Article published in BMC Infectious Diseases Journal, an impact factor open access journal

The Sulphur Cycle

THE SULPHUR CYCLE

Sulphur like nitrogen and carbon, is an essential part of all living cells and constitute about 1% of the dry matter of cell. Sulphur containing amino acids are always present in almost all kinds of proteins. Plants can absorb directly the sulphur containing amino acids, e.g., cystine, homocysteine, cysteine and methionine.  Besides S-containing amino acids, it is also an important part of growth factors like thiamin, biotin and lipoic acids. However these amino acids fulfill only a small proportion or requirements for sulphur to the plants.  To fulfill rest of the requirements of plants, sulphur passes through a cycle of transformation mediated by microorganisms. Sulphur compounds involved in the sulphur cycle are H₂S, S°, thiosulphate, sulphite (SO₃^--) and sulphate (SO₄^--). Most common forms of sulphur are H₂S, S° and SO₄^--. The greatest reservoir of sulphur in the biosphere is the sulphate in the oceans. Whereas in the soil,

 it accumulates mainly as a constituent of organic compounds and has to be converted to sulphates to become readily available to the plants. The complete cycling of sulphur is schematically represented and some important steps are discussed as under:

Figure 1: Schematic Representation of Sulphur Cycle (Source Microbial ecology, Heintz and Scott)

1. Source of Sulphur:

The major source of sulphur in marine environment is sulphate. While in the lithosphere, the sulhpur is found as sulphate and iron sulphide (FeS). The metal sulphides (FeS) are readily oxidized to sulphates b y both biological and chemical processes.

FeS      ®        oxidized to      ®        sulphate (SO₄^--) [Biologically and Chemically]

The sulphur can exist in many different oxidation states ranging from -2 to +6:

Form                           Example                     Oxidation state

S^2-                                          Sulphide                                  -2

S°                                Elemental form                       0

S₂O₄                                      Hyposulphite                           +2

SO₃^--                            Sulphite                                   +4

SO₄^--                            Sulphate                                  +6

2. Assimilatory Sulphate Reduction:

In most habitats, sulphates are available to plants and microorganisms which is assimilated into sulphydryl compounds (R-SH) that becomes a part of biomass of living organism. This reduction process in which sulphate becomes biomass is known as Assimilatory Sulphate reduction. Various microorganisms and green photosynthetic plants are involved in the process. Since animals can only uptake the reduced form of sulphur it is an important step to transfer the S into a food chain. The assimilation of inorganic sulphate involves a series of transfer reactions initiated by the reaction of sulphate with ATP to form APS (Adenosine 5 Phosphosulphate) and pyrophosphate (PPi). In second reaction, the APS is converted into PAPS (3-phospho adenosine 5-phosphosulpahte) using one more ATP.

            i. SO₄^-- + ATP             ®                    APS + PPi

                                       ATP sulphurylase

           

ii. APS + ATP             ®                    PAPS + ADP

                                         APS kinase

The active sulphate of PAPS is subsequently reduced to yield sulphite and adenosine 3-5 diphosphate (PAP). Again in the second reduction step sulphides are formed which is immediately incorporated into an amino acid, as a reduced organic S-compound.

iii. 2e^- + PAPS             ®                    SO₃^--  + PAP

                                                NADH-PAPS reductase

           

iv. SO₃^-- + PAP                       ®                    S^2- + 6H⁺ + 6e^-

                                                 NADH- SO₃^—reductase

           

v. S^2- + O-acetylserine             ®                    cysteine + H₂O

3. Release of H₂S

H₂S is release to biosphere by both aerobic and anaerobic processes. They can be either release from decomposition of organic compounds (Desulphurylation) or by reduction of inorganic sulphate (Dissimilatory sulphate reduction).

(A) Degradation of Organic Compounds to Release H₂S (Desulphurylation): Sulphur is released from organic dead matter as H₂S by a process called desulphuration.

(i) Degradation of proteins (proteolysis) liberates amino acids which generally contain sulphur.

