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 cellular 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: Mineralization of organic sulfur into inorganic forms, such as hydrogen sulfide (H2S), elemental sulfur, as well as sulfide minerals. Incorporation of sulfide into organic compounds (including metal-containing derivatives). Oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to sulfate (). Reduction of sulfate and sulfite to sulfide. Disproportionation of sulfur compounds (elemental sulfur, sulfite, thiosulfate) into sulfate and hydrogen sulfide.
These are often termed as follows: Assimilative sulfate reduction (see also sulfur assimilation) in which sulfate () is reduced by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in sulfate and –2 in R–SH. Desulfurization in which organic molecules containing sulfur can be desulfurized, producing hydrogen sulfide gas (H2S, oxidation state = –2). An analogous process for organic nitrogen compounds is deamination. Oxidation of hydrogen sulfide produces elemental sulfur (S8), oxidation state = 0. This reaction occurs in the photosynthetic green and purple sulfur bacteria and some chemolithotrophs. Often the elemental sulfur is stored as polysulfides. Oxidation of elemental sulfur by sulfur oxidizers produces sulfate. Dissimilative sulfur reduction in which elemental sulfur can be reduced to hydrogen sulfide. Dissimilative sulfate reduction in which sulfate reducers generate hydrogen sulfide from sulfate.
Sulfur oxidation states Sulfur can be found under several oxidation states in nature, mainly −2, −1, 0, +2 (apparent), +2.5 (apparent), +4, and +6. When two sulfur atoms are present in the same polyatomic oxyanion in an asymmetrical situation, i.e., each bound to different groups as in thiosulfate, the oxidation state calculated from the known oxidation state of accompanying atoms (H = +1, and O = −2) can be an apparent average (+2 as in thiosulfate), and even differ from an entire number (+2.5 as in tetrathionate). This is the direct consequence of the different valence of each sulfur atoms present in the oxyanion. The most common sulfur species participating to the sulfur cycle are listed hereafter from the most reduced to the most oxidized:
S (−2): H2S, , ; (CH3)2S
S (−1): disulfide, , as in
S (0): native, or elemental, sulfur ()
S (+2): thiosulfate, (here +2 is only an "apparent mean" oxidation state: (+5 -1)/2 = +2 because the two sulfur atoms in thiosulfate are not at the same oxidation state. In fact they are at +5 and −1 respectively).
S (+4): SO2; sulfite ()
S (+6): (H2SO4, CaSO4).
Sulfur sources and sinks Sulfur is found in oxidation states ranging from +6 in to −2 in sulfides. Thus, elemental sulfur can either give or receive electrons depending on its environment. On the anoxic early Earth, most sulfur was present in minerals such as pyrite (FeS2). Over Earth history, the amount of mobile sulfur increased through volcanic activity as well as weathering of the crust in an oxygenated atmosphere. Earth's main sulfur sink is the oceans available as electron acceptor for microorganisms in anoxic waters.
When is assimilated by organisms, it is reduced and converted to organic sulfur, which is an essential component of proteins. However, the biosphere does not act as a major sink for sulfur, instead the majority of sulfur is found in seawater or sedimentary rocks including: pyrite rich shales, evaporite rocks (anhydrite and baryte), and calcium and magnesium carbonates (i.e. carbonate-associated sulfate). The amount of sulfate in the oceans is controlled by three major processes:
input from rivers sulfate reduction and sulfide re-oxidation on continental shelves and slopes burial of anhydrite and pyrite in the oceanic crust.
The primary natural source of sulfur to the atmosphere is sea spray or windblown sulfur-rich dust, neither of which is long lived in the atmosphere. In recent times, the large annual input of sulfur from the burning of coal and other fossil fuels has added a substantial amount of SO2 which acts as an air pollutant. In the geologic past, igneous intrusions into coal measures have caused large scale burning of these measures, and consequential release of sulfur to the atmosphere. This has led to substantial disruption to the climate system, and is one of the proposed causes of the Permian–Triassic extinction event.
Dimethylsulfide [(CH3)2S or DMS] is produced by the decomposition of dimethylsulfoniopropionate (DMSP) from dying phytoplankton cells in the ocean's photic zone, and is the major biogenic gas emitted from the sea, where it is responsible for the distinctive “smell of the sea” along coastlines. DMS is the largest natural source of sulfur gas, but still only has a residence time of about one day in the atmosphere and a majority of it is redeposited in the oceans rather than making it to land. However, it is a significant factor in the climate system, as it is involved in the formation of clouds.
Biologically and thermochemically driven sulfate reduction
Through the dissimilatory sulfate reduction pathway, sulfate can be reduced either bacterially (bacterial sulfate reduction) or inorganically (thermochemical sulfate reduction). This pathway involves the reduction of sulfate by organic compounds to produce hydrogen sulfide, which occurs in both processes.
