THE MECHANISM OF PHOTOSYNTHESIS 327 



of activation of a hydrogenase in light is in agreement with the develop- 

 ment of a reducing action by the light. The following observation, made 

 by Gaffron (1944), is probably related to the one just mentioned. Cells 

 of Scenedesmus, "adapted" to hydrogen, show a photochemical evolution 

 of hydrogen. In a nitrogen atmosphere these cells produce hydrogen 

 also in the dark, but the rate is at least ten times higher in the light. 

 Gaffron assumed that the photochemical evolution of hydrogen is depend- 

 ent upon the presence of suitable hydrogen donors in the cells. The 

 observed photoacceleration of a hydrogenase activity may well be con- 

 sidered as evidence for a direct reducing effect of the light. 



A phenomenon that probably belongs to the same type is the evolution 

 of hydrogen in the light by suspensions of the purple bacterium Rhodo- 

 spirillum, as observed by Gest and Kamen (1949) and by Gest et al. 

 (1950). Since it does not occur in the dark, it may again be considered 

 as a sign of the generation of reducing power in the light. For the reduc- 

 tion of carbon dioxide, substrate hydrogen seems preferable to gaseous 

 hydrogen. Hydrogen production is inhibited by molecular nitrogen and 

 ammonium ions. It thus seems that nitrogen and ammonium compounds 

 are preferable acceptors for this form of reducing power. 



Gaffron (1937) once assumed that, at the beginning of a light period, 

 a catalyst of the Blackman system is reactivated by conversion into a 

 reduced state. In other cases, observations were made which suggest 

 that the over-all effect of light in photosynthesis of green cells results in a 

 shift to the oxidized side, just as was found in purple sulfur bacteria. 

 Fluorescence, for example, shows a downward trend, pointing to an 

 increased state of oxidation obviously related to the production of oxy- 

 gen (Wassink and Katz, 1939). Also a few direct measurements of the 

 redox potential in green-cell suspensions show a shift to the oxidized side 

 during illumination (Tang and Lin, 1937). Brief illuminations of sus- 

 pensions of Chlorella and of diatoms, i.e., during 5 or 10 sec each minute 

 with interspersed dark periods, keep fluorescence at a high level, indi- 

 cating the maintenance of a relatively reduced state. Under these con- 

 ditions a dark system appears to be inactivated, which leads to an 

 unchanged rate of oxygen production at low light intensities but decreased 

 production at high intensities (cf. Wassink and Kersten, 1943-1945, and 

 Fig. 5-17). At illuminations of 10 sec each minute, the effect is about 

 intermediate between the behavior at 5 sec each minute and that with 

 continuous illumination. The reactivation of the dark system proceeds 

 along with oxygen production, and in connection with the fluorescence 

 measurements it seems reasonable to assume that this catalyst is more 

 active in an oxidized state. The reactivation of this dark catalyst, how- 

 ever, very likely is not a direct effect of light and does not interfere with 

 the view that light produces a reducing power. It shows that in the sta- 

 tionary conditions of normal photosynthesis, with respect to the general 



