumption of methane leads to archaeal and bacterial biomarkers, specifically lipids, that are highly depleted in ¹³C. The bacteria and archaea associated with the SMTZ, have very depleted in carbon-13, with archaea generally being more depleted than bacteria.

Isotopes have also been the main tool to study ancient SMTZs. Paleo-SMTZ have been studying using a ³⁴S isotopic signature. Extremely ³⁴S depleted pyrite, which represents pore water sulfide, is correlated to AOM and suggests the presence of an SMTZ. Additionally, carbonates within an SMTZ would be come from the bicarbonate released during AOM and will record depleted ¹³C isotope ratios.