sulphur cycle

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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:

Sulphur cycleSecured image
Figure 6. Sulphur cycle
  • Simple oxygen reduction reactions in:
    • Leucothiobacteria (or colourless sulphur bacteria) such as Beggiatoa or Thiothrix:
      Formula: oxygen reduction Leucothiobacteria
    • Some Protobacteria, such as Thiobacillus thiooxydans that then oxidise the sulphur to produce sulphuric acid:
Formula: oxygen reduction protobacteria

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:

Formula:  oxygen reduction sulphureductases

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:
Formula: 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 :

Thiospirillum x 1 000Secured image
Photo 2. Thiospirillum x 1 000
Formula: Sulphur cycle - converted into sulphuric acid

The overall reaction can then be written as follows :

Formula: Sulphur cycle - overall reaction

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

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