Sulfate-methane transition zone

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The sulfate-methane transition zone (SMTZ) is a zone in oceans, lakes, and rivers typically found below the sediment surface in which sulfate and methane coexist. The formation of a SMTZ is driven by the diffusion of sulfate down the sediment column and the diffusion of methane up the sediments. At the SMTZ, their diffusion profiles meet and sulfate and methane 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 presence of AOM marks the transition from dissimilatory sulfate reduction to methanogenesis as the main metabolism utilized by organisms. [1]

Contents

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. [2] It tends to span several centimeters, but can also reach widths up to a whole meter. [2] [3] It is characterized by low concentrations of sulfate and methane because the anaerobic oxidation of methane consumes both molecules. [4]

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. [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. Taylor. [5] Following this discovery, non-zero concentration of sulfate and methane were found in the same 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. [3] Since then, many studies have been conducted to trace the sulfate and methane profiles above, in, and below the SMTZ.

Metabolic processes

All organisms need a metabolic pathway in order to generate energy. In a sediment column, the dominant metabolism used by organisms changes with depth, as the availability of different electron acceptors changes.

Above SMTZ

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:

. [6]

Within SMTZ

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:

.

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 SMTZ

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:

. [8]

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

Geochemistry

Above 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.

SMTZ profiles.png

Within SMTZ

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]

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. [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]

Microbiology

Above SMTZ

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]

Within SMTZ

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]

Under SMTZ

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]

Impacts on global carbon cycle

The SMTZ is a major sink for methane because AOM consumes mostly all of the methane produced by methanogens. [7] It has been shown that AOM takes up over 90 percent of all the methane produced in the ocean. [12] Since methane is a prominent greenhouse gas, AOM is especially vital to controlling the amount of greenhouse gases in the atmosphere. [13] Further, the inorganic carbon entering through the SMTZ via AOM, DSR, and from methanogenic depths significantly contributes to marine inorganic carbon pool and sediment carbon burial. [14]

Isotopes

Isotopic mass balance calculations have implied that sulfate reduction and anaerobic oxidation of methane can significantly fractionate sulfur and carbon isotopes. [10] During sulfate reductions, the extent of sulfur fractionation varies depending on the environment and rates of reduction. Slower reduction rates lead to higher fractionations and sulfate concentration below 1 mM lead to lower fractionations. [6] The production and consumption of methane leads to archaeal and bacterial highly depleted in 13C biomarkers, specifically lipids. [11] The bacteria and archaea associated with the SMTZ are very depleted in 13C, with archaea generally being more depleted than bacteria. [7] The dissolved inorganic carbon (DIC) stable isotopes (δ13C) also show depleted signals due to the DIC being sourced from methane oxidation.

Isotopes have also been the main tool to study ancient SMTZs. Paleo-SMTZ have been studied using a 34S isotopic signature. Extremely 34S depleted pyrite forms from the pore water sulfide, or the by product of AOM. Thus, depleted sulfur values are correlated to AOM and suggests the presence of a SMTZ. Additionally, carbonates within an SMTZ might form from the bicarbonate released during AOM and would record depleted 13C isotope ratios expected from AOM. [15]

Related Research Articles

<span class="mw-page-title-main">Chemosynthesis</span> Biological process building organic matter using inorganic compounds as the energy source

In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing molecules and nutrients into organic matter using the oxidation of inorganic compounds or ferrous ions as a source of energy, rather than sunlight, as in photosynthesis. Chemoautotrophs, organisms that obtain carbon from carbon dioxide through chemosynthesis, are phylogenetically diverse. Groups that include conspicuous or biogeochemically important taxa include the sulfur-oxidizing Gammaproteobacteria, the Campylobacterota, the Aquificota, the methanogenic archaea, and the neutrophilic iron-oxidizing bacteria.

Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.

Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They belong to the domain Archaea and are members of the phylum Euryarchaeota. Methanogens are common in wetlands, where they are responsible for marsh gas, and can occur in the digestive tracts of animals including ruminants and humans, where they are responsible for the methane content of belching and flatulence. In marine sediments, the biological production of methane, termed methanogenesis, is generally confined to where sulfates are depleted below the top layers and methanogens play an indispensable role in anaerobic wastewater treatments. Other methanogens are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of Earth's crust, kilometers below the surface.

Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism. In anoxic environments, it is the final step in the decomposition of biomass. Methanogenesis is responsible for significant amounts of natural gas accumulations, the remainder being thermogenic.

Methanotrophs are prokaryotes that metabolize methane as their source of carbon and chemical energy. They are bacteria or archaea, can grow aerobically or anaerobically, and require single-carbon compounds to survive.

<span class="mw-page-title-main">Sulfate-reducing microorganism</span> Microorganisms that "breathe" sulfates

Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO2−
4) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.

<span class="mw-page-title-main">Sulfur cycle</span> Biogeochemical cycle of sulfur

The sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes. Steps of the sulfur cycle are:

Sulfur-reducing bacteria are microorganisms able to reduce elemental sulfur (S0) to hydrogen sulfide (H2S). These microbes use inorganic sulfur compounds as electron acceptors to sustain several activities such as respiration, conserving energy and growth, in absence of oxygen. The final product of these processes, sulfide, has a considerable influence on the chemistry of the environment and, in addition, is used as electron donor for a large variety of microbial metabolisms. Several types of bacteria and many non-methanogenic archaea can reduce sulfur. Microbial sulfur reduction was already shown in early studies, which highlighted the first proof of S0 reduction in a vibrioid bacterium from mud, with sulfur as electron acceptor and H
2 as electron donor. The first pure cultured species of sulfur-reducing bacteria, Desulfuromonas acetoxidans, was discovered in 1976 and described by Pfennig Norbert and Biebel Hanno as an anaerobic sulfur-reducing and acetate-oxidizing bacterium, not able to reduce sulfate. Only few taxa are true sulfur-reducing bacteria, using sulfur reduction as the only or main catabolic reaction. Normally, they couple this reaction with the oxidation of acetate, succinate or other organic compounds. In general, sulfate-reducing bacteria are able to use both sulfate and elemental sulfur as electron acceptors. Thanks to its abundancy and thermodynamic stability, sulfate is the most studied electron acceptor for anaerobic respiration that involves sulfur compounds. Elemental sulfur, however, is very abundant and important, especially in deep-sea hydrothermal vents, hot springs and other extreme environments, making its isolation more difficult. Some bacteria – such as Proteus, Campylobacter, Pseudomonas and Salmonella – have the ability to reduce sulfur, but can also use oxygen and other terminal electron acceptors.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

Anaerobic oxidation of methane (AOM) is a methane-consuming microbial process occurring in anoxic marine and freshwater sediments. AOM is known to occur among mesophiles, but also in psychrophiles, thermophiles, halophiles, acidophiles, and alkophiles. During AOM, methane is oxidized with different terminal electron acceptors such as sulfate, nitrate, nitrite and metals, either alone or in syntrophy with a partner organism.

Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from -2 to +6 and is reduced or oxidized by a diverse range of organisms. The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.

<i>Thioploca</i> Genus of bacteria

Thioploca is a genus of filamentous sulphur-oxidizing bacteria which occurs along 3,000 kilometres (1,900 mi) of coast off the west of South America. Was discovered in 1907 by R. Lauterborn classified as belonging to the order Thiotrichales, part of the Gammaproteobacteria. They inhabit as well marine as freshwater environments, with vast communities present off the Pacific coast of South America and other areas with a high organic matter sedimentation and bottom waters rich in nitrate and poor in oxygen. A large vacuole occupies more than 80% of their cellular volume and is used as a storage for nitrate. This nitrate is used for the sulphur oxidation, an important characteristic of the genus. Due to their unique size in diameters, ranging from 15-40 µm, they are considered part of the largest bacteria known. Because they use both sulfur and nitrogen compounds they may provide an important link between the nitrogen and sulphur cycles. They secrete a sheath of mucus which they use as a tunnel to travel between the sulfide containing sediment and the nitrate containing sea water.

