The oxyhydrogen reaction in the presence and absence of CO2 was studied in H2-adapted Scenedesmus obliquus by monitoring the initial rates of H2, 02, and "4CO2 uptake and the effect of inhibitors on these rates with gas-sensing electrodes and isotopic techniques. In the presence of 0.02 atmosphere 02, the pH2 was varied from 0 to 1 atmosphere. Whereas the rate of 02 uptake increased by only 30%, the rate of H2 uptake increased severalfold over the range of pH2 values. At 0.1 atmosphere H2 and 0.02 atmosphere 02, rates for H2 and 02 uptake were between 15 and 25 micromoles per milligram chlorophyll per hour. As the pH2 was changed from 0 to 1 atmosphere, the quotient H2:02 changed from 0 to roughly 2. This change may reflect the competition between H2 and the endogenous respiratory electron donors. Respiration in the presence of glucose and acetate was also competitive with H2 uptake. KCN inhibited equally respiration (02 uptake in the absence of H2) and the oxyhydrogen reaction in the presence and absence of CO2. The uncoupler carbonyl cyanide ptrifluoromethoxyphenylhydrazone accelerated the rate of respiration and the oxyhydrogen reaction to a similar extent. It was concluded that the oxyhydrogen reaction both in the presence and absence of CO2 has properties in common with components of respiration and photosynthesis. Participation of these two processes in the oxyhydrogen reaction would require a closely linked shuttle between mitochondrion and chloroplast.The oxyhydrogen reaction in green algae, a process first noted and studied in depth by Gaffron (5), involves a simultaneous uptake of H2 and 02 under darkness. Using Warburg manometric techniques, he usually observed a quotient of H2:02 equal to 1. In the presence of CO2, uptake of CO2 was observed and the quotient H2:02 changed to 2. To account for these stoichiometries, perturbation of the quotient with inhibitors and the release of only traces of CO2 during the oxyhydrogen reaction, Gaffron (5) concluded that little of the 02 absorbed by the algal cell in the presence of H2 was used for normal respiratory processes. Horwitz (11) used a mass spectrometer to monitor 02 and CO2 levels continuously and observed that both cellular respiration and the oxyhydrogen reaction coupled to CO2 reduction had the same dependence on 02 tension and that rates of 02 uptake for the two reactions fell within the same range. The conclusion was reached that the oxyhydrogen reaction had some properties in common with respiration. Under oxyhydrogen reaction conditions, the reduction of CO2 is known to involve the reductive pentose-P cycle (2, 14 the oxyhydrogen reaction. The proposed mechanism involves considerable intracellular transport between mitochondrion and chloroplast.Here, we have studied the initial velocities of H2, 02, and CO2 uptake in hydrogen-adapted Scenedesmus obliquus and the effect of inhibitors on these uptake rates. The rates of H2 and 02 uptake were monitored continuously during the first few min of the oxyhydrogen reaction using gas-sensing electr...