ATP-independent reduction of CO2 to C1 products by VFe protein. a) Time-dependent formation of CO from CO2-reduction in H2O (▽) or D2O (○). b) Comparison of time-dependent formation of CH4 from CO2-reduction in H2O (▽) or D2O (○) with that from CO-reduction in H2O (▼) or D2O (•).The amounts of CO supplied (in b) were 136 ppm (▼) and 178 ppm (•), respectively, which simulated the amounts of CO formed in H2O (▽) and D2O (○), respectively, from CO2-reduction at 180 min (see a, arrows). The inset (in b) is a blown-up of the first 30 min of CH4 formation in H2O where CO (▼) or CO2 (▽) was supplied as a substrate. Data are presented as mean ± SD (N = 3) after background correction.