sulphur cycle

Figure 6 provides a diagram illustrating the Sulphur cycle. Chapter aquatic organisms defines the place occupied within the bacterial kingdom by the micro-organisms that are responsible for these transformations. The hydrogen sulphide, anaerobic fermentation process converts organic sulphur compounds into H₂S while other, aerobic, bacteria can oxidise H₂S to form colloidal sulphur and then, possibly, sulphuric acid:

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• Simple oxygen reduction reactions in:
  • Leucothiobacteria (or colourless sulphur bacteria) such as Beggiatoa or Thiothrix:

    

    

    

  • Some Protobacteria, such as Thiobacillus thiooxydans that then oxidise the sulphur to produce sulphuric acid:

In an aerobic medium, the final term can be the appearance of sulphates; conversely, in an anaerobic medium, these sulphates can be reduced by other bacteria (Desulfovibrio desulfuricans, some Clostridium…) that secrete sulphureductases capable of catalysing the entire reaction:

There are also sulphur reducing bacteria (some species of Clostridium and Welchia).

Some of these bacteria are involved in the corrosion phenomena that affect cast iron, steel and concete pipelines (see chapter corrosion in metal and concrete).

• Photosynthetic reactions in Rhodotiobacteria (or purple sulphur bacteria) such as Chromatium, Thiospirillum (photo 2) or Thiopedia, and also in Chlorothiobacteria (or green sulphur bacteria) such as Chlorobium or Chlorobacterium; the reduced sulphur acts as a provider of electrons for converting mineral carbon (CO₂) into organic matter accompanied by the formation of elementary sulphur:

The sulphur produced is stored in the bacteria cell or excreted depending on the species. Later on, it may be converted into sulphuric acid :

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The overall reaction can then be written as follows :

* In these reactions, (CH₂O) symbolises the organic matter that has been synthesised.