Fig. 1.
H2 production profiles of Vibrio tritonius strain AM2. Effects of pH on cell growth (A) and H2 production (B). Effect of temperature on H2 production of Vibrio tritonius strain AM2 in batch culture (C). A and B: closed bar, on glucose; open bar, on mannitol. Symbols: ●, glucose at 37 °C; ○, mannitol at 37 °C; ▲, glucose at 30 °C; ▵, mannitol at 30 °C; ■, glucose at 25 °C; □, mannitol at 25 °C. Culture conditions were set at 30 °C (A and B), and pH 6 (C). n = 3, error bars represent SD.
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Calculation of the molar yields of H2 from both substrates revealed differences when compared to the volume of H2 produced by strain AM2 (Fig. 2). The molar yields of H2 from mannitol were significantly higher than the yields from glucose (P < 0.05) in the pH range 5.5–6.5, and the cultures at pH 5.5 and 6.0 gave the maximum molar yield on both substrates: 1.2 mol H2/mol glucose and 1.4 mol H2/mol mannitol. In contrast, the pH dependency of the molar yields of formate, a precursor of H2 in the FHL pathway, revealed an opposite trend to the yields of H2 from both glucose and mannitol ( Fig. 2).