tolerance for three hydrogenases in situ. Cells expressing a hydrogenase from either C. acetobutylicum (ca HydA), C. saccharobutylicum (cs HydA) or C. reinhardtii (cr HydA) were exposed to 10% O2 at 1 atm total pressure for the indicated times. Remaining activity was assessed with methyl viologen, as described in the methods. Three biological replicates are plotted for each time point. Significant differences in both maximal activity and O2 tolerance were evident. Hydrogenase inactivation by O2 was well described by first order kinetics, and the best-fit exponential decay curves are shown. The ca HydA enzyme exhibited a characteristic half-life of 8 (± 0.8) minutes, the cs HydA enzyme 2.7 (± 0.2) minutes and cr HydA 1.0 (± 0.3) minutes, including 95% confidence intervals.