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Biologically Produced Sulfur 171 data in Table 1 should be seen only as a list of properties of some sulfur compound oxidizing bacteria. Most important in the classification of sulfur bacteria is the distinction between phototrophic and chemotrophic sulfur bacteria. Phototrophic (purple or green) bacteria use light as energy source to reduce CO2 to carbo- hydrates. Reduced sulfur compounds are used as an electron donor for this reduction, which takes place under anaerobic conditions. The oxidation re- actions of sulfide to sulfur and sulfate by phototrophic bacteria are called the Van Niel reactions: 2H2S þ CO2lightfCH2Og þ H2O þ 2S0 ð1Þ light H2S þ 2CO2 þ 2H2O ! 2fCH2Og þ H2SO4 ð2Þ The first Van Niel reaction Eq. (1) has a strong parallel with the photosyn- thesis in plants [3]: light 2H2OþCO2 !fCH2OgþH2OþO2 ð3Þ In fact, cyanobacteria are capable of both ways of photosynthesis, which is why it was found difficult to place them in the group of plants (prokary- otes) or bacteria (eukaryotes) [12]. The chemotrophic (colorless) sulfur bacteria obtain energy from the chemical aerobic oxidation of reduced sulfur compounds. The overall reac- tions occurring, concerning the biological oxidation of sulfide, are the for- mation of sulfur (at low oxygen concentrations) and the formation of sulfate (when there is an excess of oxygen): 2H2SþO2 !2S0 þ2H2O ð4Þ H2S þ 2O2 ! H2SO4 ð5Þ Apart from the above sulfide oxidation reactions, other sulfur com- pounds, such as sulfur and thiosulfate, can be oxidized by sulfur compound oxidizers. Furthermore, nitrate can be used as oxidant instead of oxygen. In Table 2 a list is shown of some other sulfur compound oxidation reactions occurring in chemotrophic sulfur compound oxidizing bacteria. It should be Table 2 Overall reactions occurring in sulfur compound oxidizing bacteria (after [5]) Overall reactions S0 þ3=2O2 þH2O!H2SO4 (6) S O2 þ2O þH O!H SO þSO2 (7) 2322244 S O2þ1=4O þ1=2H O!1=2S O2þOH (8) 232246 2S O2 þ7O þ6H O!6H SO þ2SO2 (9) 4622244 5H2Sþ2HNO3 !5S0 þN2 þ6H2O (10) 5H2Sþ8HNO3 !5H2SO4 þ4N2 þ4H2O (11)PDF Image | Topics in Current Chemistry
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