Hydrogenase-rescued growth is O
-dependent and H
-dependent. The genetically insulated EW11 strain expressing so FD, zm SIR, and hydrogenase maturation factors was transformed with zm FNR (A), empty vector (A), ca HydA (B), cs HydA (C), or cr HydA (D). Cells were plated at low density under custom atmospheres at varying O2 levels with and without H2. Colony sizes were measured after 3 days by imaging and automated analysis, as described in the methods. Representative images illustrating the effects of O2 and H2 on colony size are displayed in (E), with green circles indicating computationally identified colonies. Growth of strains rescued by each hydrogenase decreased monotonically with increasing O2 levels, becoming nearly undetectable at 10% O2. Atmospheric H2 improved growth but was not required for growth, suggesting an alternative source of reducing equivalents for the hydrogenase. Indicated for each curve is a best-fit sigmoid of the form area = a/(1+exp(b·(O2-c))). Error bars are 95% confidence intervals.