A decsription of The Sulfur Cycle

Sulfur cycle

   Despite nitrogen and carbon cycles in nature it is important to understand the role of the sulfur cycle.  The yellow, odorless, nonmetallic element plays an important role in plants and animals, forming methionine, cysteine and taurine amino acids. It is also a component of vitamin B1 and an antioxidant glutathione. Sulfur contributes to forming skin, nails and cartilage of joints and nose.

   The sulfur cycle is the sum of processes occurring in nature that moves sulfur through ecosystems. It encompasses both terrestrial and atmospheric processes. Stored in soil and rocks, sulfur is later released and comes in contact with air. Oxidation produces sulfate (SO4). Plants and microorganisms consume sulfate and convert it into organic forms. The food chain continues by animals eating organic food. Sulfur then returns back to soil as soon as animals die. Within atmospheric processes, sulfur is released into air by volcanoes, swamps and evaporation of water. It is inevitably comes back to Earth’s surface within rainfall. Within the ocean, sulfur moves through the food chain to the top layer. It is then released into atmosphere with water vapor. The remaining non-consumed sulfur resides at the bottom combined with iron (FeS) resulting in black color of the sediment. Moreover, humans have contributed to loss of sulfur into atmosphere by processing metals and burning fossil fuels. Reactions with other chemicals in the atmosphere create acid rains that can be damaging to environment. However, there is a positive effect of collection of sulfur dioxide in the atmosphere that acts as an offset of ultraviolet radiation.

   Winogradsky column is an intensified artificial model cultivating different microorganisms that can be observed in a lake ecosystem as well as under natural circumstances. The Winogradsky column may be viewed as a microcosm of the sulfur cycle as it relates to bodies of water. In the various layers of the column, sulfuric compounds undergo different reduction and oxidation states before finally reaching the top layer and releasing into the atmosphere. Different bacteria use sulfates or sulfides instead of water to gain energy or build their cells.

Beginning with the bottom layer such bacteria species as Desulfovibrio consume sulfate and reduce it to sulfide as follows:

2(CH2O) + SO4 =  HCO3 + H2S

Photoheterotrophic green sulfur bacteria of genus Chlorobium later oxidizes sulfide, produced by the previous bacteria and produces sulfur as follows:

CO2 + H2S = CH2O + S

Purple sulfur bacteria of genus Chromatium (photohetertrophic) use sulfates and reduce it to produce elemental sulfur.

Purple non-S bacteria ( Rhodospirilum, Rhodopseudomonas) are photoheterotrophs and exhibit the same nutritional mode as Green Sulfur Bacteria

   Bacteria in the bottom area of the column, such as Desulfovibrio, Desulfotomaculum and Desulfobacter, consume sulfates in large amounts, use it as an energy source and release toxic H2S as a waste product (dissimilatory sulfate reduction). By contrast, assimilatory sulfate reduction occurs when bacteria uses sulfates to build cell content and produce organic sulfur but do not resquire it as an energy source.