VOL. 12 (1953) BIOCHIMICA ET BIOPHYSICA ACTA 67 



ON THE PHOTOCHEMICAL REDUCTION OF NITRATE BY ALGAE 



by 



C. B. VAN NIEL, M. B. ALLEN and B. E. WRIGHT 

 Hopkins Marine Station of Stanford University , Pacific Grove, California [U.S.A.) 



"Punkte I und 2 [viz. that in the primary reaction Oj is not evolved, and that no substances 

 are produced from which O^ is spontaneously liberated) sind das einzige Sichere, was sich liber den 

 Primarvorgang aussagen lasst; sie machen es wahrscheinlich, dass der Primarvorgang nicht das 

 Kohlensauremolekiil betrifft." (O. Warburg^, p. 206.) 



INTRODUCTION 



Thirty-three years ago Warburg and Negelein^ found that suspensions of algae, 

 illuminated in nitrate-containing solutions, could produce Og even in the absence of 

 CO 2- This phenomenon was interpreted by relating it to the fact, established by the same 

 authors, that in darkness the algae can oxidize some of their organic components to 

 CO2 with the simultaneous reduction of nitrate, presumably to NH3. Consequently, an 

 algal suspension in the presence of nitrate can never be considered as strictly free of 

 CO a, and O2 production during a period of illumination could be ascribed to the occur- 

 rence of a normal photosynthetic reaction. Schematically, this interpretation may be 

 represented by the following equations : 



In darkness 2 C + HNO3 + HgO -> 2 COg + NH3 (a) ; 



In light 2 CO2 -f 2 H2O ^ 2 C + 2 O, (b). 



The above formulation implies that, at sufficiently high light intensities, the rate 

 of O2 production is determined by the rate of CO 2 formation, and can never exceed the 

 rate of reaction (b). It was, however, observed that Og evolution in COg-free suspensions 

 of algae supplied with nitrate might proceed as much as ten times faster than CO 2 

 production in darkness. In order to make these results compatible with the suggested 

 explanation the additional hypothesis was proposed that the rate of oxidation of organic 

 cell constituentstoCOgwith concomitant reduction of nitrate was considerably accelerated 

 by light. 



Considering the state of our knowledge of the photosynthetic reaction in 1920, it 

 must be granted that Warburg and Negelein's interpretation of the photochemical 

 nitrate reduction appeared the most feasible one. But later developments in this field 

 gradually led to the conclusion that the process in question could also be understood 

 on the basis of an entirely different mechanism. 



Studies with green and purple sulfur bacteria had revealed the existence of photo- 

 synthetic processes that differ from normal green plant photosynthesis by the absence 

 References p. 73I74. 



