72 



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



VOL. 12 (1953) 



determined at one specific intensity during the early part of an experiment differed con- 

 siderably from those measured at the same intensity after the cell suspension had 

 meanwhile been subjected to different intensities of illumination. This should be ascribed 

 to the fact that such experiments extended over many hours, especially with dilute 

 suspensions, where rates were calculated from measurements taken during a 2-hour 

 exposure at one light intensity in order to insure reliable data for computations. The 

 present report is therefore restricted to observations made at high light intensity, with 

 one companion experiment permitting comparison with rates at a light intensity below 

 saturation. 



A summary of the results is presented in Table L 



TABLE I 



ILATION IN 

 IN /iL PER HOUR 



* This experiment was carried out with a cell suspension in tap water; it is probable that the 

 relatively high value for COg assimilation is the result of the formation of alkali during the nitrate 

 reduction, with the consequent absorption of COj by the suspension fluid. An experimental determina- 

 tion of a possible increase in bicarbonate was not made. 



It will be seen that at high light intensities the rate of O^ production is greater in 

 the presence than in the absence of nitrate, while the rate of COg assimilation is approxi- 

 mately the same. Consequently, the quotient, CO^/Og, is invariably lower in the former 

 than in the latter case. In contrast, under conditions of light limitation the rate of Og 

 evolution appears not to be affected by the presence of nitrate ; nowits influence expresses 

 itself in a lower rate of CO 2 assimilation. This last result is in complete agreement with 

 the earlier and more extensive data of Cramer and Myers^*'. 



From these results it can be concluded that the photochemical nitrate reduction 

 cannot be explained by invoking the production of CO 2 from cellular constituents and 

 nitrate because in all cases photosynthesis was measured in the presence of sufficient 

 COo to eliminate this component as a limiting factor. 



One might postulate a conversion of organic cell material with nitrate as oxidant 

 to substances other than COg which could compete with the latter as hydrogen acceptors 

 in photosynthesis and thus cause an increased rate of O2 production at high light intensi- 

 ties. Such a postulate does not seem warranted, however, since it involves the hidden 

 assumption that the enzymic oxidation of cell constituents with nitrate yields oxida- 



References p. yjjy^. 



