Some Effects of Deuterium Oxide on the Quinone Hill Reaction of Chlorella pyrenoidosa Chick.

Since the discovery of the Hill reaction (9, 10, 11, 12), there has accumulated considerable evidence (5, 7, 10, 14, 15, 16, 17, 20) indicating that it is indeed a partial reaction of photosynthesis. And in particular there is evidence that the normally limiting dark reactions of photosynthesis and the Hill reaction are identical (5, 7). Numerous substances inhibitory to photosynthesis and the Hill reaction have been studied, and in certain cases the site of action of the inhibitor has been localized in more or less detail. The inhibition of photosynthesis of Chlorella by deuterium oxide was shown (6, 18) to have the characteristics of a dark reaction inhibition, but the experimental data give no indication of which dark reaction is sensitive. By means of flashing light experiments, it may be possible to determine whether or not an inhibitor affects the normally rate-limiting dark reaction; thus the inhibition by cyanide was shown (19, 23) not to affect this reaction in Chlorella and Scenedesmus. Similar studies have not been reported for other inhibitors which inhibit only at high light intensities and the reaction responsible for light saturation of Chlorella photosynthesis (and presumably also of the Hill reaction) remains one for which no specific poison has been certainly established. The experiments reported here were preliminary to an attempt to decide whether deuterium oxide is an inhibitor of this reaction. No decision can be made on the basis of data so far obtained. The present paper is particularly concerned with a comparison of the effects of deuterium oxide on the light saturation curves of photosynthesis in Chlorella and of the quinone Hill reaction of whole Chlorella cells. As an aid in interpreting these results, the effect of pH on the rate of the quinone Hill reaction of Chlorella at low and at saturating light intensities is also reported.