Sulfur is a vital element in Earth's biogeochemical systems. It transitions through various inorganic states, including sulfate (SO₄²⁻), elemental sulfur (S⁰), and sulfide (S²⁻). Abiotic and biological mechanisms across oxic and anoxic environments intricately mediate these transformations. Sulfate, the most oxidized form of sulfur, is predominantly stored in rocks, marine sediments, and oceanic waters, acting as a long-term reservoir in the global sulfur cycle.
In oxic environments, chemolithoautotrophic bacteria such as Thiobacillus spp. catalyze the oxidation of reduced sulfur compounds. Sulfide (H₂S), derived from volcanic outgassing or hydrothermal emissions, is biologically oxidized to elemental sulfur and sulfate. Conversely, in anoxic conditions, sulfate-reducing bacteria (SRB) like Desulfovibrio spp. carry out dissimilatory sulfate reduction. This anaerobic process reduces sulfate to sulfide via sulfite (SO₃²⁻) as a key intermediate, often coupled with organic carbon oxidation.
Anoxygenic phototrophs, including green and purple sulfur bacteria, oxidize sulfide into intracellular sulfur granules. These granules are further oxidized to sulfate by large chemolithotrophic bacteria such as Thiomargarita spp., which contribute to acidification by releasing protons during oxidation. At hydrothermal vent ecosystems, sulfur-oxidizing chemoautotrophs harness energy from sulfide to fix carbon dioxide, supporting complex microbial and faunal communities in deep-sea habitats.
Marine phytoplankton release dimethylsulfoniopropionate (DMSP) to regulate osmotic pressure. Bacterial cleavage of DMSP yields dimethylsulfide (DMS), a volatile compound that enters the atmosphere. Photooxidation of DMS produces sulfate aerosols, which influence cloud condensation and atmospheric sulfur deposition onto terrestrial ecosystems. Ultimately, plants and microbes assimilate and reduce this sulfate while synthesizing sulfur-containing amino acids such as cysteine and methionine, completing the cycle.
Inorganic sulfur cycles through the environment as sulfate, elemental sulfur, and sulfide.
Volcanoes and hydrothermal vents emit sulfur gases like hydrogen sulfide, making vital contributions to the sulfur cycle.
For example, in anoxic, organic-rich environments near hydrothermal vents, sulfate-reducing bacteria convert sulfate mainly to hydrogen sulfide.
This gas, toxic to most life forms, is converted by various microorganisms into less harmful sulfate or elemental sulfur.
In oxygen-rich conditions, sulfur-oxidizing bacteria like Thiobacillus convert hydrogen sulfide to sulfur or sulfate, sometimes forming sulfuric acid and lowering local pH.
On the other hand, green and purple sulfur bacteria convert sulfide into sulfur granules, which Thiomargarita and similar microbes further oxidize to sulfate.
Elemental sulfur can be reduced back to sulfide by specialized anaerobic bacteria and Archaea.
In addition to inorganic transformations, marine algae form dimethyl sulfide, a volatile organic sulfur compound, which is converted in the atmosphere into aerosols that promote cloud formation and help regulate Earth’s climate.
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