e was measured during the incubation. After incubating at 80 °C for 5 min, the WTF-156T strain showed 17.4% higher specific formate uptake rate (302.4 mmol/g/h) than the parent strain (257.6 mmol/g/h) (Fig. [6](#Fig6){ref-type="fig"}). The mutant at TON_1573 (A52T) in the parent background showed 9.3% higher specific formate consumption rate (281.6 mmol/g/h) than the parent strain. On the other hand, the deletion of TON_1573 significantly decreased formate consumption rate (187.2 mmol/g/h) and hydrogen production rate. Taken together, the A52T mutation in TON_1573 appears to be a beneficial mutation to promote cell growth by increasing formate consumption.Figure 6The effect of alteration at formate transporter (TON_1573) on formate uptake rate using resting cell suspensions. Formate consumption rate (**a**) and H~2~ production rate (**b**) of TON_1573 (A52T) mutant and TON_1573 deletion mutant were compared among the parent and WTF-156T strains using resting cell assay. Error bars indicate the standard deviations of independent duplicate experiments.

Adaptation in a bioreactor {#Sec7}
--------------------------

Previously, we reported the kinetic analysis of repeated batch culture of *T. onnurineus* NA1^[@CR24]^, where we did not consider the possibility that the strain might be modified genetically. However, we realized that cells have been serially transferred in bioreactors through anaerobically harvesting at the end of each batch culture and inoculation into fresh medium, at least more than 10 times. Therefore, we speculated that the serial transfer in bioreactors might also cause genetic changes in the parent strain. In fact, the cell stock after the serial transfer in bioreactors showed enhanced growth and H~2~ production than the parent strain (Supplementary Table [2](#MOESM1){ref-type="media"}). Based on the assumption that genomic changes might occur in cells during the time period, the genome sequence of cells was determined using PacBio Single Molecule Real-Time (SMRT) sequencing techn