mption rate, respectively, than the parent strain.Figure 1Physiological changes of *T. onnurineus* NA1 through serial transfers into fresh MM1 medium containing 147 mM sodium formate. After 2 (closed circle), 32 (open square), 62 (closed inverted triangle), 92 (open triangle), 122 (closed square) and 156 (open circle) transfers, the cell density (expressed as optical density at 600 nm) (**a**) was determined at the indicated time points. Formate consumption rates (**b**) and H~2~ production rates (**c**) were determined during the exponential phase. All experiments were conducted independently in duplicate.

Kinetic analysis of formate consumption and H~2~ production {#Sec4}
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Even though the adapted strain exhibited enhanced cell growth and hydrogen production on formate-containing medium, it was difficult to characterize the physiological changes quantitatively in a serum vial. The pH in the culture medium rapidly increased and the cells did not grow exponentially, with a final pH of approximately 8 at stationary phase. Therefore, the kinetic properties of WTF-156T were investigated in a pH-controlled bioreactor operated at pH 6.2 in comparison with those of the parent strain. Notably, WTF-156T exhibited a shorter lag time in the bioreactor culture. The WTF-156T strain reached 0.7--0.8 optical density (OD~600~) after 5 hours while the parent strain reached an OD~600~ of 0.39 at 16 h culture (Fig. [2a](#Fig2){ref-type="fig"}). Furthermore, the H~2~ production rate of the WTF-156T strain was higher than that of the parent strain, with maximum rates of 117 mmol liter^−1^ h^−1^ for the WTF-156T strain and 31 mmol liter^−1^ h^−1^ for the parent strain, respectively (Fig. [2b](#Fig2){ref-type="fig"}). While the parent strain consumed only 2% formate after 7 hours, WTF-156T consumed 84% during the same period of time (Fig. [2c](#Fig2){ref-type="fig"}). Formate consumption was well balanced with H~2~ production, and the molar ratios of 