Protein =/degradation/=Amino acid

(ii) Enzymatic activity of many heterotrophic bacteria results in the release of H2S from further degradation of sulphur containing amino acids.         

Amino acids (cysteine, methionine, cystine)              ®        H₂S

(B) Dissimilatory sulphate reduction (Sulphate respiration): It is also called sulphate respiration. Sulphates may also be reduced to H₂S by the action of Desulfotomaculum bacteria. The process occurs in anaerobic condition below zero (0) mV redox potential. The process is similar to assimilatory sulphate reduction where intermediates are thiosulphate and tetrathionate. Sulphate is first reduced to H₂S by sulphate reducing microorganisms under anaerobic conditions. Many bacteria including species of Bacillus, Pseudomonas, Desulfovibrio do this work. The mechanism of sulphate reduction to hydrogen sulphide involves, firstly, the reduction of sulphate to sulphite utilizing ATP and, secondly, reduction of sulphite to hydrogen sulphide. The whole mechanism of the reduction of sulphate to hydrogen sulphide by Desulfovibrio desulfuricans, the most important bacterium of this reduction. The only different is the H₂S released in this phenomenon is not incorporated into the biomass as in case of assimilatory sulphate reduction.

SO₄^-- + ATP                ®        APS + PPi       ®        thiosulphate     ®        tetrathionate

Tetrathionate               ®                    H₂S (evaporation)

The major genera which take part in the process are Desulfovibrio, Desulfotomaculum, Desulfomonas, Desulfobacter, Desulfococcus, Desulfonema, Desulfosarcina etc.

Example of sulphate reduction by Desulfotomaculum:

CaSO₄ + 4H₂ =/ Desulfotomaculum /= Ca(OH)₂ + H₂S + 2H₂O

Besides microbiological process, the geochemical processes such as volcanic activities also reduce sulphates into H₂S.

The dissimilatory sulphate reduction process carried out by microbes has an economically important application as it causes corrosion of iron. The reaction mechanisms are shown as;

4Fe + 8H⁺                   ®                    4Fe²⁺ + 4H₂ (Anaerobic polarization)

(H₂ produced protects the iron from further oxidation)

4H₂ + SO₄^--                  ®                    H_2­S + 2H_2­O + 2OH^-

In the presence of H₂S and OH^-, the Fe²⁺ is converted into FeS and Fe(OH)₂

4Fe²⁺ + H_2­S + 2OH^- + 4H_2­O              ®        FeS + 3Fe(OH)₂ + 6H⁺

Final Reaction:

4Fe + SO₄^--  + 2H⁺ + 2H_2­O                 ®        FeS + 3Fe(OH)₂

4. Oxidation of Hydrogen Sulphide (H₂S) to Elemental Sulphur

Hydrogen sulphide undergoes decomposition to produce elemental sulphur by the action of certain photosynthetic sulphur bacteria, e.g., members belonging to the families Chlorobiaceae (Chlorobium) and Chromatiaceae (Chromatium). Example:

CO2 +2H2S ---Photosynthetic sulphur bacteria ---CH2O + H2O +S

Some non-sulphur purple bacteria, e.g., Rhodospirillum, Rhodopseudomonas, and Rhodomicrobium which are facultative phototrophs and grow aerobically in the dark and anaerobically in the light can also degrade H₂S to elemental sulphur

5. Oxidation of Elemental sulphur to Sulphates

Elemental form of sulphur accumulated in soil by earlier described processes cannot be utilized as such by the plants. It is oxidized to sulphates by the action of chemolithotrophic bacteria of the family Thiobacteriaceae (Thiobacillus thiooxidans. Thiobacillus ferroxidans, Thiobacillus denitroficans). Example:

2S + 2H2O + 3O2 ---Thiobacillus thioxidans----2H2SO4
Sulphates are the compounds that can readily be taken by the plants and are beneficial to agriculture in the following three ways:

1.      It is the most suitable source of sulphur and is readily available to plants.

2.      Accumulation of sulphate solubilizes organic salts that contain plant nutrients such as phosphates and metals.

3.      Sulphate is the anion of a strong mineral acid (H₂SO₄) and prevents excessive alkalinity due to ammonia formation by soil microorganisms.

