FIG 7.

Influence of hydrogen oxidation on electron turnover and pH in Beggiatoa sp. 35Flor cultures. (A) Electron budgets in hydrogen-supplemented (+) and -unsupplemented (–) cultures over 4 weeks. The total electron demand (shown in red) was calculated based on the measured oxygen consumption rate and the estimated rate of CO₂ fixation into biomass (<CH₂O>). Weekly averages of CO₂ fixation rates were estimated on the basis of the increase in Beggiatoa protein levels and a cell carbon-to-protein ratio of 1.13 (wt/wt; determined for closely related strain Beggiatoa sp. MS-81-6 under similar growth conditions; [31]). According to this estimation, CO₂ fixation accounted for ≤6.4% of the total electron demand at all times. Hatched areas indicate the contribution of sulfide oxidation (2 electrons per H₂S → S⁰) to the electron supply; dotted areas represent the contribution of hydrogen oxidation (2 electrons per H₂). The electron demand, which cannot be fulfilled by the reactions described above, is most likely met by the oxidation of sulfur to sulfate (6 electrons per S⁰). (B) Average pH profiles (± standard deviation; n = 6) measured in hydrogen-supplemented (black) and -unsupplemented (white) cultures after 7 days of incubation. Mats in hydrogen-supplemented cultures were situated 5.2 to 6.2 mm below the air-agar interface; mats in hydrogen-unsupplemented cultures were located at a depth of 6.6 to 7.5 mm.