Association of the Chloroplastic Respiratory and Photosynthetic Electron Transport Chains of Chlamydomonas reinhardii with Photoreduction and the Oxyhydrogen Reaction

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Abstract
The hydrogenase-dependent processes, photoreduction and the dark oxyhydrogen reaction, both of which can support CO2 assimilation, were compared with aerobic photosynthesis and respiration for their sensitivity to electron transport inhibitors in cells and intact chloroplasts of Chlamydomonas reinhardii 11-32/6. Photoreduction but not photosynthesis was inhibited in chloroplasts and the oxyhydrogen reaction detected only in cells was inhibited up to 75 and 90%, respectively, by 150 micromolar rotenone, indicating the involvement of a NAD(P)H-plastoquinone oxidoreductase in the hydrogen utilizing pathways. The oxyhydrogen reaction coupled to CO2 fixation was inhibited more than 95% by 10 micromolar 2,5 - dibromo - 3 - methyl - 6 - isopropyl - p - benzoquinone (DBMIB), a concentration which did not affect respiratory activity. In cells, both photoreduction and the oxyhydrogen reaction exhibited a similar sensitivity to salicylhydroxamic acid (SHAM) showing approximately 90% inhibition by 7 millimolar concentration. Photosynthesis was inhibited only 30% by the same concentration of SHAM. Antimycin A (18 micromolar, 10 micrograms per milliliter) inhibited both photoreduction (80%) and the oxyhydrogen reaction (92%) in cells with the oxyhydrogen reaction being approximately 10 times more sensitive to lower concentrations of the inhibitor. Antimycin A at 18 micromolar concentration did not inhibit photosynthetic CO2 fixation unless the cells were adapted to an atmosphere of N2 and the reaction conducted anaerobically. Photosynthesis, photoreduction, and the oxyhydrogen reaction coupled to CO2 fixation were all inhibited greater than 90% by 10 micromolar carbonylcyanide-p-trifluoromethoxyphenylhydrazone. ATP added to chloroplasts adapted to an atmosphere of H2 could support CO2 uptake in the dark. These results are interpreted as evidence that photoreduction and the oxyhydrogen reaction involve some common components of thylakoidal electron transport pathways in Chlamydomonas including NAD(P)H-plastoquinone oxidoreductase and the plastoquinone pool. An O2-consuming thylakoidal or mitochondrial reaction is an additional component of the oxyhydrogen reaction.