The aerobic chemolithotrophic group contains organisms with the ability to reduce CO2 to CH2O using the Calvin cycle and energy derived from oxidation of reduced minerals. It cannot operate independent of oxygenic photosynthesis, however, because of its strict requirement for O2.
Oxidation of H2S yields SO4-, electrons, and hydrogen ions (protons) as in the half-cell reactions depicted in Table 9.5. Water contributes O during formation of SO|-. Electrons are passed down the respiratory chain from S to reduce O2 to water and generate proton motive force for synthesis of ATP (Eq. , Table 9.5) as per Figs. 9.4 and 9.5. Consequently water is both used and formed in such oxidations and there may be no change in total water. Oxidation of S0 to SO4- yields only 6e- per mole of S (Table 9.6, Eq. ) and consequently has a lower energy yield (Eq. ) than does oxidation of H2S to SO4- (Table 9.5, Eq. ). The overall reaction shows the correct stoichiometry between S, O, and H together with generation of acidity.
TABLE 9.5 Oxidation of H2S and Role of O2 as Terminal Electron Acceptor by Chemolithotrophs such as Thiobacillus spp.
H2S + 4HOH ^ SO4- + 8e- + 10H+ 8e-+ 8H+ + 2O2 ^ 4HOH - 794.5 kJ 
(Reproduced from McGill (1996), with permission from SBCS and SLCS.)
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