Sulphur is found in living organisms in the form of compounds such as amino acids, coenzymes and vitamins. It can be utilised by different types of organisms in several forms; Figure 16.4 shows the principal components of the sulphur cycle.
In its elemental form, sulphur is unavailable to most organisms; however, certain bacteria such as Acidithiobacillus are able to oxidise it to sulphate (1), a form that can be utilised by a much broader range of organisms (see Chapter 7):
Powdered sulphur is often added to alkaline soils in order to encourage this reaction and thereby reduce the pH.
Sulphate-reducing bacteria convert the sulphate to hydrogen sulphide gas (2) using either an organic compound or hydrogen gas as electron donor:
These bacteria are obligate anaerobes, and the process is termed dissimilatory sulphate reduction.
Plants are also able to utilise sulphate, incorporating it into cellular constituents such as the amino acids methionine and cysteine (3) (assimilatory sulphate reduction).
When the plants die, these compounds are broken down, again with the release of hydrogen sulphide (4) (see mineralisation, above).
Green and purple photosynthetic bacteria and some chemoautotrophs use hydrogen sulphide as an electron donor in the reduction of carbon dioxide, producing elemental sulphur and thus completing the cycle (5):
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