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Biologically Produced Sulfur 169 Fig. 1 The biological sulfur cycle. SB: sulfur compound oxidizing bacteria; SRB: sulfate reducing bacteria Sulfur compound oxidizing bacteria cover the oxidation of sulfide to sul- fate with reduced sulfur compounds such as sulfur, thiosulfate, and tetrathionate as intermediates. It should be noted that there is a large variety in sulfur compound oxidizing bacteria of which some species are only capa- ble of covering a specific step in the oxidation of sulfide to sulfate (e.g., oxi- dation of thiosulfate to sulfate). Also the pathways in which the oxidation takes place may vary so that not always the same intermediates are formed. The reduction of sulfate back to sulfide takes place in two ways. In the first way, sulfate reducing bacteria (SRB) reduce sulfate via dissimilatory reduc- tion (inorganic compounds reduced to other inorganic compounds). Sec- ond, inorganic sulfur compounds can be incorporated into organic sub- strates, such as proteins, by plants or other microorganisms (assimilatory reduction). Sulfide is then formed by mineralization of the organic matter. In natural ecosystems the sulfur cycle should be in balance, meaning that the amount of sulfide that is oxidized should correspond to the amount of sulfate that is reduced. Such a balance can be found in a sulfuretum. This is a syntrophical community of bacteria in which H2S produced by sulfate re- ducing bacteria is reoxidized by the sulfur compound oxidizing bacteria. 1.2 Sulfur Compound Oxidizing Bacteria Sulfur compound oxidizing bacteria were often called sulfur bacteria, which was used to indicate autolithotrophic (using inorganic compounds to obtain carbon and hydrogen) sulfur compound oxidizing bacteria. These sulfur bacteria consisted of phototrophic (purple and green) and chemotro- phic (colorless) bacteria. A number of sulfur bacteria were however found to be able to grow organoheterotrophically (using organic compounds as car-PDF Image | Topics in Current Chemistry
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