THE MECHANISM OF PHOTOSYNTHESIS 329 



suggested that it is vinyl phosphate (Benson and Calvin, 1950). It seems 

 unlikely that this substance as such is identical with the "reducing 

 power," but it may well be generated rather directly from this reducing 

 power, i.e., in a reaction sequence: 



dark light 



DH > D + H+-+E^EH > E + H + • • • + F -^ FH, 



in which FH eventually might represent the vinyl phosphate, EH the 

 "energy acceptor" normally present at the pigment-protein complex, and 

 DH the ultimate hydrogen donor (cf. also Wassink, 1947). 



Vogler (1942) suggested that in photosynthesis radiant energy may be 

 stored in a form available for CO2 reduction, from analogy with his 

 experiments on the metabolism of Thiohacillus. In view of the facts 

 observed by Vogler and Umbreit (1942) on the coupUng of the energy- 

 producing sulfur oxidation in Thiohacillus with carbon dioxide assimi- 

 lation by presumably energy-rich phosphate compounds, it is tempting 

 to look upon these compounds as connected with the storage of radiant 

 energy in photosynthesis also. In Thiohacillus inorganic phosphate is 

 taken up during the oxidative phase and converted into an adenosine- 

 triphosphate, whereas during the reductive phase phosphate is released 

 (LePage and Umbreit, 1943). 



A few observations on phosphate interactions with photosynthesis have 

 since been made (Aronoff and Calvin, 1948; Emerson et al., 1944), but 

 not with very conclusive results. In the writer's opinion this could be 

 due to the fact that no separation of an eventual phosphate-accumulating 

 phase and a phosphate-consuming phase had been attempted. This was 

 the basis of some studies by Wassink et al. (Wassink et al., 1949; Wassink 

 et al., 1951), who illuminated suspensions of Chromatium and of Chlorella 

 under various gas phases. In all cases the absence of carbon dioxide led 

 to a markedly increased consumption of phosphate. In Chlorella it was 

 shown that the phosphate taken up is converted into a TCA-insoluble 

 form (cf. Fig. 5-18). These findings are in accordance with studies by 

 Gest and Kamen (1948, p. 309), who, using tracer phosphate in Rhodo- 

 spirillum, concluded that uptake and turnover are both much greater in 

 the light than in the dark. Simonis and Gruber (1952) found P^' phos- 

 phate uptake by Helodea densa increased by light and carbon dioxide. 

 With the colorimetric technique used by Wassink et al., the turnover of 

 phosphate between fractions that maintain the same stationary concen- 

 trations cannot be detected. Under the conditions chosen, however, a 

 shift results, the explanation of which goes along similar Unes as that 

 given for the redox-potential shifts. 



Kandler (1950), working with Chlorella, measured short-time shifts of 

 TCA-soluble phosphate and found distinct changes in the phosphate 

 level (TCA-soluble) at the shift from dark to hght and vice versa. Since 