<span class="mw-page-title-main">Microbial oxidation of sulfur</span>

Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to build their structural components. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy to survive, grow and reproduce. Some inorganic forms of reduced sulfur, mainly sulfide (H2S/HS−) and elemental sulfur (S0), can be oxidized by chemolithotrophic sulfur-oxidizing prokaryotes, usually coupled to the reduction of oxygen (O2) or nitrate (NO3−). Anaerobic sulfur oxidizers include photolithoautotrophs that obtain their energy from sunlight, hydrogen from sulfide, and carbon from carbon dioxide (CO2).

An oxygen minimum zone (OMZ) is characterized as an oxygen-deficient layer in the world's oceans. Typically found between 200m to 1500m deep below regions of high productivity, such as the western coasts of continents. OMZs can be seasonal following the spring-summer upwelling season. Upwelling of nutrient-rich water leads to high productivity and labile organic matter, that is respired by heterotrophs as it sinks down the water column. High respiration rates deplete the oxygen in the water column to concentrations of 2 mg/L or less forming the OMZ. OMZs are expanding, with increasing ocean deoxygenation. Under these oxygen-starved conditions, energy is diverted from higher trophic levels to microbial communities that have evolved to use other biogeochemical species instead of oxygen, these species include Nitrate, Nitrite, Sulphate etc. Several Bacteria and Archea have adapted to live in these environments by using these alternate chemical species and thrive. The most abundant phyla in OMZs are Pseudomonadota, Bacteroidota, Actinomycetota, and Planctomycetota.

<span class="mw-page-title-main">Hydrothermal vent microbial communities</span> Undersea unicellular organisms

The hydrothermal vent microbial community includes all unicellular organisms that live and reproduce in a chemically distinct area around hydrothermal vents. These include organisms in the microbial mat, free floating cells, or bacteria in an endosymbiotic relationship with animals. Chemolithoautotrophic bacteria derive nutrients and energy from the geological activity at Hydrothermal vents to fix carbon into organic forms. Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.

The deep biosphere is the part of the biosphere that resides below the first few meters of the surface. It extends down at least 5 kilometers below the continental surface and 10.5 kilometers below the sea surface, at temperatures that may reach beyond 120 °C (248 °F) which is comparable to the maximum temperature where a metabolically active organism has been found. It includes all three domains of life and the genetic diversity rivals that on the surface.

Sulfur isotope biogeochemistry is the study of the distribution of sulfur isotopes in biological and geological materials. In addition to its common isotope, 32S, sulfur has three rare stable isotopes: 34S, 36S, and 33S. The distribution of these isotopes in the environment is controlled by many biochemical and physical processes, including biological metabolisms, mineral formation processes, and atmospheric chemistry. Measuring the abundance of sulfur stable isotopes in natural materials, like bacterial cultures, minerals, or seawater, can reveal information about these processes both in the modern environment and over Earth history.

<span class="mw-page-title-main">Hydroxyarchaeol</span> Chemical compound

Hydroxyarchaeol is a core lipid unique to archaea, similar to archaeol, with a hydroxide functional group at the carbon-3 position of one of its ether side chains. It is found exclusively in certain taxa of methanogenic archaea, and is a common biomarker for methanogenesis and methane-oxidation. Isotopic analysis of hydroxyarchaeol can be informative about the environment and substrates for methanogenesis.

Methanoperedens nitroreducens is a candidate species of methanotrophic archaea that oxidizes methane by coupling to nitrate reduction.

Biphytane (or bisphytane) is a C40 isoprenoid produced from glycerol dialkyl glycerol tetraether (GDGT) degradation. As a common lipid membrane component, biphytane is widely used as a biomarker for archaea. In particular, given its association with sites of active anaerobic oxidation of methane (AOM), it is considered a biomarker of methanotrophic archaea. It has been found in both marine and terrestrial environments.

