Nutrients present in the environment, such as nitrogen, sulfur, phosphorous, manganese, and iron are cycled, carried out during the course of microbial growth & metabolism. This process of nutrient cycling is termed biogeochemical cycling.
During the biochemical cycle, the nutrient characteristic is changed by the oxidation-reduction process caused by microorganisms involved in the nutrient cycle.
Sulfur is an abundant element found on earth crust as well as deep oceans; this nutrient is cycled in the oxidized and reduced valence state, by the process of sulfur cycle.
Where sulfur elements are found?
In a natural environment, sulfur can exist in the range of valence states between +6 to -2.
Most surplus forms of sulfur are sulfate, elemental sulfur, and sulfides, it also exists in the combined forms such as sulfur dioxide, and poly sulfur compounds like thiosulfates.
Generally, sulfur elements have two main reservoirs i.e., deep oceanic rocks [includes minerals, sediments] and organic reservoirs [include plants and dead organic matters.
Ocean has 90% of sulfur elements present in the form of sediments and minerals, constantly undergo the sulfur cycle.
Why do microorganisms participate in the sulfur cycle?
Sulfur is a significant component for a living cell, contains in the form of cysteine, methionine which is required in the metabolism of the microorganisms.
To acquire the simple form of sulfur, microorganisms go through biological oxidation-reduction reactions.
Photosynthetic & Chemosynthetic microorganisms contribute to the sulfur cycle and transform sulfur in large amounts by redox reactions.
Microorganisms responsible for dissimilatory sulfate reduction are Desulfovibrio, Desulfotomaculum, found in soils, where water is logged and sediments, Desulfuromonas, present in water containing H2S and marine mud.
Assimilatory sulfate reduction is carried out by plants, algae, fungi, and bacteria.
Sulfur oxidation is carried out by chemolithotrophs such as Thiobacillus, Beggiatoa, and photolithotrophs such as Chloribium and Chromatium.
Elemental sulfur (S) is converted into sulfate (SO42-) by photosynthetic and chemolithoautotrophs like Thiobacillus, sulfide acts as an e– source.
When sulfate enters the reduced habitats, sulfate reduction is carried out by other microorganisms.
For instance, during anaerobic respiration, Desulfovibrio used sulfate as an external e– acceptor, this reduction process is termed Dissimilatory sulfate reduction which causes sulfide accumulation in the surrounding.
Assimilatory sulfate reduction is carried out by microorganisms to synthesize proteins & amino acids.
Dissimilatory elemental sulfur reduction is performed by a group of bacteria which includes Desulfurous, cyanobacteria, and thermophilic archaea present in hypersaline sediments.
Microorganisms such as Alteromonas, Clostridium, and obligate anaerobes such as Desulfovibrio & Desulfotomoculum can reduce sulfite (SO32-) into sulfide (H2S).
Group of bacteria which carry out aerobic anoxygenic photosynthesis in deep water zone, also are sulfur oxidizers, they belong to photolithotrophs group which includes Chromatium and Chlorobium.
Bacterioplankton, floating bacteria, use dimethylsulfoniopropionate (DMSP) as a source of sulfur for their synthesis of protein.
In absence of microorganisms, key steps of the sulfur cycle can occur when redox reaction conditions and pH is suitable. For e.g, elemental sulfur can be oxidized into sulfide, at room temperature and neutral pH, the half-life of sulfide is 10 min.