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

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.