The main products and reactants of bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) are very similar. For both, various organic compounds and dissolved sulfate are the reactants, and the products or by-products are as follows: H2S, CO2, carbonates, elemental sulfur and metal sulfides. However, the reactive organic compounds differ for BSR and TSR because of the mutually exclusive temperature regimes. Organic acids are the main organic reactants for BSR and branched/n-alkanes are the main organic reactants for TSR. The inorganic reaction products in BSR and TSR are H2S (HS−) and (CO2).
These processes occur because there are two very different thermal regimes in which sulfate is reduced, particularly in low-temperature and high-temperature environments. BSR usually occurs at lower temperatures from 0−80 °C, while TSR happens at much higher temperatures around 100–140 °C. Temperatures for TSR are not as well defined; the lowest confirmed temperature is 127 °C and the highest temperatures occur in settings around 160−180 °C. These two different regimes appear because at higher temperatures most sulfate-reducing microbes can no longer metabolize due to the denaturation of proteins or deactivation of enzymes, so TSR takes over. However, in hot sediments around hydrothermal vents BSR can happen at temperatures up to 110 °C.
BSR and TSR occur at different depths. BSR takes place in low-temperature environments, which are shallower settings such as oil and gas fields. BSR can also take place in modern marine sedimentary environments such as stratified inland seas, continental shelves, organic-rich deltas, and hydrothermal sediments which have intense microbial sulfate reduction because of the high concentration of dissolved sulfate in the seawater. Additionally, the high amounts of hydrogen sulfide found in oil and gas fields is thought to arise from the oxidation of petroleum hydrocarbons by sulfate. Such reactions are known to occur by microbial processes but it is generally accepted that TSR is responsible for the bulk of these reactions, especially in deep or hot reservoirs. Thus, TSR occurs in deep reservoirs where the temperatures are much higher. BSR is geologically instantaneous in most geologic settings, while TSR occurs at rates in the order of hundreds of thousands of years. Although much slower than BSR, even TSR appears to be a geologically fairly fast process.
BSR in shallow environments and TSR in deep reservoirs are key processes in the oceanic sulfur cycle. Approximately, 10% (of the total gas) of H2S is produced in BSR settings, whereas 90% of the H2S is produced in TSR settings. If there is more than a few percent of H2S in any deep reservoir, then it is assumed that TSR has taken over. This is due to the fact that thermal cracking of hydrocarbons doesn't provide more than 3% of H2S. The amount of H2S is affected by several factors such as, the availability of organic reactants and sulfate and the presence/availability of base and transition metals.
Microbial sulfur oxidation Sulfide oxidation is performed by both bacteria and archaea in a variety of environmental conditions. Aerobic sulfide oxidation is usually performed by autotrophs that use sulfide or elemental sulfur to fix carbon dioxide. The oxidation pathway includes the formation of various intermediate sulfur species, including elemental sulfur and thiosulfate. Under low oxygen concentrations, microbes will oxidize to elemental sulfur. This elemental sulfur accumulates as sulfur globules, intracellularly or extracellularly, to be consumed under low sulfur concentrations.Fike, Bradley, Leavitt (Jan 1, 2015). Ehrlich's Geomicrobiology (6th ed). CRC Press. pp. 479-516. ISBN 9781466592414 To ameliorate low oxidant concentrations (that is, to find an electron sink), sulfur oxidizers like cable bacteria form long chains that span the length between oxic and sulfidic zones of the coastal sediments. The bacteria present in the sulfide rich zones oxidize the sulfide and transport the electrons to the bacteria present in the oxygen rich zone through multiple periplasmic strings where the oxygen is reduced.
Anaerobic sulfide oxidation is performed by both phototrophs and chemotrophs. Green sulfur bacteria (GSB) and purple sulfur bacteria (PSB) perform anoxygenic photosynthesis fueled by sulfide oxidation. Some PSB can also perform aerobic sulfide oxidation in the presence of oxygen and can even grow chemoautotrophically under low light conditions. GSB lack this metabolic potential and have compensated by developing efficient light harvesting systems. PSB can be found in various environments ranging from hot sulfur springs and alkaline lakes to wastewater treatment plants. GSB populate stratified lakes with high reduced sulfur concentrations and can even grow in hydrothermal vents by using infra-red light to perform photosynthesis.
Hydrothermal vents emit hydrogen sulfide that support the carbon fixation of chemolithotrophic bacteria that oxidize hydrogen sulfide with oxygen to produce elemental sulfur or sulfate. The chemical reactions are as follows:
CO2 + 4 H2S + O2 → CH2O + 4 S0 + 3 H2O
CO2 + H2S + O2 + H2O → CH2O + + 2 H+
In modern oceans, Thiomicrospira, Halothiobacillus, and Beggiatoa are primary sulfur oxidizing bacteria, and form chemosynthetic symbioses with animal hosts. The host provides metabolic substrates (e.g., CO2, O2, H2O) to the symbiont while the symbiont generates organic carbon for sustaining the metabolic activities of the host. The produced sulfate usually combines with the leached calcium ions to form gypsum, which can form widespread deposits on near mid-ocean spreading centers.