References

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  4. ↑ Bhatnagar, Gaurav; Chapman, Walter G.; Dickens, Gerald R.; Dugan, Brandon; Hirasaki, George J. (2008-02-08). "Sulfate-methane transition as a proxy for average methane hydrate saturation in marine sediments". Geophysical Research Letters. 35 (3). doi:10.1029/2007gl032500. ISSN   0094-8276.
  5. ↑ Oremland, Ronald S.; Taylor, Barrie F. (February 1978). "Sulfate reduction and methanogenesis in marine sediments". Geochimica et Cosmochimica Acta. 42 (2): 209–214. doi:10.1016/0016-7037(78)90133-3. ISSN   0016-7037.
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  8. ↑ Thauer, Rudolf K.; Hedderich, Reiner; Fischer, Reinhard (1993), "Reactions and Enzymes Involved in Methanogenesis from CO2 and H2", Methanogenesis, Springer US, pp. 209–252, doi:10.1007/978-1-4615-2391-8_5, ISBN   9781461360131
  9. ↑ Scientists, Yair Rosenthal; Ann E. Holbourn; Denise K. Kulhanek; and the Expedition 363 (2017-02-08). International Ocean Discovery Program Expedition 363 Preliminary Report. International Ocean Discovery Program Scientific Prospectus. International Ocean Discovery Program. doi:10.14379/iodp.pr.363.2017.{{cite book}}: CS1 maint: multiple names: authors list (link)
  10. 1 2 Brunner, Benjamin; Arnold, Gail L.; Røy, Hans; Müller, Inigo A.; Jørgensen, Bo B. (2016). "Off Limits: Sulfate below the Sulfate-Methane Transition". Frontiers in Earth Science. 4. doi: 10.3389/feart.2016.00075 . ISSN   2296-6463.
  11. 1 2 Boetius, Antje; Ravenschlag, Katrin; Schubert, Carsten J.; Rickert, Dirk; Widdel, Friedrich; Gieseke, Armin; Amann, Rudolf; Jørgensen, Bo Barker; Witte, Ursula (2000-10-05). "A marine microbial consortium apparently mediating anaerobic oxidation of methane". Nature. 407 (6804): 623–626. doi:10.1038/35036572. ISSN   0028-0836. PMID   11034209.
  12. ↑ Hinrichs, K.-U.; Boetius, A. (2002). Ocean Margin Systems. Springer, Berlin, Heidelberg. pp. 457–477. doi:10.1007/978-3-662-05127-6_28. ISBN   9783642078729.
  13. ↑ Lloyd, Karen G.; Lapham, Laura; Teske, Andreas (2006-11-01). "An Anaerobic Methane-Oxidizing Community of ANME-1b Archaea in Hypersaline Gulf of Mexico Sediments". Applied and Environmental Microbiology. 72 (11): 7218–7230. doi:10.1128/AEM.00886-06. ISSN   0099-2240. PMC   1636178 . PMID   16980428.
  14. ↑ Akam, Sajjad A.; Coffin, Richard; Abudlla, Hussain (2006-11-01). "Dissolved Inorganic Carbon Pump in Methane-Charged Shallow Marine Sediments: State of the Art and New Model Perspectives". Frontiers in Marine Science. 7 (206). doi: 10.3389/fmars.2020.00206 . ISSN   2296-7745.
  15. ↑ Peketi, A.; Mazumdar corresponding author, A.; Joshi, R. K.; Patil, D. J.; Srinivas, P. L.; Dayal, A. M. (October 2012). "Tracing the Paleo sulfate-methane transition zones and H2S seepage events in marine sediments: An application of C-S-Mo systematics" (PDF). Geochemistry, Geophysics, Geosystems. 13 (10): n/a. doi:10.1029/2012gc004288. ISSN   1525-2027.{{cite journal}}: |last2= has generic name (help)
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