Sulphur Cycle

Sulphur Cycle

Sulphur is present in sulphur containing aminoacids. The sulphur cycle involves oxidation – reduction reaction between Sulphate (SO₄), Elemental S and H₂S and hence there is change in the valence states of sulphur from -2 to +6.

The basic steps involved in sulphur cycle are

1.     Sulphide/ sulphur oxidation

2.     Sulphate reduction

3.     Sulphur reduction

4.     Organicsulphur compound oxidation or reduction

5.     Desulfurylation

Sulphide/Sulphur oxidation (H₂S→S⁰→SO₄^2-)

It is carried out by prokaryotes under aerobic and anaerobic conditions. Under aerobic conditions, H₂S is spontaneously oxidized at neutral pH to elemental sulphur. Elemental sulphur is oxidized to suphates by chemolithotrophic bacteria like Thiobacillus, Beggiatoa.

If light is available, H₂S can be used as electron donor to carry out photosynthesis under anoxic conditions by phototrophic sulphur bacteria like Chromatium and Chlorobium.

Sulphate reduction

When sulphate is present in habitats, different groups of microorganisms are capable of carrying out sulphate reduction.

Beijerinck described the use of sulphate (SO₄^2-) as a terminal electron acceptor during anaerobic respiration to form sulphide (H₂S). This process is called Dissimilatory Sulphate Reduction(DSR). The anerobic bacteria capable of carrying out DSR are Desulfovibrio, Desulfococcus, Desulfotomaculum (Figure 11.6) This reaction by sulphate reducers requires organic carbon sources like pyruvate or lactate. H₂S accumulated in such habitats by the action of sulphate reducers is toxic to aerobic organisms.

The reduction of sulphate to H₂S, for building up aminoacids and proteins

is called as assimilatory sulphate reduction. The H₂S thus produced is immediately incorporated into organic compounds.

Sulphur reduction (S⁰→H₂S):

The dissimilative sulphur reducing bacteria can reduce elemental sulphur to hydrogen sulphide. Example: Desulfuromonas, an obligate anaerobe. Under aerobic conditions, organisms like Pseudomonas, Proteus and Salmonella are also capable of performing this reaction.

Organic sulphur compounds reduction/oxidation

Organic sulphur compounds like dimethyl sulphide can be used as carbon and energy source for many microorganisms.

Desulfurylation

It is a process where organic sulphur compounds are used up by microorganisms for energy to produce H₂S.