User:VishalliAlagappan/Sulfur cycle

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. To ameliorate low oxidant concentrations, 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 + SO2–

4 + 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.

Sulfur metabolizing microbes are often engaged in close symbiotic relationships with other microbes, and even animals. PSB and sulfate reducers form microbial aggregates called “pink berries” in the salt marshes of Massachusetts within which sulfur cycling occurs through the direct exchange of sulfur species. The Vestimentiferan tube worms that grow around hydrothermal vents lack a digestive tract but contain specialized organelles called trophosomes within which autotrophic, sulfide oxidizing bacteria are housed. The tube worms provide the bacteria with sulfide and the bacteria shares the fixed carbon with the worms.

Marine sulfur cycle
The marine sulfur cycle is driven by sulfate reduction because hydrogen sulfide is oxidized by microbes for energy or is oxidized abiotically. Dissimilatory sulfate reduction is driven by the degradation of buried organic matter and anaerobic oxidation of methane (AOM)  both of which produce carbon dioxide. At depths where sulfate is depleted, methanogenesis is prevalent. At the sulfate-methane transition zone (SMTZ), the upwelling of methane produced by the methanogens is met by the anaerobic methanotrophic archaea in the SMTZ which oxidize it using sulfate as an electron acceptor. More sulfate is present at the SMTZ than methane. A 4:1 ratio of sulfate: methane is observed and the excess sulfate is directed towards organic matter degradation. Syntrophic aggregates of sulfate reducers and methanotrophs have been discovered and the underlying mechanisms observed include direct interspecies electron transfer using large multi heme complexes.

Sulfide produced by sulfate reduction can be oxidized by iron minerals to make iron sulfides and pyrite or used as electron donor or to sulfurize organic matter by microbes. Pyrite is formed through two pathways: the polysulfide and the hydrogen sulfide pathway. The polysulfide pathway is dominant until the depletion of elemental sulfur since elemental sulfur is necessary in the formation of polysulfides, then the hydrogen sulfide pathway takes over. Microbial sulfur oxidation utilizes multiple oxidants because the concentrations of the electron acceptors are depth dependent. In the upper sediment layers oxygen and nitrate are the preferred oxidants because of the high energy yield from the reaction, and in the suboxic zones iron and manganese take on the role. Sulfide oxidation yields various sulfur intermediates such as elemental sulfur, thiosulfate, sulfite, and sulfate.The sulfur intermediates formed during sulfide oxidation are unique to this process and thus are indicative of sulfide oxidation when found in environmental samples. Sulfur isotope fractionation of these intermediates and other sulfur species has been a useful tool in the study of sulfide oxidation.

The sulfur cycle in marine environments has been well-studied via the tool of sulfur isotope systematics expressed as δ34S. The modern global oceans have sulfur storage of 1.3×1018 kg, mainly occurring as sulfate with the δ34S value of +21‰. The overall input flux is 1.0×1011 kg/a with the sulfur isotope composition of ~3‰. Riverine sulfate derived from the terrestrial weathering of sulfide minerals (δ34S = +6‰) is the primary input of sulfur to the oceans. Other sources are metamorphic and volcanic degassing and hydrothermal activity (δ34S = 0‰), which release reduced sulfur species (such as H2S and S0). There are two major outputs of sulfur from the oceans. The first sink is the burial of sulfate either as marine evaporites (such as gypsum) or carbonate-associated sulfate (CAS), which accounts for 6×1010 kg/a (δ34S = +21‰). The second sulfur sink is pyrite burial in shelf sediments or deep seafloor sediments (4×1010 kg/a; δ34S = −20‰). The total marine sulfur output flux is 1.0×1011 kg/a which matches the input fluxes, implying the modern marine sulfur budget is at steady state. The residence time of sulfur in modern global oceans is 13,000,000 years.

Sulfurization of organic matter is a significant sulfur pool, containing 35-80% of the reduced sulfur in marine sediments. These organo-sulfur molecules are also desulfurized to release oxidized sulfur species like sulfite and sulfate. This desulfurization may allow degradation of the organic matter and thus this process determines if the organic matter is assimilated or buried. Sulfurization increases molecular weight and introduces a new moiety to the organic molecule which may inhibit its recognition by catabolic enzymes that degrade organic matter. Microbial ability for desulfurization is reflected by the presence of sulfatase genes.