Sulfur cycle

The biogeochemical cycling of sulfur is extremely complex and cannot be simply described diagrammatically. The complexity of the cycle is dictated through the relatively high number of oxidation states which sulfur can have in solution and in the presence of oxygen.  The most significant carbon compound in the atmosphere is probably carbon dioxide serving as a link among terrestrial and marine environments but there is no direct sulfur equivalent. In prioritizing volatile atmospheric sulfur compounds sulfur dioxide SO₂, hydrogen sulfide H₂S and carbonyl sulfide COS carbon disulfide CS₂ dimethyl sulfide CH₃SCH₃ dimethyl disulfide CH₃SSCH₃ and methane thiol CH₃SH may be significant depending on the geographical location examined. The nature of sulfur’s chemistry may mean that nonvolatile molecules such as methane sulfonate CH₃SO₃ - and sulfate SO₄ - may also be present in quantity in the atmosphere as dissolved aqueous ions.

The biogeochemical cycling of sulfur serves to describe regulation on a global scale. The sulfur compound dimethylsulfonium propionate DMSP is produced intracellularly through marine photosynthetic algae.  During and after algal blooms in the open oceans this DMSP is released into solution on cell death where it is metabolized by marine microorganisms to produce dimethyl sulfide DMS. The generation of DMS is sufficiently high for some of the compound to enter the atmosphere as a gas above the ocean where photooxidation through the sun’s rays lead to the breakdown of DMS into two solid forms of sulfur:  sulfate and methane sulfonate. These solids act as nuclei for the condensation of water and clouds are formed.

Clouds reduce the sunlight reaching the ocean surface and the growth of the photosynthetic algae that produced the DMSP in the first place is reduced. This regulatory cycle stops algae covering the entire open ocean although only in regions where man has not upset this cycle by the dumping of sewerage or other com- pounds in which the algae might feed on.Sulfur   cycling   also  occurs by  terrestrial  ecosystems where a  combination  of sulfate-reducing  Bacteria  such  as   Desulfobacter  and   Desulfovibrio  species  and sulfate-oxidizing Bacteria  such as Thiobacillus species interconvert hydrogen sulfide, elemental sulfur sulfate, thiosulfate and  polythionates. Sulfur is also assimilated through all organisms into the amino acids cysteine and methionine and so ultimately into proteins. Sulfur is also assimilated for the prosthetic protein groups known as iron-sulfur clusters.