Sulphate is assimilated by plants and is incorporated into sulphur amino acids and then into proteins. Animal fulfill their demand or sulphur by feeding on plants and plant products.

Winogradsky Column: The Winogradsky column, which is named after the Russian Microbiologist Sergei Winogradsky, is a model ecosystem that is used in the study of aquatic and sediment microorganism. A Winogradsky column consists of mud or sediment placed within a glass or clear plastic cylinder. The height of column allows the development of an aerobic zone at the surfaces and microaerophillic and anoxic zones below the surface. The column is exposed to light so that various photosynthetic populations develop at differing depths in the column. Various microbial zones developed in the column as below;

1. Photosynthetic zone: The top layer of the column is the photosynthetic zone. The predominant organisms in this zone are Algae and cyanobacteria. In presence of sunlight, these oraganisms photosynthesize to prepare their own food and oxygen is evolved in the reaction.

CO₂     ®        CH₂O,             H₂O     ®        O₂

                        Carbohydrates

2. Aerobic heterotrophic zone: Beneath the photosynthetic zone there is highly aerated zone. The oxygen and organic compounds in the zone is supplied by photosynthetic zone. A large number of heterotrophic bacteria are present in this zone. Carbohydrates and organic compounds are utilized in this zone.

CH₂O ®        CO₂,                O₂        ®        H₂O

            Carbohydrates

Figure 2: Winogradsky column (source Microbial Ecology; Atlas and Bartha)

3. Microaerophillic zone: Low oxygen level is preferred by sulphide oxidizers such as Beggiatoa and Thiothrix, the gradient organisms with white gray filamentous growth. Non acid tolerant Thiobacilli, Thiobacillus thioparus also grow in this zone.

H₂S      ®        S°                           CO₂     ®        CH₂O,            

                        Elemental sulphur                                     Carbohydrates

4. Facultative anaerobic zone: Organic compounds are utilized in this zone in presence or absence of oxygen. The oxygen level decreases more beneath this zone causing anaerobic zone. Enterobacter, Klebseilla, Citrobacter are predominant in this zone.

CH₂O ®        CO₂ + H₂

Carbohydrates

5. Red Brown zone: The upper portion of sand column is reddish brown with the growth of non-sulfur anaerobic photoheterotrophs (Rhodospirilliaceae). E.g. Rhodospirillum

 CH₂O             ®        H₂                         CO₂     ®        CH₂O

Carbohydrates                                                                        Carbohydrates

6. Red-purple zone: Below the red-brown zone, there is a red-purple zone which indicates the growth of purple sulfur bacteria (Chromatiaceae and Ectothiorhodospiraceae). They are photosynthetic organism. E. g. Chromatium spp. and Ectothiorhodospirillum spp. These organisms oxidize H₂S into elemental sulfur.

H₂S      ®        S°                          CO₂     ®        CH₂O

Carbohydrates                                                                        Carbohydrates

7. Green-gray zone: Even lower. A greenish zone indicates the growth of the green sulfur bacteria (Cholobiaceae). These bacteria grow using sulphide or elemental sulfur as the electron donor. E. g.: Chlorobium spp.

H₂S      ®        S°                          CO₂     ®        CH₂O

Carbohydrates                                                                        Carbohydrates

8. Black zone: The intensely black zone extending upward from the bottom of the column which shows the activity of sulphate reducers. The black coloration is due to metal sulphides principally ferrous sulphide (FeS). Some of sulphate reducers are Desulfovibrio, Desulfotomaculum, Desulfomonas, Desulfobacter, Desulfococcus, Desulfonema, Desulfosarcina etc.

SO₄^-- ®           H₂S                      CH₂O              ®        CO₂

                                                                Carbohydrates

The zone also signifies the fermentative heterotrophic zone:

CH₂O             ®        CO₂ + H₂

Carbohydrates