68 C. B. VAN NIEL, M. B. ALLEN, B. E. WRIGHT VOL. 12 (1953) 



of Og production and the requirement for an external supply of reducing agents^' '^. 

 Together with the fundamental contributions of Kluyver and Donker^ describing all 

 biochemical reactions as special cases of hydrogen (electron) transfer, these findings 

 resulted in the formulation of photosynthesis as a photochemical conversion of COg into 

 organic matter with the simultaneous oxidation of any one of a number of hydrogen 

 (electron) donors, in accord with the general equation : 



COo ^ 2 H.A i^-> (CH,0; + HgO + 2 A. 



This concept of photosynthesis suggested that the evolution of Og by green plants 

 during illumination is the result of a photochemical dehydrogenation of HgO, a possibilitv 

 which received strong experimental support, if not confirmation, from the studies of 

 Ruben et al.^, and of Vinogradov and Teis^ carried out with oxygen-i8 labeled HoO 

 and COg. The results showed convincingly that the Og evolved is derived exclusively 

 from HgO. 



A further consequence of this interpretation is that the actual assimilation or 

 reduction of CO 2 in photosynthesis should be considered as the result of cnzymic, non- 

 photochemical reactions. That such an assimilation of CO2 in the absence of light can be 

 accomplished had been definitively shown by Winogradsky's discovery of the chemo- 

 autrotrophic bacteria, as early as 1890^. That also in green plant photosynthesis COo 

 does not participate directly in the primary photochemical reaction was unequivocally 

 demonstrated when Hill^"^^ succeeded in separating the photochemical mechanism from 

 the non-photochemical enzyme systems in his epoch-making experiments with isolated 

 chloroplasts. These organs can produce O2 when illuminated, but completely lack the 

 ability to reduce CO 2. 



This need for relegating the assimilation of COo to non-photochemical reactions 

 suggested the possibility that in photosynthesizing organisms reduction processes 

 might occur in which compounds other than COg are involved. In that event the photo- 

 chemical reduction of nitrate could be interpreted as a reaction essentially analogous 

 to the photochemical reduction of CO 3, with nitrate replacing CO 2 as the final hydrogen 

 acceptor. This idea was expressed in 1941 in the following passage: 



"Green plant photosynthesis is thus considered as a complex of photochemical and dark reactions 

 in which the former consist of a photodecomposition of water, with the aid of chlorophyll and enzymes 

 of unknown nature. One series of dark reactions proceeds from here by transferring hydrogen to the 

 ultimate acceptor (COj, nitrate, hydroxylamine, etc.), while a second series results in the formation 

 of a peroxidic compound and its decomposition with the liberation of oxygen". (12, p. 323). 



Rabinowitch^^, in discussing the production of Og by illuminated algae suspensions 

 in the presence of nitrate, also offered this interpretation as a possible alternative to the 

 one advanced by Warburg and Negelein. And Warburg himself, in the second 

 edition of his book, "Schwermetallc als Wirkungsgruppen von Fermenten'^^, has revised 

 his earlier explanation in a passage which reads, in part, as follows: 



"Betrachtet man jcdoch die photochemische Wirkung der Chlorella auf Chinon und auf Nitrat 

 als analogc Vorgangc, so wird man annehmen, dass auch die photochemische Nitratreduktion nicht 

 (lurch Vcrmittlung der Kohlensaurc, sondern direktcr crfolgt. Es ware also zu untersuchcn, ob griinc 

 Granula, in Nitratlosung suspendiert, bei Bclichtung Sauerstoff entwickeln . . . lintwickeln die 

 Granula Sauerstoff, so ist der Wcg iibcr die Kohlensaurc, geradeso wie obcn in Fall dcs Chinons, aus- 

 geschlossen, da die Granula Kohlensaurc nicht reduzieren konnen". (p. 184). 



In spite of these attempts to formulate a concept of the photochemical nitrate 

 reduction that is integrated with present knowledge of various aspects of the photo- 



Rcjerences p. 73J74- 



